Code Bubbles - Cool Java IDE

Seeing is believing so just visit http://www.cs.brown.edu/people/acb/codebubbles_site.htm and watch the video.
What an awesome IDE!!!

Download Page: Google Collections Library 1.0 – FINAL

Maven users can simply add the following dependency info to use Google Collections Library 1.0 – FINAL.

<dependency>
	<groupId>com.google.collections</groupId>
	<artifactId>google-collections</artifactId>
	<version>1.0</version>
	<type>jar</type>
	<scope>compile</scope>
</dependency>

It’s a question to find an algorithm that gives the highest floor where you can drop a water balloon without breaking it at a 100 story building. You have only two balloons. I had actually seen this problem before and solved it already. It was not balloons but eggs. The solution is simple. Since if you miss two chances then have no balloons available, you need to try from the lowest safe level which means throw it from the first floor then if it does not pop, pick it up and go up to the level 2 and repeat it until the balloon pops. When you get the level from which the balloon bursts after dropping it, one level below this level is the highest safe level. (e.g. If the balloon first breaks on the 20th floor, the highest safe floor is the 19th).

It’s easy isn’t it? Yet the post above has one more condition to make it a little bit more difficult which is that you can try only 15 times. This means if I try the solution I’ve just described, I can test up to only the 15th floor. If the breaking point is the 40th floor or even the 16th floor, I’ll never know it. When I first saw it, I tried to solve it for about five minutes yet couldn’t solve it. That time, I was too busy to think about it further thus couldn’t be bothered to do it. So I saved the URI of the post and forgot about it. Last night, no actually today morning, at approximately 6 am, I went to bed (yeah, am I a vampire or what? ). Although I didn’t even try to think about the problem yet all of sudden I came up with the answer of that question. It was just like the ‘Eureka‘ story of Archimedes. Just like he figured out how to assess the purity of a golden crown when he stepped into a bath. OK, stop flattering myself I figured it out as the question was easy to solve.

Well, an important point here is that I in fact often find solutions to some difficult problems when I am not struggling to solve it yet am doing something irrelevant to the problem such as working out at the gym, washing dishes after a meal, lying on the bed to sleep or even when I’m dreaming. So if you can’t solve some problem then why don’t you try something else rather than thinking about the solution all the time. Your brain may need to relax or when you use the other parts of the brain by doing something else, it might figure it out.

Anyway, here is what my brain told me that time.

Since I have only two balloons, if I choose a wrong floor twice, I lose all the chances. Then what if I check every second level as I have two balloons. So first, if I drop it from the second level and it breaks then I still have one more balloon so that I can try it from the first level and I can find if the highest safe level is the first or there is no safe level. If the balloon is fine after dropping it from the second level then I can pick it up and try it from the fourth level and so on. However, with this solution I can check at most until the 30th floor which is better than checking up to the 15th one but still not enough to check the 100 stories. This means that it is not sufficient to check ever second level in order to find the floor and I should focus on not only the two balloons but also the 15 times of attempts. From the first solution above, I know that I can definitely have 15 times if I try from the lowest safe floor and move up. It means if the number of floors between the already known highest safe level and the breaking point level found is the same as or fewer than the number of attempts I still have, I can find the very first level where the balloon bursts after dropping it (In order to reduce the number of attempts, of course, I have to make it the same as the number of attempts still left). Therefore I can tell the highest safe level that is one level below the breaking point level found. This should be easier to understand with an example.

Let’s assume that the breaking point is the 17th floor.

At first, I have 15 tries to drop one balloon from the 15th floor.
The balloon’s still OK and I now have 14 times of attempts. Since the number of attempts I still have is 14 time, I can’t try it from the 30th floor which is 15 + 15 yet I have to try it from the 29th floor (15 + 14). Because the breaking point is the 17th floor, the balloon breaks on the 29th floor. Now I have 13 times of attempts and 1 balloon left. From the first try, I know the 14th floor is fine so I try from the next floor that is floor 15 to see if floor 14 is the highest safe floor. It turns out the 15th one is fine and now I have 12 times left. 16th -> OK / 11 times left, 17th -> it breaks! / 10 times left
So I can tell the 16th floor is the highest safe floor and I didn’t even use all the 15 times of attempts.

The post that has the question specifies the floor starts from 0 so let’s make the first floor is floor 0 and see have it works. I made a table which shows the number of attempts required to find the highest safe floor.

====================================================
 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15   tries
====================================================
 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14	- 1
15 16 17 18 19 20 21 22 23 24 25 26 27 28		- 2
29 30 31 32 33 34 35 36 37 38 39 40 41			- 3
42 43 44 45 46 47 48 49 50 51 52 53				- 4
54 55 56 57 58 59 60 61 62 63 64				- 5
65 66 67 68 69 70 71 72 73 74					- 6
75 76 77 78 79 80 81 82 83						- 7
84 85 86 87 88 89 90 91							- 8
92 93 94 95 96 97 98							- 9
99												- 10

So if the highest safe floor is 73. The first balloon breaks on the floor 74 after 6 times of attempts. After that try from the floor 65 (attempts: 7) through 66 (8), 67 (9), 68 (10), 69 (11), 70 (12), 71 (13), 72 (14) to 73 (15) and the second balloon hasn’t broken so I can tell the highest safe floor is the floor 73.

====================================================
 1  2  3  4  5  6  7  8[ 9]10 11 12 13 14 15
====================================================
65 66 67 68 69 70 71 72[73]74					-[6]

attempts: 6 times to get the floor 74 + 9 times to check until the floor 73 = 15 times.

Once I found the solution writing code is a piece of cake.
I roughly wrote the main part of the code like this.

private static final int MAX_ATTEMPTS = 15;

private int theFloor = (int) (Math.random() * 100);
private int numberOfBalloons = 2;
private int numberOfAttempts = 0;

// ...

public void findHighestSafeFloor()
{
	int previousFloorIndex = -1;
	int floorIndex = MAX_ATTEMPTS - 1;
	int highestSafeFloor = 0;

	while (MAX_ATTEMPTS > numberOfAttempts)
	{
		numberOfAttempts++;
		if (theFloor <= floorIndex)
		{
			numberOfBalloons--;
			highestSafeFloor = findHighestSafeFloor(previousFloorIndex + 1, floorIndex);
			break;
		}
		previousFloorIndex = floorIndex;
		floorIndex += (MAX_ATTEMPTS - numberOfAttempts);
		if (99 < floorIndex)
		{
			floorIndex = 99;
		}
	}

	/* print the result. */
}

private int findHighestSafeFloor(final int start, final int end)
{
	for (int i = start; i < end; i++)
	{
		numberOfAttempts++;
		if (theFloor == i)
		{
			numberOfBalloons--;
			return (i - 1);
		}
	}
	return (end - 1);
}

I am sure that the findHighestSafeFloor() method above works but I decided to make it recursion. Why? No special reasons. I just like recursion as I haven’t used it for approximately four years since I last programmed in a functional programming language like Haskell in my math class thus miss it.

Although the code can be very simple as the following code with recursion, it has to increase the number of attempts and to decrease the number of balloons left, so it would be a bit more verbose as the second code.

private int findHighestSafeFloor(final int start, final int end)
{
	return (start < end ? (theFloor == start ? (start - 1) : findHighestSafeFloor((start + 1), end)) : (end - 1));
}

private int findHighestSafeFloor(final int start, final int end)
{
	if (start < end)
	{
		numberOfAttempts++;
		if (theFloor == start)
		{
			numberOfBalloons--;
			return (start - 1);
		}
		else
		{
			return findHighestSafeFloor((start + 1), end);
		}
	}
	else
	{
		return (end - 1);
	}
}

Now it’s time to code the complete programme and test it. Well, since it’s a very small programme, I didn’t do object-oriented programming.

package com.lckymn.kevin.findingfloor;

public final class HighestSafeFloorFinder
{
	private static final int TOP_LEVEL = 99;
	private static final int MAX_ATTEMPTS = 15;

	private int theBreakingPointFloor;
	private int numberOfBalloons;
	private int numberOfAttemptsMade;

	public void init(final int theBreakingPointFloor)
	{
		this.theBreakingPointFloor = theBreakingPointFloor;
		System.out.println("The breaking point: " + theBreakingPointFloor);
		numberOfBalloons = 2;
		numberOfAttemptsMade = 0;
	}

	public void findHighestSafeFloor()
	{
		int previousFloorIndex = -1;
		int floorIndex = MAX_ATTEMPTS - 1;
		int highestSafeFloor = 0;

		while (MAX_ATTEMPTS > numberOfAttemptsMade)
		{
			dropBalloon();
			if (theBreakingPointFloor <= floorIndex)
			{
				breakBalloon();
				highestSafeFloor = findHighestSafeFloor(previousFloorIndex + 1, floorIndex);
				break;
			}
			previousFloorIndex = floorIndex;
			floorIndex += (MAX_ATTEMPTS - numberOfAttemptsMade);
			if (TOP_LEVEL < floorIndex)
			{
				floorIndex = TOP_LEVEL;
			}
		}

		if (0 > highestSafeFloor)
		{
			System.out.println("There is no floor that you can drop a balloon without breaking it. Even the first floor (floor 0) is not safe.");
		}
		else
		{
			System.out.println("The highest safe floor: " + highestSafeFloor);
		}

		System.out.println("The number of attempts made: " + numberOfAttemptsMade);
	}

	private int findHighestSafeFloor(final int start, final int end)
	{
		if (start < end)
		{
			dropBalloon();
			if (theBreakingPointFloor == start)
			{
				breakBalloon();
				return (start - 1);
			}
			else
			{
				return findHighestSafeFloor((start + 1), end);
			}
		}
		else
		{
			return (end - 1);
		}
	}

	private void dropBalloon()
	{
		if (MAX_ATTEMPTS == numberOfAttemptsMade)
		{
			throw new IllegalStateException("You've already tried 15 times.");
		}
		numberOfAttemptsMade++;
	}

	private void breakBalloon()
	{
		if (0 == numberOfBalloons)
		{
			throw new IllegalStateException("You've already broken all the balloons available.");
		}
		numberOfBalloons--;
	}

	public static void main(String[] args)
	{
		HighestSafeFloorFinder highestSafeFloorFinder = new HighestSafeFloorFinder();
		for (int i = 0; i < 100; i++)
		{
			System.out.println("[Test Number: " + i + "]");
			highestSafeFloorFinder.init(i);
			highestSafeFloorFinder.findHighestSafeFloor();
			System.out.println("=======================================");
		}
		System.exit(0);
	}
}

Well, the code looks a bit ugly but in my defence, I’d just like to see the result and am busy so had no stomach for making it pretty. (Oh! such a poor excuse / but really busy )

The dropBalloon() and the breakBalloon() are added to increase the number of attempts made and to decrease the number of balloons left respectively. The dropBalloon() method also checks if the number of attempts already made is 15 and it tries to make another one than throws an exception telling that all the chances have been used. Similarly the breakBalloon() method checks if all the available balloons have broken if so, it also throws an exception saying there are no available balloons left. Thus I can see if the programme can find the highest safe floor with 15 times or fewer times of attempts and only two water balloons. If it cannot, it immediately throws an exception and stop the programme so I can see my programme fails to prove my algorithm.

The init() method is to initialise all the variables so that I can test it many times. It checks all the floors from the first one (floor 0) to the last one (floor 99) so if the programme works fine without any exception thrown then it might prove my answer is correct.

Let’s check out the result!

[Test Number: 0]
The breaking point: 0
There is no floor that you can drop a balloon without breaking it. Even the first floor (floor 0) is not safe.
The number of attempts made: 2
=======================================
[Test Number: 1]
The breaking point: 1
The highest safe floor: 0
The number of attempts made: 3
=======================================
[Test Number: 2]
The breaking point: 2
The highest safe floor: 1
The number of attempts made: 4
=======================================
[Test Number: 3]
The breaking point: 3
The highest safe floor: 2
The number of attempts made: 5
=======================================
[Test Number: 4]
The breaking point: 4
The highest safe floor: 3
The number of attempts made: 6
=======================================
[Test Number: 5]
The breaking point: 5
The highest safe floor: 4
The number of attempts made: 7
=======================================
[Test Number: 6]
The breaking point: 6
The highest safe floor: 5
The number of attempts made: 8
=======================================
[Test Number: 7]
The breaking point: 7
The highest safe floor: 6
The number of attempts made: 9
=======================================
[Test Number: 8]
The breaking point: 8
The highest safe floor: 7
The number of attempts made: 10
=======================================
[Test Number: 9]
The breaking point: 9
The highest safe floor: 8
The number of attempts made: 11
=======================================
[Test Number: 10]
The breaking point: 10
The highest safe floor: 9
The number of attempts made: 12
=======================================
[Test Number: 11]
The breaking point: 11
The highest safe floor: 10
The number of attempts made: 13
=======================================
[Test Number: 12]
The breaking point: 12
The highest safe floor: 11
The number of attempts made: 14
=======================================
[Test Number: 13]
The breaking point: 13
The highest safe floor: 12
The number of attempts made: 15
=======================================
[Test Number: 14]
The breaking point: 14
The highest safe floor: 13
The number of attempts made: 15
=======================================
[Test Number: 15]
The breaking point: 15
The highest safe floor: 14
The number of attempts made: 3
=======================================
[Test Number: 16]
The breaking point: 16
The highest safe floor: 15
The number of attempts made: 4
=======================================
[Test Number: 17]
The breaking point: 17
The highest safe floor: 16
The number of attempts made: 5
=======================================
[Test Number: 18]
The breaking point: 18
The highest safe floor: 17
The number of attempts made: 6
=======================================
[Test Number: 19]
The breaking point: 19
The highest safe floor: 18
The number of attempts made: 7
=======================================
[Test Number: 20]
The breaking point: 20
The highest safe floor: 19
The number of attempts made: 8
=======================================
[Test Number: 21]
The breaking point: 21
The highest safe floor: 20
The number of attempts made: 9
=======================================
[Test Number: 22]
The breaking point: 22
The highest safe floor: 21
The number of attempts made: 10
=======================================
[Test Number: 23]
The breaking point: 23
The highest safe floor: 22
The number of attempts made: 11
=======================================
[Test Number: 24]
The breaking point: 24
The highest safe floor: 23
The number of attempts made: 12
=======================================
[Test Number: 25]
The breaking point: 25
The highest safe floor: 24
The number of attempts made: 13
=======================================
[Test Number: 26]
The breaking point: 26
The highest safe floor: 25
The number of attempts made: 14
=======================================
[Test Number: 27]
The breaking point: 27
The highest safe floor: 26
The number of attempts made: 15
=======================================
[Test Number: 28]
The breaking point: 28
The highest safe floor: 27
The number of attempts made: 15
=======================================
[Test Number: 29]
The breaking point: 29
The highest safe floor: 28
The number of attempts made: 4
=======================================
[Test Number: 30]
The breaking point: 30
The highest safe floor: 29
The number of attempts made: 5
=======================================
[Test Number: 31]
The breaking point: 31
The highest safe floor: 30
The number of attempts made: 6
=======================================
[Test Number: 32]
The breaking point: 32
The highest safe floor: 31
The number of attempts made: 7
=======================================
[Test Number: 33]
The breaking point: 33
The highest safe floor: 32
The number of attempts made: 8
=======================================
[Test Number: 34]
The breaking point: 34
The highest safe floor: 33
The number of attempts made: 9
=======================================
[Test Number: 35]
The breaking point: 35
The highest safe floor: 34
The number of attempts made: 10
=======================================
[Test Number: 36]
The breaking point: 36
The highest safe floor: 35
The number of attempts made: 11
=======================================
[Test Number: 37]
The breaking point: 37
The highest safe floor: 36
The number of attempts made: 12
=======================================
[Test Number: 38]
The breaking point: 38
The highest safe floor: 37
The number of attempts made: 13
=======================================
[Test Number: 39]
The breaking point: 39
The highest safe floor: 38
The number of attempts made: 14
=======================================
[Test Number: 40]
The breaking point: 40
The highest safe floor: 39
The number of attempts made: 15
=======================================
[Test Number: 41]
The breaking point: 41
The highest safe floor: 40
The number of attempts made: 15
=======================================
[Test Number: 42]
The breaking point: 42
The highest safe floor: 41
The number of attempts made: 5
=======================================
[Test Number: 43]
The breaking point: 43
The highest safe floor: 42
The number of attempts made: 6
=======================================
[Test Number: 44]
The breaking point: 44
The highest safe floor: 43
The number of attempts made: 7
=======================================
[Test Number: 45]
The breaking point: 45
The highest safe floor: 44
The number of attempts made: 8
=======================================
[Test Number: 46]
The breaking point: 46
The highest safe floor: 45
The number of attempts made: 9
=======================================
[Test Number: 47]
The breaking point: 47
The highest safe floor: 46
The number of attempts made: 10
=======================================
[Test Number: 48]
The breaking point: 48
The highest safe floor: 47
The number of attempts made: 11
=======================================
[Test Number: 49]
The breaking point: 49
The highest safe floor: 48
The number of attempts made: 12
=======================================
[Test Number: 50]
The breaking point: 50
The highest safe floor: 49
The number of attempts made: 13
=======================================
[Test Number: 51]
The breaking point: 51
The highest safe floor: 50
The number of attempts made: 14
=======================================
[Test Number: 52]
The breaking point: 52
The highest safe floor: 51
The number of attempts made: 15
=======================================
[Test Number: 53]
The breaking point: 53
The highest safe floor: 52
The number of attempts made: 15
=======================================
[Test Number: 54]
The breaking point: 54
The highest safe floor: 53
The number of attempts made: 6
=======================================
[Test Number: 55]
The breaking point: 55
The highest safe floor: 54
The number of attempts made: 7
=======================================
[Test Number: 56]
The breaking point: 56
The highest safe floor: 55
The number of attempts made: 8
=======================================
[Test Number: 57]
The breaking point: 57
The highest safe floor: 56
The number of attempts made: 9
=======================================
[Test Number: 58]
The breaking point: 58
The highest safe floor: 57
The number of attempts made: 10
=======================================
[Test Number: 59]
The breaking point: 59
The highest safe floor: 58
The number of attempts made: 11
=======================================
[Test Number: 60]
The breaking point: 60
The highest safe floor: 59
The number of attempts made: 12
=======================================
[Test Number: 61]
The breaking point: 61
The highest safe floor: 60
The number of attempts made: 13
=======================================
[Test Number: 62]
The breaking point: 62
The highest safe floor: 61
The number of attempts made: 14
=======================================
[Test Number: 63]
The breaking point: 63
The highest safe floor: 62
The number of attempts made: 15
=======================================
[Test Number: 64]
The breaking point: 64
The highest safe floor: 63
The number of attempts made: 15
=======================================
[Test Number: 65]
The breaking point: 65
The highest safe floor: 64
The number of attempts made: 7
=======================================
[Test Number: 66]
The breaking point: 66
The highest safe floor: 65
The number of attempts made: 8
=======================================
[Test Number: 67]
The breaking point: 67
The highest safe floor: 66
The number of attempts made: 9
=======================================
[Test Number: 68]
The breaking point: 68
The highest safe floor: 67
The number of attempts made: 10
=======================================
[Test Number: 69]
The breaking point: 69
The highest safe floor: 68
The number of attempts made: 11
=======================================
[Test Number: 70]
The breaking point: 70
The highest safe floor: 69
The number of attempts made: 12
=======================================
[Test Number: 71]
The breaking point: 71
The highest safe floor: 70
The number of attempts made: 13
=======================================
[Test Number: 72]
The breaking point: 72
The highest safe floor: 71
The number of attempts made: 14
=======================================
[Test Number: 73]
The breaking point: 73
The highest safe floor: 72
The number of attempts made: 15
=======================================
[Test Number: 74]
The breaking point: 74
The highest safe floor: 73
The number of attempts made: 15
=======================================
[Test Number: 75]
The breaking point: 75
The highest safe floor: 74
The number of attempts made: 8
=======================================
[Test Number: 76]
The breaking point: 76
The highest safe floor: 75
The number of attempts made: 9
=======================================
[Test Number: 77]
The breaking point: 77
The highest safe floor: 76
The number of attempts made: 10
=======================================
[Test Number: 78]
The breaking point: 78
The highest safe floor: 77
The number of attempts made: 11
=======================================
[Test Number: 79]
The breaking point: 79
The highest safe floor: 78
The number of attempts made: 12
=======================================
[Test Number: 80]
The breaking point: 80
The highest safe floor: 79
The number of attempts made: 13
=======================================
[Test Number: 81]
The breaking point: 81
The highest safe floor: 80
The number of attempts made: 14
=======================================
[Test Number: 82]
The breaking point: 82
The highest safe floor: 81
The number of attempts made: 15
=======================================
[Test Number: 83]
The breaking point: 83
The highest safe floor: 82
The number of attempts made: 15
=======================================
[Test Number: 84]
The breaking point: 84
The highest safe floor: 83
The number of attempts made: 9
=======================================
[Test Number: 85]
The breaking point: 85
The highest safe floor: 84
The number of attempts made: 10
=======================================
[Test Number: 86]
The breaking point: 86
The highest safe floor: 85
The number of attempts made: 11
=======================================
[Test Number: 87]
The breaking point: 87
The highest safe floor: 86
The number of attempts made: 12
=======================================
[Test Number: 88]
The breaking point: 88
The highest safe floor: 87
The number of attempts made: 13
=======================================
[Test Number: 89]
The breaking point: 89
The highest safe floor: 88
The number of attempts made: 14
=======================================
[Test Number: 90]
The breaking point: 90
The highest safe floor: 89
The number of attempts made: 15
=======================================
[Test Number: 91]
The breaking point: 91
The highest safe floor: 90
The number of attempts made: 15
=======================================
[Test Number: 92]
The breaking point: 92
The highest safe floor: 91
The number of attempts made: 10
=======================================
[Test Number: 93]
The breaking point: 93
The highest safe floor: 92
The number of attempts made: 11
=======================================
[Test Number: 94]
The breaking point: 94
The highest safe floor: 93
The number of attempts made: 12
=======================================
[Test Number: 95]
The breaking point: 95
The highest safe floor: 94
The number of attempts made: 13
=======================================
[Test Number: 96]
The breaking point: 96
The highest safe floor: 95
The number of attempts made: 14
=======================================
[Test Number: 97]
The breaking point: 97
The highest safe floor: 96
The number of attempts made: 15
=======================================
[Test Number: 98]
The breaking point: 98
The highest safe floor: 97
The number of attempts made: 15
=======================================
[Test Number: 99]
The breaking point: 99
The highest safe floor: 98
The number of attempts made: 10
=======================================

It works!

Firstly, create a web service interface then put the @WebService annotation.

package com.lckymn.kevin.test.openejb.webservice;

import javax.jws.WebService;

import com.lckymn.kevin.test.openejb.domain.User;

@WebService
public interface TestWebService
{
	User getUser(Long id);
}

Secondly, create a class implmenting the web service interface with also the @WebService annotation then specify the targetNamespace and the serviceName as you wish. In my case, these are

targetNamespace: "http://webservice.webhibernate.test"
serviceName: "testWebService"

To use the UserService EJB, injected by OpenEJB, make the web service class session EJB with the @Stateless annotation.

package com.lckymn.kevin.test.openejb.webservice;

import javax.ejb.EJB;
import javax.ejb.Local;
import javax.ejb.Stateless;
import javax.jws.WebService;

import com.lckymn.kevin.test.openejb.domain.User;
import com.lckymn.kevin.test.openejb.service.UserService;

@Local
@Stateless
@WebService(targetNamespace = "http://webservice.webhibernate.test", serviceName = "testWebService")
public class TestWebServiceImpl implements TestWebService
{
	@EJB
	private UserService userService;

	@Override
	public User getUser(Long id)
	{
		return userService.getUser(id);
	}
}

The class above simply returns a User entity object acquired from the UserService EJB, made in the previous post.

That’s it. You have just made your web service. When the server starts and the application is deployed, the EJB container (OpenEJB) registers the above EJB as a web service.

Now, let’s make a web service client. To make a very simple example client, I’m going to create a servlet in the same web application to which the web service belongs, yet it can of course be another web application, Java desktop application, Java console application and so on.

Here is a simple servlet which gets the user ID from the client-side then accesses the web service to get the User. After that it passes the User entity object through the HttpSession to the JSP page to display.

package com.lckymn.kevin.test.openejb.web;

import java.io.IOException;
import java.net.URL;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import javax.xml.namespace.QName;
import javax.xml.ws.Service;

import com.lckymn.kevin.test.openejb.domain.User;
import com.lckymn.kevin.test.openejb.webservice.TestWebService;

public class TestWebServiceClientServlet extends HttpServlet
{
	private static final long serialVersionUID = 1L;

	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
	{
		doPost(request, response);
	}

	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
	{
		Service service = Service.create(new URL("http://localhost:8080/TestWebServiceImpl?wsdl"), new QName(
				"http://webservice.webhibernate.test", "testWebService"));
		TestWebService testWebService = service.getPort(TestWebService.class);

		String userIdParam = request.getParameter("userId");

		HttpSession session = request.getSession();
		if (null == userIdParam || 0 == userIdParam.length())
		{
			session.removeAttribute("userFound");
		}
		else
		{
			Long userId = Long.parseLong(userIdParam);
			User user = testWebService.getUser(userId);
			session.setAttribute("userFound", user);
		}
		getServletContext().getRequestDispatcher("/WEB-INF/jsp/webServiceClient.jsp")
				.forward(request, response);
	}
}

This is just a simple example thus I omitted validation (e.g. checking whether the userId is ‘long’ type or not) and exception handling.

The WSDL location is the server URI + “/” + web service class name + “?wsdl”.
http://localhost:8080/TestWebServiceImpl?wsdl
The parameters of the QName constructor are namespaceURI and localPart, and these are defined in the web service. If you look at the web service code again, these can easily be found from the @WebService annotation.

@WebService(targetNamespace = "http://webservice.webhibernate.test", serviceName = "testWebService")
public class TestWebServiceImpl implements TestWebService
{
	...
}

The value of the targetNamespace element is the namespaceURI of the QName constructor, and the value of the serviceNmae element is the localPart.

This is web.xml. The information of the new servlet, the web service client just created, is added to the web.xml, made in the previous post.

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>test-web-hibernate</display-name>
  <servlet>
    <description></description>
    <display-name>TestServlet</display-name>
    <servlet-name>TestServlet</servlet-name>
    <servlet-class>com.lckymn.kevin.test.openejb.web.TestServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>TestServlet</servlet-name>
    <url-pattern>/Test</url-pattern>
  </servlet-mapping>
  <servlet>
    <description></description>
    <display-name>TestWebServiceClientServlet</display-name>
    <servlet-name>TestWebServiceClientServlet</servlet-name>
    <servlet-class>com.lckymn.kevin.test.openejb.web.TestWebServiceClientServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>TestWebServiceClientServlet</servlet-name>
    <url-pattern>/TestWebServiceClient</url-pattern>
  </servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>

Finally, add a JSP file to enter the userId to search a user and to display the user info.
In my case, I added the file to the application/WebContent/WEB-INF/jsp directory which is the same location that I set in the TestWebServiceClientServlet (look at the TestWebServiceClientServlet code above).
test-web-hibernate/WebContent/WEB-INF/jsp/webServiceClient.jsp

<?xml version="1.0" encoding="UTF-8" ?>
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Insert title here</title>
</head>
<body>
<div>
<form name="userForm" action="TestWebServiceClient" method="post" >
	<input type="text" name="userId" value="" /> <input type="submit" name="userIdSubmit" value="Search" />
</form>
</div>
<div>
	<table>
		<tr>
			<td>User ID: </td><td>${userFound.id }</td>
		</tr>
		<tr>
			<td>Username: </td><td>${userFound.username }</td>
		</tr>
		<tr>
			<td>Surname: </td><td>${userFound.surname }</td>
		</tr>
		<tr>
			<td>Given name: </td><td>${userFound.givenName }</td>
		</tr>
		<tr>
			<td>Email: </td><td>${userFound.email }</td>
		</tr>
	</table>
</div>
</body>
</html>

To test it, open the browser and enter the following URI.

http://localhost:8080/test-web-hibernate/TestWebServiceClient

It displays the screen like below

Enter userId and click the 'Search' button

It displays the result yet there is one problem. It doesn’t display the userId.

The Search Result: userId is not displayed

There are two simple ways to solve this problem. Either way works so choose whichever you like.

1. Add the setter method. If there is setId() method, the id field is included when the object is passed through the web service. The reason why I did not write the setter method is that the id is supposed to be set by Hibernate and to avoid any problems caused by setting it manually, I did not write it. However, it is required in order to include the field when passing object through the web service. So adding setter can solve this problem. If you do not like this solution as you do not like to put the setter method due to the reason I explained, you can try the second solution.

@Entity
@Table(name = "users")
public class User implements Serializable
{
	private static final long serialVersionUID = 1L;

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	@Column(name = "user_id")
	private Long id;

	...

	public Long getId()
	{
		return id;
	}

	public void setId(Long id)
	{
		this.id = id;
	}

	...
}

OR

2. If you add the @XmlElement annotation to the id field and set the value of the required element to true, the id field is included even without the setter method.

...

import javax.xml.bind.annotation.XmlElement;

...
@Entity
@Table(name = "users")
public class User implements Serializable
{
	private static final long serialVersionUID = 1L;

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	@Column(name = "user_id")
	@XmlElement(required = true)
	private Long id;

	...

	public Long getId()
	{
		return id;
	}

	// No setId() required

	...
}

Now, test if it works.

Enter userId and click the 'Search' button

It works!

The Search Result: userId is displayed correctly

There are also other ways and tools to create a web service in Java. I used to use Apache Axis then later moved to XFire. Now, I use Apache CXF which is considered as XFire 2.0. It is, as explained here, very easy to create a web service client, yet it can be even easier with WSDL2Java from Apache Axis or WSDL2Java from Apache Axis2.

Before I start writing this blog entry, I’d better point out a few things.

• This blog entry is not about how to make a good Java EE application yet is about how to use OpenEJB on Apache Tomcat server with Hibernate as an implementation of the JPA. If you are looking for better Java EE development or similar, this one is not for you.
• I don’t have enough time to explain all the details of how to install JDK, Eclipse, Tomcat and so on. So this post doesn’t have that level of details. I simply assume that you’ve already known how to install JDK, Eclipse and Tomcat. Well, anyway, installing Eclipse and Tomcat are as easy as extracting a zip or gzip file and JDK installation is also not difficult at all.
• This post does not cover how to use those tools and frameworks in the production environment. In other words, it is only for the development. Setting up the environment for development and for the production use have some differences. It is, however not that hard to figure out once you know how to use those in your development environment.

1. Development Environment

First of all, I’m going to list the development environment and tools I have for this introductory tutorial post.
(Click each name and it will take you to the download page).

OS: Ubuntu Linux Jaunty Jackalope 9.04 Desktop 64bit
Java: Sun JDK 1.6.0_16 (64 bit) (it’s from the Ubuntu repository).
Database: MySQL Community Server 5.0 (it’s from the Ubuntu repository).
JDBC Driver: MySQL Connector/J 5.1.10
Eclipse: Eclipse IDE for Java EE Developer Ganymede SR2 (Eclipse 3.4.2)
Tomcat: Tomcat 6.0.20
OpenEJB: OpenEJB 3.1.1 (openejb.war)
Hibernate: Hibernate 3.2.1GA (including hibernate-3.2.1.ga, hibernate-annotations-3.2.1.ga, hibernate-entitymanager-3.2.1.ga)

2. Installing OpenEJB

Java EE server normally means a Java application server which consists of a Servlet container and an EJB container. Apache Tomcat is a servlet container but not an EJB container so you need to have an EJB container like OpenEJB or use a Java EE server instead of Tomcat in order to use EJB unless you’re using the frameworks supporting EJB such as Spring framework. “So if I use Tomcat server and Spring framework, do I not need OpenEJB or other Java EE servers to use EJB?” No, you don’t.

Since this post is, as already mentioned, about using using EJB on Tomcat with OpenEJB and Hibernate, I will first show you how to install OpenEJB.

Before installing OpenEJB, don’t forget to copy JDBC driver that is, in this post, MySQL Connector/J to the $TOMCAT/lib directory.
-Copy the Connector/J jar file to the Tomcat’s library directory ($TOMCAT_HOME/lib)

Select Connector/J and Extract to the $TOMCAT/lib directory.

Make sure the Connector/J jar file is in the $TOMCAT/lib directory.

-Run Eclipse and add Tomcat server: Menu – Window -> Preferences -> Server -> Runtime Environments -> Add

-Download the openejb.war, OpenEJB for Tomcat, and import the file from Eclipse.
-Right click on the project explorer -> Import -> WAR file

Right Click on the project explorer -> Import -> WAR File

-Click the ‘Browse’ button and select the openejb.war file -> Select your Tomcat 6.0 as the target runtime. -> Click the ‘Finish’ button.

Click the 'Browse' button and select the openejb.war file -> Select your Tomcat 6.0 as the target runtime. -> Click the 'Finish' button.

3. Add OpenEJB project to the Server to deploy

-Right click on the server name in the ‘Server’ view -> Select the ‘Add and Remove Projects…’

Right click on the server name in the 'Server' view -> Select the 'Add and Remove Projects...'

-Select openejb -> Click the ‘Add’ button -> Click the ‘Finish’ button

Select openejb -> Click the 'Add' button -> Click the 'Finish' button

-openejb is ready to be deployed.

openejb is deployed.

-Now run the server to deploy openejb.

Run the server to deploy openejb

-openejb is successfully deployed.

the openejb is successfully deployed.

4. Set up DataSource

-openejb.xml has to be copied to the Tomcat configuration folder of the Eclipse workspace.

openejb.xml has to be copied to the Tomcat configuration folder of the Eclipse workspace.

If you are using Tomcat without OpenEJB and want to create a DataSource, you might do by putting the DataSource info to Tomcat’s server.xml file or your application’s context configuration file (your_app/META-INF/context.xml). Yet to create the DataSource for JPA, you need to do through openejb.xml file. It is created in the Tomcat folder in the .metadata folder of your Eclipse workspace when the openejb project is deployed. However, it is not copied automatically to the configuration folder of your Eclipse workspace, you should copy the openejb.xml file to the Server configuration folder manually. If the location of the workspace is ‘/home/username/workspace’, the Tomcat is in ‘/home/username/test-workspace/.metadata/.plugins/org.eclipse.wst.server.core’ and the openejb.xml file can be found in the ‘/home/username/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/conf’ directory.

-Find the openejb.xml file and copy to the Tomcat configuration folder of the Eclipse workspace.

Find the openejb.xml file and copy to the Tomcat configuration folder of the Eclipse workspace.

Paste the file to the Tomcat configuration folder of the Eclipse workspace.

-Open the ‘openejb.xml’ file then you can find the default DataSources.

Open the 'openejb.xml' file then you can find the default DataSources

-Add the following lines and modify for your own database.

<Resource id="mysqlDataSource" type="javax.sql.DataSource">
	JdbcDriver      	com.mysql.jdbc.Driver
	JdbcUrl         	jdbc:mysql://localhost:3306/test_db
	UserName        	test_user
	Password        	1234
	JtaManaged      	true
	DefaultAutoCommit 	true
	InitialSize     	3
	MaxActive       	20
	MinIdle         	20
	MaxIdle         	0
	MaxWait         	50000
	ValidationQuery 	SELECT 1
	TestOnBorrow    	true
	TestOnReturn    	false
	TestWhileIdle   	false
</Resource>

DataSource to access the MySQL database.

If you want to use OpenJPA as an implmentation of the JPA, you can do now. However, to use Hibernate there is one more step to do.

5. Install Hibernate

If you just have the Hibernate jar files in your application directory (e.g. your_app/WEB-INF/lib), the EJB container that is OpenEJB cannot find the hibernate classes as it is the container, it tries to find the hibernate class from the server’s lib directory. Thus just like you need to copy the JDBC driver, in this post it’s ‘Connector/J’, to the tomcat’s lib directory, the hibernate jar files should be placed in the Tomcat’s lib directory.

Otherwise, you will get an error like this.

ERROR - Unable to deploy collapsed ear in war /test-web-hibernate: Exception: Creating application failed: /home/username/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/test-web-hibernate: java.lang.ClassNotFoundException: org.hibernate.ejb.HibernatePersistence: org.hibernate.ejb.HibernatePersistence
org.apache.openejb.OpenEJBException: Creating application failed: /home/username/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/test-web-hibernate: java.lang.ClassNotFoundException: org.hibernate.ejb.HibernatePersistence: org.hibernate.ejb.HibernatePersistence
	at org.apache.openejb.assembler.classic.Assembler.createApplication(Assembler.java:658)
	at org.apache.openejb.assembler.classic.Assembler.createApplication(Assembler.java:442)
	at org.apache.openejb.tomcat.catalina.TomcatWebAppBuilder.start(TomcatWebAppBuilder.java:249)
	at org.apache.openejb.tomcat.catalina.GlobalListenerSupport.lifecycleEvent(GlobalListenerSupport.java:58)
	at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)
	at org.apache.catalina.core.StandardContext.start(StandardContext.java:4339)
	at org.apache.catalina.core.StandardContext.reload(StandardContext.java:3190)
	at org.apache.catalina.loader.WebappLoader.backgroundProcess(WebappLoader.java:404)
	at org.apache.catalina.core.ContainerBase.backgroundProcess(ContainerBase.java:1309)
	at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1601)
	at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1610)
	at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1610)
	at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.run(ContainerBase.java:1590)
	at java.lang.Thread.run(Thread.java:619)
Caused by: org.apache.openejb.OpenEJBException: java.lang.ClassNotFoundException: org.hibernate.ejb.HibernatePersistence: org.hibernate.ejb.HibernatePersistence
	at org.apache.openejb.assembler.classic.Assembler.createApplication(Assembler.java:487)
	... 13 more
Caused by: java.lang.ClassNotFoundException: org.hibernate.ejb.HibernatePersistence
	at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1387)
	at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1233)
	at org.apache.openejb.assembler.classic.PersistenceBuilder.createEntityManagerFactory(PersistenceBuilder.java:177)
	at org.apache.openejb.assembler.classic.Assembler.createApplication(Assembler.java:482)
	... 13 more

6. Adding logger configuration (Optional)

By default, OpenEJB creates a log file in the Eclipse’s $TOMCAT/logs directory. It is very inconvenient as you have to open the file when you want to get information from the log. However, it is very easy to change this to make it displayed on the Console view of Eclipse.

-Right click on the Tomcat configuration folder -> Select ‘New’ -> Select ‘File’ -> crated a file with the name ‘logging.properties’.

Right click on the Tomcat configuration folder -> Select 'New' -> Select 'File' -> crated a file with the name 'logging.properties'.

-Open the file and put the following lines

log4j.rootLogger                   = fatal,C
log4j.category.OpenEJB             = warn
log4j.category.OpenEJB.options     = info
log4j.category.OpenEJB.server      = info
log4j.category.OpenEJB.startup     = info
log4j.category.OpenEJB.startup.service = warn
log4j.category.OpenEJB.startup.config = info
log4j.category.OpenEJB.hsql        = info
log4j.category.CORBA-Adapter       = info
log4j.category.Transaction         = warn
log4j.category.org.apache.activemq = error
log4j.category.org.apache.geronimo = error
log4j.category.openjpa             = error

log4j.appender.C                   = org.apache.log4j.ConsoleAppender
log4j.appender.C.layout            = org.apache.log4j.SimpleLayout

Put the logger config details

7. Create Web Application Project

Now, you can develop a web application using EJB3 and Hibernate as the implementation of the JPA.

-Right click on the project explorer -> Select ‘New’ -> Select ‘Dynamic Web Project’

Right click on the project explorer -> Select 'New' -> Select 'Dynamic Web Project'

-Put the name you like -> Select the ‘Apache Tomcat v6.0′ as the target runtime -> Select ‘2.5′ as the version of ‘Dynamic Web Module’ -> Select the default Tomcat configuration or your own one -> Click the ‘Next’ button.

Put the name you like -> Select the 'Apache Tomcat v6.0' as the target runtime -> Select '2.5' as the version of 'Dynamic Web Module' -> Select the default Tomcat configuration or your own one -> Click the 'Next' button.

-Change the project context root and directory names if you like -> Click the ‘Finish’ button.

Change the project context root and directory names if you like -> Click the 'Finish' button.

-Right click on the Server to add the project -> Select the ‘Add and Remove Projects…’.

Right click on the Server to add the project -> Select the 'Add and Remove Projects...'.

-Select the project you created -> Click the ‘Add’ button -> Click the ‘Finish’ button.

Select the project you created -> Click the 'Add' button -> Click the 'Finish' button.

-Both openejb and your project are added.

Both openejb and your project are added.

8. Make JPA Project

To use the JPA support feature of Eclipse, you need to change the project facet configuration.

-Right click on your project -> Select the ‘Properties’.

Right click on your project -> Select the 'Properties'.

-Select the ‘Project Facets’ -> Check ‘Java Persistence 1.0′ -> Click the ‘Further configuration available…’ link.

Select the 'Project Facets' -> Check 'Java Persistence 1.0' -> Click the 'Further configuration available...' link.

-Select ‘Generic’ -> Select ‘None’ or your own connection or add a new connection if you wish -> Select ‘Use implementation provided by server runtime’ -> Select ‘Discover annotated classes automatically’ -> Uncheck ‘Create orm.xml’ option -> Click the ‘OK’ button.

Select 'Generic' -> Select 'None' or your own connection or add a new connection if you wish -> Select 'Use implementation provided by server runtime' -> Select 'Discover annotated classes automatically' -> Uncheck 'Create orm.xml' option -> Click the 'OK' button.

9. Add Java EE 5 API library file

You also need to add the Java EE 5 API library file so you can use all the necessary annotations required for EJB3 and JPA.

-Select ‘Java Build Path on the left-hand side menu -> Select the ‘Libraries’ tab -> Click the ‘Add JARs…’ button.

Select 'Java Build Path on the left-hand side menu -> Select the 'Libraries' tab -> Click the 'Add JARs...' button.

-Select ‘javaee-api-5.0-2.jar in the openejb/WebContent/lib directory -> Click the ‘OK’ button.

Select 'javaee-api-5.0-2.jar in the openejb/WebContent/lib directory -> Click the 'OK' button.

-Click the ‘OK’ button.

Click the 'OK' button.

JPA configuration file (persistence.xml) is added

10. Configure persistence.xml file

It’s the last step before starting to programme the actual application.

-Open the persistence.xml file -> Type ‘org.hibernate.ejb.HibernatePersistence’ to the ‘Persistence Provider’.

org.hibernate.ejb.HibernatePersistence

Open the persistence.xml file -> put 'org.hibernate.ejb.HibernatePersistence' to the 'Persistence Provider'.

-Click the ‘Connection’ tab -> Select the ‘JTA’ as the ‘Transaction Type’ -> Type ‘mysqlDataSource’ or your datasource name added in the previous steps. -> Press ‘Ctrl + S’ keys to save the file.

Click the 'Connection' tab -> Select the 'JTA' as the 'Transaction Type' -> Type 'mysqlDataSource' or your datasource name added in the previous steps. -> Press 'Ctrl + S' keys to save the file.

-If you select the ‘Source’ tab, you should see the XML like this.

If you select the 'Source' tab, you should see the XML like above.

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0"
	xmlns="http://java.sun.com/xml/ns/persistence"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
	<persistence-unit name="test-web-hibernate" transaction-type="JTA">
		<provider>org.hibernate.ejb.HibernatePersistence</provider>
		<jta-data-source>mysqlDataSource</jta-data-source>
	</persistence-unit>
</persistence>

-If you want Hibernate to automatically create the database tables based on your entity classes every time the server is restarted (in other words, the application is re-deployed), You can add Hibernate specific properties.

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0"
	xmlns="http://java.sun.com/xml/ns/persistence"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
	<persistence-unit name="test-web-hibernate" transaction-type="JTA">
		<provider>org.hibernate.ejb.HibernatePersistence</provider>
		<jta-data-source>mysqlDataSource</jta-data-source>
		<properties>
			<property name="hibernate.hbm2ddl.auto" value="create" />
		</properties>
	</persistence-unit>
</persistence>

11. Test

Finally, we have the development environment ready. To test, if I can use EJB3 and JPA with Hibernate on Tomcat, I made a very simple application. The way I design it is not my usual way yet I used Generic DAO pattern which I usually use.

11.1. Entity classes

Here is my only entity class in this test.

package com.lckymn.kevin.test.openejb.domain;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "users")
public class User implements Serializable
{
	private static final long serialVersionUID = 1L;

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	@Column(name = "user_id")
	private Long id;

	@Column(name = "username", nullable = false, length = 30)
	private String username;

	@Column(name = "surname", nullable = false, length = 50)
	private String surname;

	@Column(name = "given_name", nullable = false, length = 50)
	private String givenName;

	@Column(name = "email", nullable = true, length = 255)
	private String email;

	public Long getId()
	{
		return id;
	}

	public String getUsername()
	{
		return username;
	}

	public void setUsername(String username)
	{
		this.username = username;
	}

	public String getSurname()
	{
		return surname;
	}

	public void setSurname(String surname)
	{
		this.surname = surname;
	}

	public String getGivenName()
	{
		return givenName;
	}

	public void setGivenName(String givenName)
	{
		this.givenName = givenName;
	}

	public String getEmail()
	{
		return email;
	}

	public void setEmail(String email)
	{
		this.email = email;
	}

	@Override
	public boolean equals(Object obj)
	{
		if (this == obj)
		{
			return true;
		}

		if (!(obj instanceof User))
		{
			return false;
		}
		User that = (User) obj;
		return (username == that.getUsername() || (null != username && username.equals(that.getUsername())));
	}

	@Override
	public int hashCode()
	{
		return (null == username ? 0 : username.hashCode());
	}
}

11.2. Generic DAOs

package com.lckymn.kevin.test.openejb.dao;

public interface GenericDao<E, K>
{
	E find(K id);

	void persist(E e);

	void remove(E e);
}

package com.lckymn.kevin.test.openejb.dao;

import java.lang.reflect.ParameterizedType;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

public abstract class AbstractGenericDao<E, K> implements GenericDao<E, K>
{
	private Class<E> classType;

	@PersistenceContext(unitName = "test-web-hibernate")
	private EntityManager entityManager;

	@SuppressWarnings("unchecked")
	public AbstractGenericDao()
	{
		ParameterizedType parameterizedType = (ParameterizedType) getClass().getGenericSuperclass();
		classType = (Class<E>) parameterizedType.getActualTypeArguments()[0];
	}

	protected final EntityManager getEntityManager()
	{
		if (null == entityManager)
		{
			throw new IllegalStateException("EntityManager is not injected.");
		}
		return entityManager;
	}

	@Override
	public E find(K id)
	{
		return getEntityManager().find(classType, id);
	}

	@Override
	public void persist(E e)
	{
		getEntityManager().persist(e);
	}

	@Override
	public void remove(E e)
	{
		getEntityManager().remove(e);
	}
}

package com.lckymn.kevin.test.openejb.dao;

import java.util.List;

import javax.ejb.Local;

import com.lckymn.kevin.test.openejb.domain.User;

@Local
public interface UserDao extends GenericDao<User, Long>
{
	List<User> getUsersByGivenName(String givenName);
}

package com.lckymn.kevin.test.openejb.dao;

import java.util.List;

import javax.ejb.Stateless;

import com.lckymn.kevin.test.openejb.domain.User;

@Stateless
public class UserDaoImpl extends AbstractGenericDao<User, Long> implements UserDao
{
	@SuppressWarnings("unchecked")
	@Override
	public List<User> getUsersByGivenName(String givenName)
	{
		return getEntityManager().createQuery("from User where givenName = ?")
				.setParameter(1, givenName)
				.getResultList();
	}
}

11.3. Services

package com.lckymn.kevin.test.openejb.service;

import java.util.List;

import javax.ejb.Local;

import com.lckymn.kevin.test.openejb.domain.User;

@Local
public interface UserService
{
	User getUser(long id);

	void AddUser(User user);

	List<User> getUserByGivenName(String givenName);
}

package com.lckymn.kevin.test.openejb.service;

import java.util.List;

import javax.ejb.EJB;
import javax.ejb.Stateless;

import com.lckymn.kevin.test.openejb.dao.UserDao;
import com.lckymn.kevin.test.openejb.domain.User;

@Stateless
public class UserServiceBean implements UserService
{
	@EJB
	private UserDao userDao;

	@Override
	public User getUser(long id)
	{
		return userDao.find(id);
	}

	@Override
	public void AddUser(User user)
	{
		userDao.persist(user);
	}

	@Override
	public List<User> getUserByGivenName(String givenName)
	{
		return userDao.getUsersByGivenName(givenName);
	}

}

11. 4. Servlets

package com.lckymn.kevin.test.openejb.web;

import java.io.IOException;

import javax.ejb.EJB;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import com.lckymn.kevin.test.openejb.domain.User;
import com.lckymn.kevin.test.openejb.service.UserService;

public class TestServlet extends HttpServlet
{
	private static final long serialVersionUID = 1L;

	@EJB
	private UserService userService;

	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
	{
		doPost(request, response);
	}

	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
	{
		User user = new User();
		user.setUsername("kevinlee");
		user.setSurname("Lee");
		user.setGivenName("Kevin");
		user.setEmail("test@test.test");

		userService.AddUser(user);

		HttpSession session = request.getSession();
		session.setAttribute("user", user);

		getServletContext().getRequestDispatcher("/index.jsp")
				.forward(request, response);
	}

}

11.5. Deployment Descriptor (web.xml)

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
	id="WebApp_ID" version="2.5">
	<display-name>test-web-hibernate</display-name>
	<servlet>
		<description></description>
		<display-name>TestServlet</display-name>
		<servlet-name>TestServlet</servlet-name>
		<servlet-class>com.lckymn.kevin.test.openejb.web.TestServlet</servlet-class>
	</servlet>
	<servlet-mapping>
		<servlet-name>TestServlet</servlet-name>
		<url-pattern>/Test</url-pattern>
	</servlet-mapping>
	<welcome-file-list>
		<welcome-file>index.jsp</welcome-file>
	</welcome-file-list>
</web-app>

11. 6. index.jsp

<?xml version="1.0" encoding="UTF-8" ?>
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Insert title here</title>
</head>
<body>
<div>
	<table>
		<tr>
			<td>User ID: </td><td>${user.id }</td>
		</tr>
		<tr>
			<td>Username: </td><td>${user.username }</td>
		</tr>
		<tr>
			<td>Surname: </td><td>${user.surname }</td>
		</tr>
		<tr>
			<td>Given name: </td><td>${user.givenName }</td>
		</tr>
		<tr>
			<td>Email: </td><td>${user.email }</td>
		</tr>
	</table>
</div>
</body>
</html>

12. Run the test application

Access the application URI

http://localhost:8080/test-web-hibernate/Test

It gives me this result.

User ID: 	1
Username: 	kevinlee
Surname: 	Lee
Given name: 	Kevin
Email: 	test@test.test

http://blog.springsource.com/2009/08/10/springsource-chapter-two/
http://www.vmware.com/company/news/releases/springsource.html
http://blogs.vmware.com/console/2009/08/vmware-acquires-springsource.html
http://www.earthtimes.org/articles/show/vmware-to-acquire-springsource,921934.shtml

Looks like a good deal for both companies.

VMware recently hired Mark Lucovsky from Google and now has SpringSource. I guess VMware is really doing well.

Plug-in org.eclipse.jst.server.tomcat.core was unable to load class org.eclipse.jst.server.tomcat.core.internal.TomcatLaunchConfigurationDelegate.
 /eclipse_installed_path/eclipse/configuration/org.eclipse.osgi/.lazy.15 (Too many open files)

or

Problems occurred while trying to save the state of the workbench.
 Could not read master table.
 /your_workspace/.metadata/.plugins/org.eclipse.core.resources/.safetable/org.eclipse.core.resources (Too many open files)

or

java.util.zip.ZipException: error in opening zip file

This is because there are too many files opened and these are more files than the number of open files allowed . So Eclipse cannot open more files and displays the errors above.

Let’s see the number of open files.

$ lsof | wc -l

e.g.)

$ lsof | wc -l
8965

In my case, it was 8965.

What about the number of files Eclipse opens. To see it, use

$ lsof | grep eclipse | wc -l

In my case,

$ lsof | grep eclipse | wc -l
2094

2094 files are opened.

Now check the limitation of open files

$ ulimit -a
core file size          (blocks, -c) #
data seg size           (kbytes, -d) #
scheduling priority             (-e) #
file size               (blocks, -f) #
pending signals                 (-i) #
max locked memory       (kbytes, -l) #
max memory size         (kbytes, -m) #
open files                      (-n) 1024
pipe size            (512 bytes, -p) #
POSIX message queues     (bytes, -q) #
real-time priority              (-r) #
stack size              (kbytes, -s) #
cpu time               (seconds, -t) #
max user processes              (-u) #
virtual memory          (kbytes, -v) #
file locks                      (-x) #

or just use

$ ulimit -n
1024

To change it, open the file /etc/security/limits.conf and put a greater number than 1024 depending on the number of open files you checked with lsof | wc -l just before.
For example,
Open the file

$ gksudo gedit /etc/security/limits.conf
add these lines
*                soft    nofile          9216
*                hard    nofile          9216

I just chose some big number that is 9216 (9 * 1024) as it’s greater than 8965

Log out and in then check with ulimit. It should show like this.

$ ulimit -n
9216

You may try this

$ ulimit -n 9216

yet I don’t believe it changes the limit for open files permanently. So you’d better modify /etc/security/limits.conf file.

If it is still not changed. Restart the computer and check again. If it still doesn’t show the changed value, open /etc/pam.d/common-session file and add session required pam_limits.so.

Open the file to edit

$ gksudo gedit /etc/pam.d/common-session

Add the following line

session required pam_limits.so

Log out and in. Now it should work!

$ ulimit -n
9216

Bauer and King in their book entitled ‘Java Persistence with Hibernate’ (2007), point out that method chaining is convenient in some cases and is more popular in Smalltalk than in Java for Smalltalk, unlike Java, does not have void type. Thus when a method is invoked, it normally returns the object itself in which the method is placed.

Although it is used to improve readability and to reduce the amount of source code, I didn’t really like this style as it can be less readable and might make code difficult to debug. How can the technique to improve readability make the code less readable? By less readable, I mean the code may possibly be unpredictable or make the programmer confused in some cases.

Let’s look at some example.

This is a simple JavaBean named Item

package com.lckymn.kevin.test.methodchaining;

/**
 * @author Lee, SeongHyun (Kevin)
 */
public class Item
{
	private int id;
	private String name;

	public Item(int id, String name)
	{
		this.id = id;
		this.name = name;
	}

	public int getId()
	{
		return id;
	}

	public void setId(int id)
	{
		this.id = id;
	}

	public String getName()
	{
		return name;
	}

	public void setName(String name)
	{
		this.name = name;
	}

	@Override
	public String toString()
	{
		return "ID: " + id + "\n" + "Name: " + name + "\n";
	}
}

This is a Storage class designed to use method chaining.

package com.lckymn.kevin.test.methodchaining;

import java.util.ArrayList;
import java.util.List;

/**
 * @author Lee, SeongHyun (Kevin)
 */
public class Storage
{
	private List<Item> items;

	public Storage()
	{
		this.items = new ArrayList<Item>();
	}

	public Storage put(String name)
	{
		items.clear();
		return and(name);
	}

	public Storage and(String name)
	{
		items.add(new Item(name.hashCode(), name));
		return this;
	}

	public Item get(int index)
	{
		return items.get(index);
	}

	public List<Item> getAll()
	{
		return items;
	}
}

Now, let’s use it.

package com.lckymn.kevin.test.methodchaining;

/**
 * @author Lee, SeongHyun (Kevin)
 */
public final class MethodChainingTest
{
	public static void main(String[] args)
	{
		String name = new Storage().put("A").and("B").and(null).and("D").get(0).getName();
		System.out.println("Name: " + name + "\n");

		System.exit(0);
	}

}

What is this? It looks like instantiating a Storage object yet the end it assigns String value to String variable named name?

It seems confusing. First, instantiate Storage. Then call several methods in it. Finally call the method which returns a String value. However, the Storage class doesn’t even have any method returning the String value. It is the method in the Item class.

OK, move to the other problem that is difficulty in debuging.
The example code above does not have any compile time error yet when it runs the result is

Exception in thread "main" java.lang.NullPointerException
	at com.lckymn.kevin.test.methodchaining.Storage.and(Storage.java:26)
	at com.lckymn.kevin.test.methodchaining.MethodChainingTest.main(MethodChainingTest.java:10)

The tenth line in the MethodChainingTest.main method is this.

		String name = new Storage().put("A").and("B").and(null).and("D").get(0).getName();

The twenty-sixth line in the Storage.and method is this.

		items.add(new Item(name.hashCode(), name));

So this line of the code causes NullPointerException but and() method is called three times in the tenth line of the MethodChainingTest class. Which one of these causes the error? In the example, it is very obvious that the and() method call with null parameter is the one. However, in real-life programme, it is usually much more difficult to find.

So, as already mentioned, I did not like using method chaining. Then again there came a time when it was very convenient and useful to use method chaining. I found it very useful when I was making a XML generator programme for an Ajax application. I made it using Java API for XML Processing (JAXP). It required to generate simple XML based on a given object so it might be too much to use XML data binding frameworks and tools such as Java Architecture for XML Binding (JAXB) and XStream. Although both JAXB and XStream are simple and easy to use and can be used to serialise objects, I wanted to have more control than what the framework provides. JAXP with Simple API for XML (SAX) parsing interface was just suitable for what I needed, yet it is not very pleasant to use the SAX interface. It uses ContentHandler interface when making XML contents and the following code is what it looks like when using it.

contentHandler.startDocument();

AttributesImpl atts = new AttributesImpl();
atts.addAttribute(null, null, "type", null, "User");
contentHandler.startElement(null, null, "users", atts);

atts.clear();
atts.addAttribute(null, null, "id", null, "kevin");
atts.addAttribute(null, null, "surname", null, "Lee");
atts.addAttribute(null, null, "givenName", null, "Kevin");
contentHandler.startElement(null, null, "user", atts);
contentHandler.characters("Test value".toCharArray(), 0, "Test value".length());
contentHandler.endElement(null, null, "user");

atts.clear();
atts.addAttribute(null, null, "id", null, "john");
atts.addAttribute(null, null, "surname", null, "Doe");
atts.addAttribute(null, null, "givenName", null, "John");
contentHandler.startElement(null, null, "user", atts);
contentHandler.characters("Blah Blah".toCharArray(), 0, "Blah Blah".length());
contentHandler.endElement(null, null, "user");

atts.clear();
atts.addAttribute(null, null, "id", null, "tom");
atts.addAttribute(null, null, "surname", null, "Smith");
atts.addAttribute(null, null, "givenName", null, "Tom");
contentHandler.startElement(null, null, "user", atts);
contentHandler.characters("12345".toCharArray(), 0, "12345".length());
contentHandler.endElement(null, null, "user");

contentHandler.endElement(null, null, "users");

contentHandler.endDocument();

As all I want was simple XML for an Ajax application, I had to type ‘null‘ many times as parameter values for namespace URI and local name which were definitely unnecessary for my programme. Since the data transfered through network need to be small, XML sent to the front-end Ajax application had better not have the data such as namespace and schema location information and so on.

However, what I all had was, as shown above, the ugly code which repeatedly calls same methods with many ‘null’ parameters. So I tried to find a better way and eventually came up with that it might be a good idea to use method chaining. Even so, there were still the two problems I mentioned.

The method chaining code example that I showed earlier is, in fact, a misuse of method chaining. That can be much better if it is used properly. After all, it is not the technique that makes the code less maintainable but how it is used that makes the code less maintainable.

Think about this. If a knife is used by a murderer, the result of using it would be a dead body while if it is used by a chef, the result would be a delicious meal unless the chef is the murderer. (I think I used this sentence when I had my presentation in the third year in my undergraduate days. The subject was about IT and ethics, and I was emphasising that technologies have nothing to do with ethics yet how we use these is important when it comes to ethics).

So, how can those two problems be solved? First of all, please don’t get me wrong. I am not saying that the way I am going to tell here is the best, but it is just what I do. That’s it.

I chose to implement fluent interface yet with my own taste.

So my tastes are
1. The prefix ‘Fluent’ shall be used to tell it is a fluent interface.
e.g.)
public interface FluentContentHandler
public interface FluentStorage

2. void return type shall be used to stop method chaining if the method should not be used with other methods in the fluent interface or if it has some side-effect when using with other methods so that the programmer should be noticed it by void return type.
e.g.)
void endDoc()
void finish()

3. Otherwise, all the methods shall return the type of interface itself, and the name of the normal methods which return the type of the interface itself shall begin with verb.
e.g.)
FluentContentHandler openElem(String qName)
FluentContentHandler setText(String qName)
FluentSaxAttributes create(String qName, String value)

OR
The name of the method which must be used before the other methods shall begin with a verb.
e.g.)
FluentSaxAttributes create(String qName, String value)
The name of the method which must be used after the method the name of which begins with a verb shall be an appropriate preposition or conjunction.
e.g.)
FluentSaxAttributes with(String qName, String value)
FluentSaxAttributes and(String qName, String value)

4. If other types than the interface itself need to be returned, distinguishable method names shall be used. The name of the methods which return other type than the type of the interface itself shall begin with a preposition followed by a noun.
e.g.)
List toList()
Map toMap()
Collection toCollection()

I believe, these can help me to get over the first matter, and the code would be more readable as expected.

So let’s have a look at the new fluent interfaces for my XML generator programme.

package com.lckymn.kevin.test.xml;

import org.xml.sax.ContentHandler;
import org.xml.sax.SAXException;

/**
 * @author Lee, SeongHyun (Kevin)
 */
public interface FluentSaxContentHandler
{
	void startDoc() throws SAXException;

	void endDoc() throws SAXException;

	FluentSaxContentHandler openElem(String qName) throws SAXException;

	FluentSaxContentHandler openElem(String qName, FluentSaxAttributes attributes) throws SAXException;

	FluentSaxContentHandler setText(String text) throws SAXException;

	void closeElem(String qName) throws SAXException;
}

The programmer using this interface had better be aware of that startDoc() and endDoc() methods are to start and to end the XML document so use it once and therefore these return void type to warn. Similarly, closeElem() method the type of which is void closes the element with the name given as the parameter so the programmer had better stop method chaining here and open a new element with another method chaining.

package com.lckymn.kevin.test.xml;

import org.xml.sax.helpers.AttributesImpl;

/**
 * @author Lee, SeongHyun (Kevin)
 */
public interface FluentSaxAttributes
{
	FluentSaxAttributes create(String qName, String value);

	FluentSaxAttributes add(String qName, String value);

	AttributesImpl toAttributesImpl();
}

Since the ContentHandler requires AttributesImpl as attributes toAttributesImpl() method needs to return AttributesImpl and therefore its name consists of the preposition ‘to’ and the noun ‘AttributesImpl’.

Now let’s use these.

	contentHandler.startDoc();

	contentHandler.openElem("users", atts.create("type", "User"));

	contentHandler.openElem("user", atts.create("id", "kevin").add("surname", "Lee").add("givenName", "Kevin")).setText("Test value").closeElem("user");
	contentHandler.openElem("user", atts.create("id", "john").add("surname", "Doe").add("givenName", "John")).setText("Blah Blah").closeElem("user");
	contentHandler.openElem("user", atts.create("id", "tom").add("surname", "Smith").add("givenName", "Tom")).setText("12345").closeElem("user");

	contentHandler.closeElem("users");

	contentHandler.endDoc();

It looks much simpler and readable than the previous code below.

	contentHandler.startDocument();

	AttributesImpl atts = new AttributesImpl();
	atts.addAttribute(null, null, "type", null, "User");
	contentHandler.startElement(null, null, "users", atts);

	atts.clear();
	atts.addAttribute(null, null, "id", null, "kevin");
	atts.addAttribute(null, null, "surname", null, "Lee");
	atts.addAttribute(null, null, "givenName", null, "Kevin");
	contentHandler.startElement(null, null, "user", atts);
	contentHandler.characters("Test value".toCharArray(), 0, "Test value".length());
	contentHandler.endElement(null, null, "user");

	atts.clear();
	atts.addAttribute(null, null, "id", null, "john");
	atts.addAttribute(null, null, "surname", null, "Doe");
	atts.addAttribute(null, null, "givenName", null, "John");
	contentHandler.startElement(null, null, "user", atts);
	contentHandler.characters("Blah Blah".toCharArray(), 0, "Blah Blah".length());
	contentHandler.endElement(null, null, "user");

	atts.clear();
	atts.addAttribute(null, null, "id", null, "tom");
	atts.addAttribute(null, null, "surname", null, "Smith");
	atts.addAttribute(null, null, "givenName", null, "Tom");
	contentHandler.startElement(null, null, "user", atts);
	contentHandler.characters("12345".toCharArray(), 0, "12345".length());
	contentHandler.endElement(null, null, "user");

	contentHandler.endElement(null, null, "users");

	contentHandler.endDocument();

Yet there is one more problem still left that is difficulty in debugging.

King and Bauer (2007) suggests that “it’s better to write each method invocation on a different line”.

So rewrite the code

	contentHandler.startDoc();

	contentHandler.openElem("users", atts.create("type", "User"));

	contentHandler.openElem("user", atts.create("id", "kevin")
			.add("surname", "Lee")
			.add("givenName", "Kevin"))
			.setText("Test value")
			.closeElem("user");
	contentHandler.openElem("user", atts.create("id", "john")
			.add("surname", "Doe")
			.add("givenName", "John"))
			.setText("Blah Blah")
			.closeElem("user");
	contentHandler.openElem("user", atts.create("id", "tom")
			.add("surname", "Smith")
			.add("givenName", "Tom"))
			.setText("12345")
			.closeElem("user");

	contentHandler.closeElem("users");

	contentHandler.endDoc();

Now not only does it solve the problem in debugging but it is also even more readable. My problem solved!
Well, unfortunately there is one more problem.

I am using Eclipse and it has a nice automatic formatting feature. So whenever I press SHIFT+CTRL+F, it automatically formats the code opened in the editor based on the format configuration. This means if I use that function, I lose the coding style of writing each method invocation on a different line as Eclipse formatter puts all the method on a different line together on one line. So do I have to reformat by myself after every automatic formatting? It’s really annoying.

Fortunately, a few changes in formatter configuration can solve this problem.

Click the ‘Window’ menu -> ‘Preferences’

When the ‘Preferences’ menu pops up
-Expand the ‘Java’ -> Expand the ‘Code Style’ -> Select the ‘Fomatter’

-Click the ‘Edit’ button on the top right-hand side.

Expand the 'Java' -> Expand the 'Code Style' -> Select the 'Fomatter' -> Click the 'Edit' button on the top right-hand side

-The profile window appears -> Select the ‘Line Wrapping’ -> Expand the ‘Function Calls’ -> Select the ‘Qualified invocations’ -> Select the ‘Wrap all elements, except first element if not necessary’ as the ‘Line wrapping policy’ -> Select the ‘Default indentation’ as the ‘Indentation policy’ -> Check the ‘Force split’ -> Click the ‘OK’ button.

Profile window appears -> Select the 'Line Wrapping' -> Expand the 'Function Calls' -> Select the 'Qualified invocations' -> Select the 'Wrap all elements, except first element if not necessary' as the 'Line wrapping policy' -> Select the 'Default indentation' as the 'Indentation policy' -> Check the 'Force split' -> Click the 'OK' button

-Click the ‘OK’ button to apply the changes.

Now, the Eclipse Java formatter formats the code, using method chaining, as what I want.

Finally, how can I improve my first example of method chaining?
The Item class does not need to be changed.

Write the fluent interface, FluentStorage.

package com.lckymn.kevin.test.fluentinterface;

import java.util.List;

/**
 * @author Lee, SeongHyun (Kevin)
 */
public interface FluentStorage
{
	FluentStorage add(String name);

	List<Item> toList();
}

Write the class implements it.

package com.lckymn.kevin.test.fluentinterface;

import java.util.ArrayList;
import java.util.List;

/**
 * @author Lee, SeongHyun (Kevin)
 */
public class FluentListStorage implements FluentStorage
{
	private List<Item> items;

	public FluentListStorage()
	{
		items = new ArrayList<Item>();
	}

	@Override
	public FluentStorage add(String name)
	{
		items.add(new Item(name.hashCode(), name));
		return this;
	}

	@Override
	public List<Item> toList()
	{
		return items;
	}

}

Now use the fluent interface.

package com.lckymn.kevin.test.fluentinterface;

import java.util.List;

/**
 * @author Lee, SeongHyun (Kevin)
 */
public final class MethodChainingTest
{
	public static void main(String[] args)
	{
		FluentStorage fluentStorage = new FluentListStorage().add("A")
				.add("B")
				.add(null)
				.add("D");

		List<Item> items = fluentStorage.toList();
		for (Item item : items)
		{
			System.out.println(item);
		}
		int howMany = items.size();
		System.out.println("There " + (1 < howMany ? "are " + howMany + " items" : "is " + howMany + " item") + " in the storage.");
		System.exit(0);
	}

}

When it runs, it displays the following error messages yet now I know that the fourteenth line causes the error.

Exception in thread "main" java.lang.NullPointerException
	at com.lckymn.kevin.test.fluentinterface.FluentListStorage.add(FluentListStorage.java:21)
	at com.lckymn.kevin.test.fluentinterface.MethodChainingTest.main(MethodChainingTest.java:14)

The fourteenth line is this.

				.add(null)

So change it to

				.add("C")

I could finally have the correct result.

ID: 65
Name: A

ID: 66
Name: B

ID: 67
Name: C

ID: 68
Name: D

There are 4 items in the storage.

So method chaining, to use, or not to use? It’s all up to you.

References
Bauer, C. and King, G. 2007, Java Persistence with Hibernate, Manning Publications Co., New York.

Cho, Y. 2008, Applicaiton of Domain-Driven Design 2. Aggregate and Repository #3, viewed 29 June 2009, <http://aeternum.egloos.com/1173825>.

Adobe has released an alpha version of Flash Player for 64bit Linux.
http://labs.adobe.com/downloads/flashplayer10.html
http://labs.adobe.com/downloads/flashplayer10_64bit.html
At the bottom of the web page, you can find the download link.

Download 64-bit Plugin for Linux (TAR.GZ, 3.56 MB)

In my case, the latest alpha version of 64 bit Flash player does not work well (e.g. Video on youtube constantly freezes) yet the previous alpha one works better. So first, try the latest one and if it doesn’t work well, download the following one.

http://download.macromedia.com/pub/labs/flashplayer10/libflashplayer-10.0.d21.1.linux-x86_64.so.tar.gz

Before installing it, if there is a previously installed Flash player, it has to be removed first.

To check it, open ‘Synaptic Package Manager’.
System -> Administration -> Synaptic Package Manager

Search by 'flash' and make sure neither flashplugin-nonfree nor flashplugin-installer is installed. If any of these are installed, remove first.

Extract the libflashplayer file downloaded to the firefox ‘plugins’ directory.
(libflashplayer-10.0.d21.1.linux-x86_64.so.tar.gz file is in the /home/username/Desktop directory).

$ cd /usr/lib/firefox/plugins
$ sudo tar -zxvf ~/Desktop/libflashplayer-10.0.d21.1.linux-x86_64.so.tar.gz

To use this Flash player in other browsers such as Opera, create the symbolic link to the /usr/lib/firefox/plugins/libflashplayer.so file in the ‘/usr/lib/mozilla/plugins’ directory.

$ cd /usr/lib/mozilla/plugins
$ sudo ln -s /usr/lib/firefox/plugins/libflashplayer.so

If the file with the same name already exists, user ‘f’ option to overwrite it.

$ sudo ln -sf /usr/lib/firefox/plugins/libflashplayer.so

Now, open the Firefox and test if it works well.