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Java Generics: Generics in Real Life Programming

It is surprising to see that there are still many people who do not use generics even when they are using JDK 5 or higher. I can often see it especially when I teach some Java programming subjects at uni. Generics can be very useful and I use it a lot in my libraries and projects. So I’d like to talk about generics but not a basic knowledge of it. I will talk about Java generics in real life programming and how it can be useful as well as some unusual cases may occur and annoy you. I am also going to explain some common cases of generics usage in my next post that I have plan to write soon (hopefully). Then I’m going to write about more complex cases in the sense that it’s complex when it’s designed but rather simple when it’s used, and it’s also more practical. It means I’ll probably write two more posts about generics so stay tuned if you’re interested.

Before I start to talk about usefulness of it, let’s start with problems that can happen if you don’t use generics when using generified types.
If your code looks like this, you’re making your code error-prone.

List list = new ArrayList();

As it discards all the generic type information in the class. To see what exactly happens, better take a look at some example.

If you have type like

public class SomeType<T>
{
  public <E> void test(final Collection<E> collection)
  {
    for (final E object : collection)
    {
      System.out.println("E: " + object);
    }
  }

  public void test(final Set<T> set)
  {
    for (final T t : set)
    {
      System.out.println("T from set: " + t);
    }
  }

  public void test(final List<Integer> integerList)
  {
    int result = 0;
    for (final Integer integer : integerList)
    {
      result += integer.intValue();
    }
    System.out.println("result: " + result);
  }
}

and execute code like

final SomeType someType = new SomeType();
final List<String> list = Arrays.asList("some", "test", "value");
someType.test(list);

Then you will get ClassCastException saying “java.lang.String cannot be cast to java.lang.Integer” in runtime but no errors in compile-time. Why? because it uses public void test(final List<Integer> integerList) method instead of public <E> void test(final Collection<E> collection). But why does it use the one taking List of Integer instead of the one with the Collection of some type E when the passed parameter is a List of String which is a Collection of some type E, in this case, String? The reason is that SomeType is generified but when using it, no generics are used. If generified type is used without generic type info specified, the Java compiler makes it like the old type in JDK prior to 5.0 which doesn’t have generics.

Thus, the methods above all become like

  public void test(final Collection collection)

  public void test(final Set set)

  public void test(final List integerList)

So the closest type matched with a List of String is a List and that’s why public void test(final List<Integer> integerList) is called. But the method, public <E> void test(final Collection<E> collection), has nothing to do with the generic type specified in the class SomeType<T>. It’s just a generic method and the E has nothing to do with the T. Yeah, but if the type is generified, yet no generic type is specified when it is used, all the generics info including the ones in the generic methods is gone.

To solve the problem, you just need to specify the generic type. In the case above, you can even use just ‘?‘ (wildcard) since you’re not using the generic type specified in the class but the generic method, public <E> void test(final Collection<E> collection).

final SomeType<?> someType = new SomeType<Object>();
final List<String> list = Arrays.asList("some", "test", "value");
someType.test(list);

Now, with generics, you don’t have the problem you had before without generics.

So, it’s clear why you should use generics when using generified types. What I just explained is actually not new but well known to most Java programmers. Now, let’s talk about some benefits from using generics.

You can avoid a problem that you have with arrays if you use generics. Arrays of reference types in Java are covariant so you may have a problem like this.

  final Integer[] integerArray = { 1, 2, 3, 4, 5 };
  final Object[] objectArray = integerArray;
  objectArray[0] = "test";

There is no compile-time error, yet running this code results in java.lang.ArrayStoreException: java.lang.String when objectArray[0] = "test"; is executed.

Unlike arrays, generics are invariant, so if you use generics like,

  final List<Integer> integerList = Arrays.asList(1, 2, 3, 4, 5);
  final List<Object> objectList = integerList;

You get a compile-time error.

  final List<Object> objectList = (List<Integer>) integerList; // compile-time error

Even this one causes the compile-time error too since a List of Integer is not a subtype of List of Object (In arrays, an Integer array is a subtype of Object array).

On the other hand, you may need to cast it although it happens very occasionally. Here is an example taken from one of my libraries.
In KommonLee ASM, there is a visitor class, MethodAnalysisClassVisitor.

public class MethodAnalysisClassVisitor<T, M extends Member> extends EmptyVisitor
{
  private final MemberCollector<M> memberCollector;
  private final Class<T> theClass;
  private final Map<M, String[]> memberToParameterNamesMap;

  public MethodAnalysisClassVisitor(final MemberCollector<M> memberCollector, final Class<T> theClass,
      final Map<M, String[]> memberToParameterNamesMap)
  {
    this.memberCollector = memberCollector;
    this.theClass = theClass;
    this.memberToParameterNamesMap = memberToParameterNamesMap;
  }
  // Remainder omitted...
}

This visitor collects all the methods / constructors and their parameter names. Why do I need this kind of tool? It’s because I needed to get the parameter names of constructors in a class for one of my libraries, JSON Statham, but there was no easy way to get the parameter names of methods or constructors in Java. Reflection in Java certainly doesn’t have this convenience which is, I believe, an integral part for library / framework development although it would be depending on what sort of library it is. So I had to use ASM to get the method parameter names. I was shocked when I first discovered it. Anyway, back to the original topic again, the visitor is used by AsmMethodAndConstructorAnalyser.

public class AsmMethodAndConstructorAnalyser implements MethodAndConstructorAnalyser
{
  // ...

  @Override
  public <T> Map<Constructor<T>, String[]> findConstructorsWithParameterNames(final Class<T> theClass)
      throws IllegalArgumentException
  {
    final Map<Constructor<T>, String[]> constructorToParameterNamesMap = new LinkedHashMap<Constructor<T>, String[]>();

    @SuppressWarnings({ "cast", "unchecked", "rawtypes" })
    final Map<Constructor<?>, String[]> map = (Map<Constructor<?>, String[]>) ((Map) constructorToParameterNamesMap);

    getClassReader(theClass).accept(
        new MethodAnalysisClassVisitor<T, Constructor<?>>(constructorCollector, theClass, map), 0);
    return constructorToParameterNamesMap;
  }
  // ...
}

I know that the type M defined in the MethodAnalysisClassVisitor is Constructor<T> in this particular method as I’m passing Class<T> and want to collect all the constructors in it which are all Constructor<T>, yet the Java compiler can’t figure it out so I get a compile-time error if I just do

    final Map<Constructor<T>, String[]> constructorToParameterNamesMap = new LinkedHashMap<Constructor<T>, String[]>();

    getClassReader(theClass).accept(
        new MethodAnalysisClassVisitor<T, Constructor<?>>(constructorCollector, theClass, constructorToParameterNamesMap), 0);

Therefore, what I did was:
first, cast Map<Constructor<T>, String[]> constructorToParameterNamesMap to a raw type Map then it will lose all the generic type info.
second, I can cast it to Map of any key value pair since the Java compiler erases all the generic types when it’s compiled.
finally, I ended up having the following lines of code. I had to add @SuppressWarnings({ "cast", "unchecked", "rawtypes" }) to make the compiler happy. I could do it because I know the constructors stored in the Map are all Constructors of T type.

  @SuppressWarnings({ "cast", "unchecked", "rawtypes" })
  final Map<Constructor<?>, String[]> map = (Map<Constructor<?>, String[]>) ((Map) constructorToParameterNamesMap);

  getClassReader(theClass).accept(
      new MethodAnalysisClassVisitor<T, Constructor<?>>(constructorCollector, theClass, map), 0);

This doesn’t happen often though. So most of the time, you don’t need to do it and can enjoy type safety that generics offer.

Along with the benefit I mentioned above with the case compared with arrays, generics offer convenience of postponing specifying the type information until it is used with compile-time type safety. One example that everyone knows is Java’s collections framework. The programmer of collections didn’t know what types would be stored in the collections when it was created, thus when it came out, it stored just an object type as there were no generics. This is the part where Java loses static typing although it’s a static type language. However, after introduction of generics, the users of collections can have compile-time type safety. Ironically, the collections are one of the most popular examples of how generics can be useful, yet according to Neal Gafter, it’s one of the most important reasons why generics are implemented using type erasure and wild-card, which make generics more complex. If you’re interested in it, you’d better watch his presentation.

Although Generics were introduced in JDK 5.0, the GJ compiler that can handle generics were introduced much earlier in JDK 1.3 but it was disabled according to the Java language specification.

Generics give us compile-time type safety but it’s compile-time only because, as I just mentioned, it does type erasure which means the compiler removes all the generic type information when it compiles, and that’s why we can’t create a generic type object or array like.

T t = new T();
T[] t = new T[10];

This is impossible in Java and that’s why you don’t see method like E[] toArray() but Object[] toArray() and <T> T[] toArray(T[] a) in the Collection class and its sub classes. You can create an array of E type if an instance of array of E is given as a parameter value. For example,

public <E> E[] copyArray(final E[] array)
{
  @SuppressWarnings("unchecked")
  final E[] copiedArray = (E[]) Array
                          .newInstance(array
                                        .getClass()
                                        .getComponentType(),
                                      array.length);
  System.arraycopy(array, 0, copiedArray, 0, array.length);
  return copiedArray;
}

It takes an array of E object then creates a new array object of E then copies all the elements in the given one to the new one then returns it.

How is it possible? It’s possible because now I’m dealing with an ‘array object’ of E type not the generic type E directly so I can get the class of the array object in order to take the component type of the element stored in the array. So first, get the class of E[] ‘object’ (not class) which is a parameter value and given in runtime, then I can get the component type of it. However, the following case is impossible.

public <E, NE, F extends Function1<E, NE>> NE[] apply(final F function, final E[] source)
{
  final List<NE> list = new ArrayList<NE>();
  for (final E element : source)
  {
    list.add(function.apply(element));
  }
  @SuppressWarnings("unchecked")
  final NE[] resultArray = list.toArray(
      (NE[]) Array
          .newInstance(NE[].class // <- compile time error here: Illegal class literal for the type parameter NE!!!
                      .getComponentType(),
                       list.size()));
  return resultArray;
}

Because there is no such thing as an NE[] ‘class’ (not object), there is no way to get the class. When it’s compiled, NE[] becomes an Object array (Object[]).

I used to hate this type erasure as I had some issues when I developed my libraries. However, I do not anymore. It’s not just because of the reason mentioned here. I have mine and will explain it later in this post.

So let me talk about why we use generics again. We use it to have compile-time type safety without having to decide the type when designing the API. One example can be found in my old post about Easier and Better Way to Use JDBC. Simply, it makes use of callback function object much more useful.

Let’s say you want to map a type stored in a List to some other type. If you don’t use generics, you probably need to make a method for each mapped type like

public List<BigDecimal> takePricesOf(List<Product> list)
{
  List<BigDecimal> newList = new ArrayList<BigDecimal>();
  for (Product product : list)
  {
    newList.add(product.getPrice());
  }
  return newList;
}

public List<DiscountedProduct> toDiscountedProducts(int discountRate, List<Product> list)
{
  List<DiscountedProduct> newList = new ArrayList<DiscountedProduct>();
  for (Product product : list)
  {
    newList.add(new DiscountedProduct(product, discountRate));
  }
  return newList;
}

// and more...

(OK, here are actually generics used but it’s only for using List).

It can become one reusable method with some callback function object like.

public interface Mapper
{
  Object map(Object input);
}
public List<Object> mapWithoutGenerics(Mapper mapperWithoutGenerics, List<?> list) // <- this cannot be List<Object> since you want to pass a List of any type not just List<Object>.
{
  List<Object> newList = new ArrayList<Object>();
  for (Object object : list)
  {
    newList.add(mapperWithoutGenerics.map(object));
  }
  return newList;
}

When you use it.

List<Object> productPriceList = mapWithoutGenerics(new Mapper() {
  public Object map(Object input)
  {
    Product product = (Product) input;
    return product.getPrice();
  }
}, productList);

Now, you don’t have compile-time type safety anymore so you may pass a List of any type.
So the following code doesn’t cause any compile-time error but the runtime exception that is java.lang.ClassCastException.

List<Object> productPriceList = mapWithoutGenerics(new Mapper() {
  public Object map(Object input)
  {
    Product product = (Product) input;
    return product.getPrice();
  }
}, Arrays.asList("Some", "String", "List"));

 

It can surely become much better with generics.

public interface Function1<T, R>
{
  R apply(T input);
}
public <T, R, F extends Function1<T, R>> List<R> map(F mapper, List<T> list)
{
  final List<R> newList = new ArrayList<R>();
  for (final T t : list)
  {
    newList.add(mapper.apply(t));
  }
  return newList;
}

Then when you use it.

final List<BigDecimal> productPriceList = map(new Function1<Product, BigDecimal>() {
  @Override
  public BigDecimal apply(final Product product)
  {
    return product.getPrice();
  }
}, productList);

Or the function object can be reused.

private static final Function1<Product, BigDecimal> PRODUCT_TO_PRICE_MAPPER = new Function1<Product, BigDecimal>() {
  @Override
  public BigDecimal apply(final Product product)
  {
    return product.getPrice();
  }
};

//...

final List<BigDecimal> productPriceList        = map(PRODUCT_TO_PRICE_MAPPER, productList);
final List<BigDecimal> anotherProductPriceList = map(PRODUCT_TO_PRICE_MAPPER, anotherProductList);

The other method that is toDiscountedProducts() can be done with this generic map method and closure.

final int discountRate = 15;

final List<DiscountedProduct> discountedProductList = map(new Function1<Product, DiscountedProduct>() {
  @Override
  public DiscountedProduct apply(final Product product)
  {
    return new DiscountedProduct(product, discountRate);
  }
}, productList);
Although Java doesn’t really have closure, the Function1 above is used as a closure because it’s accessing a non-local variable, and in almost all the cases when people say that Java doesn’t have closure, they actually mean a way to support first-class functions (e.g. lambda expression to support first-class functions in Java).

Speaking of generics used for collections, I should point out that it would have been much more useful, if the collections had got methods like select (or filter), map, etc.

So if you had got a List of Integer and wanted to get all the positive ones, you would have done like.

List<Integer> positiveIntegerList = integerList.select(new Condition1<Integer>() {
  public boolean isMet(Integer integer)
  {
    return 0 < integer.intValue();
  }
});

Or getting all the prices from the List of Product object as I already showed. Unfortunately Java’s Collections don’t have those methods. Well, my collection library has it, but it’s incomplete and not so compatible with existing code using Java’s collections although it has a way to convert from and to Java ones. So to keep using Java’s collections, I made some helper methods to achieve the goal which is having one generic method to apply different kinds of functions to all the elements in the collections. By the way, why do I bother about it? Can’t I just use for or foreach loop? Sure, I can. So why? With the generic methods mentioned above and function objects, I can focus on the actual problems. For instance, to get all the prices from the list of Product, my concern should be getting the price of each Product not how to use for or foreach loop.

Here, I need to worry about using foreach and creating ArrayList.

List<BigDecimal> productPriceList = new ArrayList<BigDecimal>();
for (Product product : productList)
{
  productPriceList.add(product.getPrice());
}

With the generic one (if there were one), I wouldn’t.

List<BigDecimal> productPriceList = productList.map(new Function1<Product, BigDecimal>() {
  public BigDecimal apply(Product product)
  {
    return product.getPrice(); // <- here I'm focusing on getting the price.
  }
});

Since Java doesn’t support first-class function, the syntax is not so pleasant. However, JDK 8 will have lambda expression to support it so when it comes out, it can probably be like.

List<BigDecimal> productPriceList = productList.map((product) -> product.getPrice());
// It hasn't been finalised yet so the syntax can be different.

Anyway, Java collections still don’t have those methods so, as I said already, I made the ones for the collections. So let me talk about one of these that is Selector. What Selector does is checking all the elements in a collection and takes only the ones that meet the given Condition. I wanted to have only one Selector that can easily deal with all the classes extends Collection instead of having one for each (e.g. ListToArrayListSelector, ListToHashSetSetSelector, SetToListSelector, and so on). Thus I made it like this.

public class IterableToCollectionSelector<E, T extends Iterable<? extends E>, C extends Condition1<? super E>, R extends Collection<E>>
    implements Selector1<T, C, R>
{
  @Override
  public R select(final C condition, final T source)
  {
    final R result = // <- How can I get the Collection instance of R?
    for (final E element : source)
    {
      if (condition.isMet(element))
      {
        result.add(element);
      }
    }
    return result;
  }
}

Here are Condition1 and Selector1 in my library.

public interface Condition1<T>
{
  boolean isMet(T input);
}
public interface Selector1<T, C, R>
{
  R select(C condition, T source);
}

Let me explain it step by step. First,

public class IterableToCollectionSelector<E, T extends Iterable<? extends E>, C extends Condition1<? super E>, R extends Collection<E>>
    implements Selector1<T, C, R>
E
E is a type of an element stored in the given collection.
T

T is any type extends Iterable of the type extends E.

  1. Why is it Iterable of any type extends E (? extneds E)? Otherwise, you can’t use a List of any sub type of E.
    It would be easier to understand with an example. Let’s say you need to get a List of Product the price of which is greater than 50 dollars. If it’s just Iterable<E> then you have no problem with passing a List of Product yet you do have when you try to pass a List of DiscountedProduct which is a sub class of Product and it should be perfectly valid to pass it as a parameter of the selector made for Product (Think about Liskov substitution principle).
    If you have Iterable<? extends E>, you can pass E type and any sub-types of E, in this case, Product and DiscountedProduct are all fine.
  2. Then why T extends Iterable? Can I just have Iterable for the input type then I can remove the generic type T? Yes, it’s OK, but what if there is anyone who wants to extends this class and wants to restrict the input type to only List instead of any sub-type of Iterable (probably if you care about Open/closed principle)?
    If you have a class like this.

    // Notice that T extends Iterable<? extends E> is gone, and it has Iterable<? extends E> instead (No T).
    public class IterableToCollectionSelector<E, C extends Condition1<? super E>, R extends Collection<E>> implements
      Selector1<Iterable<? extends E>, C, R>
    {
    }
    

    You can’t restrict the input type to something other than Iterable<? extends E>, and if you try to override the select method in the sub-class of it.
    e.g.)

    public class IterableToArrayListSelector<E> extends
      IterableToCollectionSelector<E, Condition1<? super E>, ArrayList<E>>
    {
      @Override
      public ArrayList<E> select(final Condition1<? super E> condition, final List<? extends E> source)
      {
        return super.select(condition, source);
      }
    }
    

    You will get a compile-time error because T is not the same as “? extends T“. In this case, “Iterable<? extends E>” specified as a generic type T for the Selector1<T, C, R> is not the same as “? extends T” that is, in here, List<? extends E> (one of “T extends Iterable<? extends E>“) in the overridden method select() (It’s not actually overridden as List<? extends E> can’t be a substitute for Iterable<? extends E> here). Still not sure why List can’t be used? Well, think about this. List<String> is not the same as and can’t be a substitute for List<Object>. You know it, and I already explained it earlier when I talked about the side-effect of using arrays. Then think about this one. Can List<List<?>> be a substitute for List<Iterable<?>>? No, it’s just like List<String> and List<Object>, and the problem we have here is the same.

    So

    public class IterableToCollectionSelector<E, T extends Iterable<? extends E>, C extends Condition1<? super E>, R extends Collection<E>>
      implements Selector1<T, C, R>
    

    gives me freedom to restrict the input iterable to whatever extends Iterable I want.
    e.g.)

    public class IterableToArrayListSelector<E> extends
        IterableToCollectionSelector<E, List<? extends E>, Condition1<? super E>, ArrayList<E>>
    

    There will be a compile-time error when passing a Set instead of a List for the input value of the select() method.

C extends Condition1<? super E>
Next one is C extends Condition1<? super E>. What? Why is the input type of the Condition1? super E“? Because, with “? super E“, I can have one Condition1 object for many sub-types of the type E. Imagine that you want to get a List of Product the price of which is greater than 20, you can have a Condition1 for Product, then if you also need to get a List of DiscountedProduct the price of which is also greater than 20, you can reuse that Condition1 object for it as DiscountedProduct is a sub-type of Product which means Condition1<Product> is a kind of Condition1<? super DiscountedProduct>. If I have “C extends Condition1<E>” then I can’t do that, but because it’s “C extends Condition1<? super E>“, I can.
R
The last one is R. R extends Collection<E> so it can be any sub-type of Collection.

Now let’s look at the method. Here I have some problem.

  public R select(final C condition, final T source)
  {
    final R result = // <- How can I get the Collection instance of R???
    for (final E element : source)
    {
      if (condition.isMet(element))
      {
        result.add(element);
      }
    }
    return result;
  }

If it’s one type of Collection, I can easily create it but it’s not determined yet. It will be set when it’s used. Then why don’t I postpone instantiation of it until it’s used. More precisely when the selector is instantiated. So I just create another type to create one of Collection also using generics.

public interface CollectionCreator<E, T extends Collection<? extends E>>
{
  T createCollection();
}

So it can be used to create any sub-types of Collection.
e.g.) for ArrayList

public class ArrayListCreator<E> implements CollectionCreator<E, ArrayList<E>>
{
  @Override
  public ArrayList<E> createCollection()
  {
    return new ArrayList<E>();
  }
}

for HashSet

public class HashSetCreator<E> implements CollectionCreator<E, HashSet<E>>
{
  @Override
  public HashSet<E> createCollection()
  {
    return new HashSet<E>();
  }
}

With all these, the final version of IterableToCollectionSelector is

public class IterableToCollectionSelector<E, T extends Iterable<? extends E>, C extends Condition1<? super E>, R extends Collection<E>>
    implements Selector1<T, C, R>
{
  private final CollectionCreator<E, ? extends R> collectionCreator;

  public <CC extends CollectionCreator<E, ? extends R>> IterableToCollectionSelector(final CC collectionCreator)
  {
    this.collectionCreator = collectionCreator;
  }

  @Override
  public R select(final C condition, final T source)
  {
    final R result = collectionCreator.createCollection();
    for (final E element : source)
    {
      if (condition.isMet(element))
      {
        result.add(element);
      }
    }
    return result;
  }
}

It needs to have CollectionCreator but it’s immutable so I don’t need to worry about the state of the selector object once it’s created.

When IterableToCollectionSelector is used, it looks

// selector for the List of Product.
final IterableToCollectionSelector<Product, Iterable<Product>, Condition1<Product>, ArrayList<Product>> collectionSelector =
  new IterableToCollectionSelector<Product, Iterable<Product>, Condition1<Product>, ArrayList<Product>>(new ArrayListCreator());
final List<Product> resultList = collectionSelector.select(greaterThan20, productList);

// selector for the List of DiscountedProduct
final IterableToCollectionSelector<DiscountedProduct, Iterable<DiscountedProduct>, Condition1<Product>, ArrayList<DiscountedProduct>> collectionSelector2 =
  new IterableToCollectionSelector<DiscountedProduct, Iterable<DiscountedProduct>, Condition1<Product>, ArrayList<DiscountedProduct>>(new ArrayListCreator());
final List<DiscountedProduct> resultList2 = collectionSelector2.select(greaterThan20, discountedProductList)

Wait, do I have to create the IterableToCollectionSelector whenever I need to use it for a different type? It seems like it’s waste of memory and not to mention of boilerplate code. OK, here comes why I don’t hate generics’ type erasure anymore. Considering type erasure, both objects, used above, actually have no difference in runtime.

So what I can do is that I can create a helper class containing one really generic IterableToCollectionSelector then cast it using a generic method so that it always returns the same instance but can be used for different types with compile-time type safety. It would be clear if I just show the code.

public class CollectionUtil
{
  private static final IterableToCollectionSelector<?, ? extends Iterable<?>, ? extends Condition1<?>, ? extends ArrayList<?>> ITERABLE_TO_ARRAY_LIST_SELECTOR =
    new IterableToCollectionSelector<Object, Iterable<?>, Condition1<Object>, ArrayList<Object>>(
        new ArrayListCreator<Object>());

  public static <E, T extends Iterable<? extends E>, C extends Condition1<? super E>> IterableToCollectionSelector getIterableToCollectionSelector()
  {
    @SuppressWarnings("unchecked")
    final IterableToCollectionSelector<E, T, C, ArrayList<E>> iterableToCollectionSelector =
      (IterableToCollectionSelector<E, T, C, ArrayList<E>>) ITERABLE_TO_ARRAY_LIST_SELECTOR;
    return iterableToCollectionSelector;
  }
}

And similarly, ArrayListCreator doesn’t have to be created more than once. It doesn’t have any state so one instance can be and should be reused.

public class ArrayListCreator<E> implements CollectionCreator<E, ArrayList<E>>
{
  public static final ArrayListCreator<Object> ARRAY_LIST_CREATOR = new ArrayListCreator<Object>();

  @Override
  public ArrayList<E> createCollection()
  {
    return newArrayList();
  }

  public static <E> ArrayListCreator<E> getInstance()
  {
    @SuppressWarnings("unchecked")
    final ArrayListCreator<E> arrayListCreator = (ArrayListCreator<E>) ARRAY_LIST_CREATOR;
    return arrayListCreator;
  }
}

So CollectionUtil is rewritten.

public class CollectionUtil
{
  private static final IterableToCollectionSelector<?, ? extends Iterable<?>, ? extends Condition1<?>, ? extends ArrayList<?>> ITERABLE_TO_ARRAY_LIST_SELECTOR =
    new IterableToCollectionSelector<Object, Iterable<?>, Condition1<Object>, ArrayList<Object>>(
        ArrayListCreator.getInstance());

  public static <E, T extends Iterable<? extends E>, C extends Condition1<? super E>> IterableToCollectionSelector getIterableToCollectionSelector()
  {
    @SuppressWarnings("unchecked")
    final IterableToCollectionSelector<E, T, C, ArrayList<E>> iterableToCollectionSelector =
      (IterableToCollectionSelector<E, T, C, ArrayList<E>>) ITERABLE_TO_ARRAY_LIST_SELECTOR;
    return iterableToCollectionSelector;
  }
}
@SuppressWarnings("unchecked")
final IterableToCollectionSelector<E, T, C, ArrayList<E>> iterableToCollectionSelector =
  (IterableToCollectionSelector<E, T, C, ArrayList<E>>) ITERABLE_TO_ARRAY_LIST_SELECTOR;

This is possible due to type erasure, so you don’t have to create many objects of the same type with different generic type info (e.g. instances of the same type, ‘IterableToCollectionSelector‘, with ‘different generic types’ like different E, T, C) which will be all the same in runtime. That’s why I do not hate type erasure anymore. Of course, you need to @SuppressWarnings to make the compiler quiet, but it’s not a big deal and I know what I’m doing. It’s perfectly valid.

The condition can be reused.

final Condition1<Product> greaterThan20 = new Condition1<Product>() {
  private final BigDecimal twenty = new BigDecimal("20");
  @Override
  public boolean isMet(final Product input)
  {
    return 0 > twenty.compareTo(input.getPrice());
  }
};

Now I can just do,

final IterableToCollectionSelector<Product, Iterable<Product>, Condition1<Product>, ArrayList<Product>> collectionSelectorForProduct =
  CollectionUtil.getIterableToCollectionSelector();
final List<Product> resultForProduct = collectionSelectorForProduct.select(greaterThan20, productList);

final IterableToCollectionSelector<DiscountedProduct, Iterable<DiscountedProduct>, Condition1<Product>, ArrayList<DiscountedProduct>> collectionSelectorForDiscountedProduct =
  CollectionUtil.getIterableToCollectionSelector();
final List<DiscountedProduct> resultForDiscountedProduct = collectionSelectorForDiscountedProduct.select(greaterThan20, discountedProductList);

Each time when you get an instance of IterableToCollectionSelector using the getIterableToCollectionSelector() method, you get the same IterableToCollectionSelector instance.

OK, that sounds good but do I really need to know all those complex E, T, C, R and even wildcard like “? extends E” or “? super E“? Well, you don’t have to. It depends on what you do. If you’re a library or framework developer, you should probably. Otherwise, not really. The library or API designer should carefully design it to offer APIs that are easy to use and understand for the users of the library. The designer or architect, on the other hand, should know the things I talked about here to make complex things simple. That’s our job, isn’t it? Making a complex thing simple then solve it. Anyway, most of the time, you don’t need to deal with that kind of complex API if you’re an application developer. Your company should have at least one person who can deal with it and he/she should provide easy to use APIs. If your company doesn’t have any, ask your boss to hire one. :) Using well designed good APIs can surely save your time which means saving your company’s money. Also less error-prone.

So how can the IterableToCollectionSelector be simple so that the users wouldn’t need to care about E, T, C, etc.

The library designer should create more specific type of selector like

public final class IterableToArrayListSelector<E> extends
    IterableToCollectionSelector<E, Iterable<? extends E>, Condition1<? super E>, ArrayList<E>>
{
  IterableToArrayListSelector(final ArrayListCreator<E> collectionCreator)
  {
    super(collectionCreator);
  }
}

Then CollectionUtil becomes

public class CollectionUtil
{
  private static final IterableToArrayListSelector<?> ITERABLE_TO_ARRAY_LIST_SELECTOR =
    new IterableToArrayListSelector<Object>(ArrayListCreator.getInstance());

  public static <E> IterableToArrayListSelector<E> getIterableToArrayListSelector()
  {
    @SuppressWarnings("unchecked")
    final IterableToArrayListSelector<E> iterableToCollectionSelector =
      (IterableToArrayListSelector<E>) ITERABLE_TO_ARRAY_LIST_SELECTOR;
    return iterableToCollectionSelector;
  }
}

Now using it is much easier.
(with static import)

import static your.package_name.here.CollectionUtil.*;

// ...

final IterableToArrayListSelector<Product> arrayListSelectorForProduct =
  getIterableToArrayListSelector();
final List<Product> resultForProduct = arrayListSelectorForProduct.select(greaterThan20, productList);

final IterableToArrayListSelector<DiscountedProduct> arrayListSelectorForDiscountedProduct =
  getIterableToArrayListSelector();
final List<DiscountedProduct> resultForDiscountedProduct =
  arrayListSelectorForDiscountedProduct.select(greaterThan20, discountedProductList);

You may not be satisfied yet, because you have the variables for the IterableToArrayListSelector objects. Should you have these? No, you don’t have to. So you probably try like this.

final List<Product> resultForProduct = getIterableToArrayListSelector()
                                         .select(greaterThan20, productList);

Yet, this doesn’t work. When getIterableToArrayListSelector() is called there is no generic type info given so it returns just an object of IterableToArrayListSelector<Object>. Thus it causes a compile-time error on calling select(greaterThan20, productList).

It can be solved by specifying the generic type like.

final List<Product> resultForProduct2 = CollectionUtil.<Product> getIterableToArrayListSelector()
                                          .select(greaterThan20, productList);

It’s not so elegant though. It can be solved as well. How? instead of having the getIterableToArrayListSelector() method to get an instance of IterableToArrayListSelector, why don’t I just create another generic select() method which does both getting the instance and calling the select() method in it.

So CollectionUtil is re-written.

public class CollectionUtil
{
  private static final IterableToArrayListSelector<?> ITERABLE_TO_ARRAY_LIST_SELECTOR =
    new IterableToArrayListSelector<Object>(ArrayListCreator.getInstance());

  public static <E, T extends Iterable<? extends E>, C extends Condition1<? super E>> ArrayList<E> select(
      final C condition, final T iterable)
  {
    final IterableToArrayListSelector<E> iterableToCollectionSelector =
      (IterableToArrayListSelector<E>) ITERABLE_TO_ARRAY_LIST_SELECTOR;
    return iterableToCollectionSelector.select(condition, iterable);
  }
}

How to use? Really simple.
(also with static import)

import static your.package_name.here.CollectionUtil.*;

// ...

final List<Product> resultForProduct = select(greaterThan20, productList);

final List<DiscountedProduct> resultForDiscountedProduct = select(greaterThan20, discountedProduct);

So it’s

final IterableToCollectionSelector<Product, Iterable<Product>, Condition1<Product>, ArrayList<Product>> collectionSelectorForProduct =
  getIterableToCollectionSelector();
final List<Product> resultForProduct = collectionSelectorForProduct.select(greaterThan20, productList);

VS

final List<Product> resultForProduct = select(greaterThan20, productList);

As you can see, depending on how you use generics it can give you a really nice, simple and easy to use API.

Just a moment. It might not be enough. That select takes an Iterable object and returns an ArrayList object, but what if I want to get HashSet instead, or what about taking an array as an input parameter instead of an Iterable object? It can be accomplished too, but I’m not going to explain it here. I’ll do it later when I talk about good API design in another post not in the generics series.

In the meantime, you can use what I’ve already provided in my library that is KommonLee.

If you use it, you can simply do

final List<Product> anotherPositiveIntegerList = selector()
        .fromIterable()
        .toArrayList()
        .select(greaterThan20, productList);

OR

final Set<Product> anotherPositiveIntegerList = selector()
        .fromIterable()
        .toHashSet()
        .select(greaterThan20, productList);

OR

Product[] productArray = // ...
final List<Product> anotherPositiveIntegerList = selector()
        .fromArray()
        .toArrayList()
        .select(greaterThan20, productArray);

and so on…
More examples are >>here<<
It also has Mappers and SelectableMappers so you can convert one type stored in a Collection to another.

Also the example code I used here is available on Github. Example Code: CLICK HERE!!!

Next post about generics would be about something easier than the selector example. So it would be some practical but easier, then the final post would be about really complex one when it’s designed but easy when it’s used. It is also very practical and useful. Unfortunately, however, I can’t tell when I can write it as I need to think about good examples to explain. As you can see above, it can be difficult to understand without example code (or even with it, still difficult). I do actually have some examples but it’s the code I made for my work so I can’t just use it. Anyway, I will figure it out so stay tuned.
 
 

Thanks for those who retweeted and favorited this post.

 

Quizzes for Primary Schoolchildren

Yesterday (the 5th of May) was the children’s day, which is an official holiday, in South Korea, (by the way, the day before yesterday was Star Wars Day :)) and I found that Xeraph, who is the CEO of the IT security company named Nchovy, as well as the creator & core programmer of the OSGi based opensource security platform, Kraken, posted some quiz for kids. :) His blog also has other quizzes for the primary schoolchildren.

These quizzes remind me of my childhood as these kinds of quizzes are what I used to solve when I was a primary schoolchild. It was approximately 22 years ago. That time, I learned Basic (Not Visual Basic (VB), VB didn’t exist back then), Fortran, COBOL and C. When I was learning Basic, those kinds of problems were what I solved. After I graduated from the primary school, I had not touched any computer programming languages at all for 13 years. I started it again as I came to Sydney and entered the university to study IT. Then I realised that I could remember nothing about what I learned before. Nothing at all. That’s why my ‘about me‘ page doesn’t contain any of Basic, Fortran and COBOL since I’ve completely forgot about these (it does have Visual Basic though as it’s my first programming language that I learned at the diploma school, UTS:INSEARCH). So my point is, doesn’t matter whether it’s a programming language or a human language, if you don’t practice or keep studying constantly, you will lose your skills and fluency of the language.

Anyway, I saw those quizzes and solved it for fun yet feel like I did better when I was a kid. :( Back then, I didn’t even use a computer to solve it but only pen and paper.
The followings are the quizzes and my answers, yet there can be much better answers of course, so why don’t you try by yourself before you check out mine.

This is the prompt utility class, used for the other programmes, to get input values. Notice that it doesn’t check whether the input value is number, integer more precisely, or not although it does check if the given integer is a positive one.

package com.lckymn.kevin.simplequiz;

import java.util.Scanner;

/**
 * @author Lee, SeongHyun (Kevin)
 * @version 0.0.1 (2010-05-05)
 */
public final class PromptUtil
{
    private PromptUtil()
    {
        throw new IllegalStateException(getClass().getName() + " cannot be instantiated.");
    }

    public static int askForPositiveInt(String message)
    {
        final Scanner scanner = new Scanner(System.in);
        System.out.print(message);
        int number = Integer.parseInt(scanner.nextLine());

        while (0 >= number)
        {
            System.out.println("Please enter a position integer.");
            System.out.print(message);
            number = Integer.parseInt(scanner.nextLine());
        }
        return number;
    }
}

 

Quiz 1. (taken from http://xeraph.com/5251248)

n for diamond? 3
  *
 ***
*****
 ***
  *

n for diamond? 5
    *
   ***
  *****
 *******
*********
 *******
  *****
   ***
    *

n for diamond? 7
      *
     ***
    *****
   *******
  *********
 ***********
*************
 ***********
  *********
   *******
    *****
     ***
      *

n for diamond? 9
        *
       ***
      *****
     *******
    *********
   ***********
  *************
 ***************
*****************
 ***************
  *************
   ***********
    *********
     *******
      *****
       ***
        *

* Kevin’s Answer1

Show »

package com.lckymn.kevin.simplequiz;

import static com.lckymn.kevin.simplequiz.PromptUtil.*;

/**
 * @author Lee, SeongHyun (Kevin)
 * @version 0.0.1 (2010-05-05)
 */
public final class Diamond
{
    public static void main(String[] args)
    {
        int n = askForPositiveInt("n for diamond? ");
        int startPoint = --n;
        int endPoint = startPoint;
        for (int outer = 0, outerSize = n << 1; outer <= outerSize; outer++)
        {
            for (int inner = 0; inner <= endPoint; inner++)
            {
                System.out.print(inner >= startPoint ? "*" : " ");
            }
            if (outer < n)
            {
                startPoint--;
                endPoint++;
            }
            else
            {
                startPoint++;
                endPoint--;
            }
            System.out.print("\n");
        }
        System.exit(0);
    }

}

* Kevin’s Answer2

Show »

package com.lckymn.kevin.simplequiz;

import static com.lckymn.kevin.simplequiz.PromptUtil.*;

/**
 * @author Lee, SeongHyun (Kevin)
 * @version 0.0.1 (2010-05-05)
 */
public final class Diamond2
{
    public static void main(String[] args)
    {
        int n = askForPositiveInt("n for diamond? ");
        int startPoint = --n;
        int endPoint = startPoint;
        for (int outer = 0; outer < n; outer++)
        {
            for (int inner = 0; inner <= endPoint; inner++)
            {
                System.out.print(inner >= startPoint ? "*" : " ");
            }
            startPoint--;
            endPoint++;
            System.out.print("\n");
        }
        for (int outer = 0; outer <= n; outer++)
        {
            for (int inner = 0; inner <= endPoint; inner++)
            {
                System.out.print(inner >= startPoint ? "*" : " ");
            }
            startPoint++;
            endPoint--;
            System.out.print("\n");
        }
        System.exit(0);
    }

}

 

 

Quiz 2. (taken from http://xeraph.com/5251248)

n for outer box? 4
m for inner box? 2
****
*  *
*  *
****

n for outer box? 7
m for inner box? 3
*******
*******
**   **
**   **
**   **
*******
*******

n for outer box? 10
m for inner box? 4
**********
**********
**********
***    ***
***    ***
***    ***
***    ***
**********
**********
**********

n for outer box? 12
m for inner box? 6
************
************
************
***      ***
***      ***
***      ***
***      ***
***      ***
***      ***
************
************
************

* Kevin’s Answer

Show »

package com.lckymn.kevin.simplequiz;

import static com.lckymn.kevin.simplequiz.PromptUtil.*;

/**
 * @author Lee, SeongHyun (Kevin)
 * @version 0.0.1 (2010-05-05)
 */
public final class Box
{
    public static void main(String[] args)
    {
        int n = askForPositiveInt("n for outer box? ");
        int m = askForPositiveInt("m for inner box? ");

        while (n <= m || (0 != ((n - m) & 1)))
        {
            System.out.println("n must be greater than m and the result of (n - m) must be an even number.");
            n = askForPositiveInt("n for outer box? ");
            m = askForPositiveInt("m for inner box? ");
        }

        int halfOfDifference = n - m >>> 1;
        int startPoint = halfOfDifference - 1;
        int endPoint = n - halfOfDifference;
        for (int outer = 0; outer < n; outer++)
        {
            for (int inner = 0; inner < n; inner++)
            {
                System.out.print((outer > startPoint && outer < endPoint && inner > startPoint && inner < endPoint) ? " " : "*");
            }
            System.out.print("\n");
        }
        System.exit(0);
    }
}

 

 

Quiz 3. (taken from http://xeraph.com/5251248)

n for outer triangle? 3
m for inner triangle? 1
  *
 ***
** **

n for outer triangle? 5
m for inner triangle? 3
    *
   ***
  ** **
 **   **
**     **

n for outer triangle? 8
m for inner triangle? 4
       *
      ***
     *****
    *******
   **** ****
  ****   ****
 ****     ****
****       ****

n for outer triangle? 10
m for inner triangle? 5
         *
        ***
       *****
      *******
     *********
    ***** *****
   *****   *****
  *****     *****
 *****       *****
*****         *****

* Kevin’s Answer

Show »

package com.lckymn.kevin.simplequiz;

import static com.lckymn.kevin.simplequiz.PromptUtil.*;

/**
 * @author Lee, SeongHyun (Kevin)
 * @version 0.0.1 (2010-05-05)
 */
public final class Triangle
{
    public static void main(String[] args)
    {
        int n = askForPositiveInt("n for outer triangle? ");
        int m = askForPositiveInt("m for inner triangle? ");

        while (n <= m)
        {
            System.out.println("n must be greater than m.");
            n = askForPositiveInt("n for outer triangle? ");
            m = askForPositiveInt("m for inner triangle? ");
        }

        int startPoint = (n - 1);
        int endPoint = startPoint;

        final int innerHeightStartPoint = startPoint - (m - 1);
        int innerWidthStartPoint = startPoint;
        int innerWidthEndPoint = startPoint;
        for (int outer = 0; outer < n; outer++)
        {
            for (int inner = 0; inner <= endPoint; inner++)
            {
                System.out.print((outer >= innerHeightStartPoint && inner >= innerWidthStartPoint && inner <= innerWidthEndPoint) ? " "
                        : inner >= startPoint ? "*" : " ");
            }
            startPoint--;
            endPoint++;
            if (outer >= innerHeightStartPoint)
            {
                innerWidthStartPoint--;
                innerWidthEndPoint++;
            }
            System.out.print("\n");
        }
        System.exit(0);
    }
}

 

 

Quiz 4. (taken from http://xeraph.com/5261697)

matrix size n? 5
  1   2   3   4   5
 16  17  18  19   6
 15  24  25  20   7
 14  23  22  21   8
 13  12  11  10   9

matrix size n? 6
  1   2   3   4   5   6
 20  21  22  23  24   7
 19  32  33  34  25   8
 18  31  36  35  26   9
 17  30  29  28  27  10
 16  15  14  13  12  11

matrix size n? 7
  1   2   3   4   5   6   7
 24  25  26  27  28  29   8
 23  40  41  42  43  30   9
 22  39  48  49  44  31  10
 21  38  47  46  45  32  11
 20  37  36  35  34  33  12
 19  18  17  16  15  14  13

matrix size n? 8
  1   2   3   4   5   6   7   8
 28  29  30  31  32  33  34   9
 27  48  49  50  51  52  35  10
 26  47  60  61  62  53  36  11
 25  46  59  64  63  54  37  12
 24  45  58  57  56  55  38  13
 23  44  43  42  41  40  39  14
 22  21  20  19  18  17  16  15

* Kevin’s Answer1

Show »

package com.lckymn.kevin.simplequiz;

import static com.lckymn.kevin.simplequiz.PromptUtil.*;

/**
 * @author Lee, SeongHyun (Kevin)
 * @version 0.0.1 (2010-05-05)
 */
public final class Matrix
{
    public static void main(String[] args)
    {
        final int size = askForPositiveInt("matrix size n? ");
        final int sizeSquare = size * size;
        final int[][] matrix = new int[size][size];
        int number = 0;
        int row = 0;
        int column = 0;
        int addColumn = 1;
        int addRow = 0;
        int max = size;
        boolean isColumn = true;
        while (number < sizeSquare)
        {
            for (int filled = 0; filled < max; filled++)
            {
                matrix[row][column] = ++number;
                row += addRow;
                column += addColumn;
            }
            row += (isColumn ? (addColumn > 0 ? (addRow = 1) : (addRow = -1)) : -addRow);
            column += (isColumn ? -addColumn : (addRow > 0 ? (addColumn = -1) : (addColumn = 1)));
            if (isColumn)
            {
                max--;
                addColumn = 0;
            }
            else
            {
                addRow = 0;
            }
            isColumn = !isColumn;
        }
        final String format = "%" + (String.valueOf(sizeSquare).length()) + "d ";
        for (row = 0; row < size; row++)
        {
            for (column = 0; column < size; column++)
            {
                System.out.printf(format, Integer.valueOf(matrix[row][column]));
            }
            System.out.println();
        }
        System.exit(0);
    }

}

* Kevin’s Answer2

Show »

package com.lckymn.kevin.simplequiz;

import static com.lckymn.kevin.simplequiz.PromptUtil.*;

/**
 * @author Lee, SeongHyun (Kevin)
 * @version 0.0.1 (2010-05-05)
 */
public final class Matrix2
{
    enum Direction
    {
        UP, LEFT, DOWN, RIGHT;

        private static Direction current = UP;
        static
        {
            UP.next = RIGHT;
            DOWN.next = LEFT;
            LEFT.next = UP;
            RIGHT.next = DOWN;
        }
        private Direction next;

        static Direction next()
        {
            current = current.next;
            return current;
        }
    }

    public static void main(String[] args)
    {
        final int size = askForPositiveInt("matrix size n? ");
        int[][] matrix = new int[size][size];
        final int sizeSquare = size * size;
        int number = 0;
        int row = 0;
        int column = 0;
        int max = size;
        int filled = 0;
        int addColumn = 1;
        int addRow = 0;

        while (number < sizeSquare)
        {
            switch (Direction.next())
            {
                case RIGHT:
                case LEFT:
                    while (filled < max)
                    {
                        matrix[row][column] = ++number;
                        column += addColumn;
                        filled++;
                    }
                    row += addColumn;
                    column += -addColumn;
                    addRow = (addColumn > 0 ? 1 : -1);
                    addColumn = 0;
                    max--;
                    break;
                case DOWN:
                case UP:
                    while (filled < max)
                    {
                        matrix[row][column] = ++number;
                        row += addRow;
                        filled++;
                    }
                    row += -addRow;
                    column += -addRow;
                    addColumn = (addRow > 0 ? -1 : 1);
                    addRow = 0;
                    break;
                default:
                    throw new IllegalStateException("No one can get here!");
            }
            filled = 0;
        }
        final String format = "%" + (String.valueOf(sizeSquare).length()) + "d ";
        for (row = 0; row < size; row++)
        {
            for (column = 0; column < size; column++)
            {
                System.out.printf(format, Integer.valueOf(matrix[row][column]));
            }
            System.out.println();
        }
        System.exit(0);
    }
}

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