Generics

List myList = new ArrayList() can accept any type of object (never primitives - well, with wrappers). With the usage of generics you are bound to declare the type of the Elements contained at compile time so we get compilation errors if we try to add a something different from what we defined.

Mixing Generics with non Generics

public List insert(List a) { a.add(new String("62"); } it’s a method that would compile without error and we would not get any type of runtime exception because of the type erasure. At runtime we basically do not have any kind of protection. The compilers remove the generics declaration and automatically puts every necessary casts that a compile time we don’t use because of the generics.

Be aware that:

A sample code to understand type erasure is available on stackoverflow. Basically let’s say I have the following codeList<String> list = new ArrayList<String>(); list.add("Hi"); String x = list.get(0);, the compiler actually transform it like this List list = new ArrayList(); list.add("Hi"); String x = (String) list.get(0); so basically you don’t have any way to know at runtime that the object list contains strings. Also check this link from Angelika Langer.

Polymorphism with Generics

With a simple array is perfectly fine to do something like Parent[] arr = new Child(3), but you will get beaten if you try to do List<Parent> list = new ArrayList<Child>();.

When you declare a method accepting as argument a List<Parent> I can use as a parameter only a variable declared List<Parent>. Obviously at runtime the list can contain Child elements.

If we declare Animal[] arr = new Dog[3]; and I can pass it to a method having as argument a Animal[] and I can even do arr[0] = new Cat() and it would compile fine. I will have an ArrayStoreException.

If we want to pass a subtype of the argument we can use the keywords extends and super in the definition of the method. An example is public void addAnimal(List<? extends Animal> list) {} where I can pass without problem a List<Dog> but we cannot use the method add inside the method:
java public class Animal { public void addAnimal(List<? extends Animal> list){ list.add(new Animal()); } }

with the following error:
> The method add(capture#1-of ? extends Animal) in the type List<capture#1-of ? extends Animal> is not applicable for the arguments (Animal)
It’s actually quite simple to explain it. The JVM understand that you are passing some kind of list containing Animal or one of its children, like Dog. Can you add an Animal to a List<Dog>? Nope, you can’t, Dog is more specific, so you should explicitly cast it. You’re saying to the JVM that you can pass to the method a wide range of List and cannot assure the safety of the List.

With List<? super Dog> you can do something like that because you can pass to as argument of the method at minimum a List<Dog> or something higher in the hierarchy, like List<Animal> you have guarantee that Dog IS-A Animal` so it’s ok.

What will happen here?

public class Animal {
	public void addAnimal(List<? super SpecificDog> list){
		list.add(new SpecificDog());
		list.add(new Dog()); // same error as above!!
		// the JVM cannot know if the list can accept Dogs
		// because you have a probability there
		// to have a List<SpecificDog>
		// YOU CAN ADD A SPECIFICDOG or its subtypes!
	}
}
class Dog extends Animal {}
class SpecificDog extends Dog {}

Then you can use just <?> and that’s is like saying that you can pass a list of any type. It’s not like saying List<Object> because you can pass only a List<Object>as parameter of a method having that as argument. With List<?> you can pass any List you like but you cannot add anything because a Dog !IS-A a Church (you cannot add even a simple Object).

Generics declarations

// it's possible to declase several generics like <E,T>
// you can use extends and super but not ?, you'll get a compiler error
public interface List<E> {
	boolean add(E e);
	public E get(int i);
	// you can declare generics directly in a method. This interface compiles.
	<T> T someMethod();
}
public interface Animal<E> {
	<E> void someMethod(E e);  
	// type shadowing!! This <E> is different from the one above
}