Java Generics (Wildcards)

I have a couple of questions about generic wildcards in Java:

1. What is the difference between List<? extends T> and List<? super T>?

2. What is a bounded wildcard and what is an unbounded wildcard?

In your first question, <? extends T> and <? super T> are examples of bounded wildcards. An unbounded wildcard looks like <?>, and basically means <? extends Object>. It loosely means the generic can be any type. A bounded wildcard (<? extends T> or <? super T>) places a restriction on the type by saying that it either has to extend a specific type (<? extends T> is known as an upper bound), or has to be an ancestor of a specific type (<? super T> is known as a lower bound).

The Java Tutorials have some pretty good explanations of generics in the articles Wildcards and More Fun with Wildcards.

If you have a class hierarchy A, B is a subclass of A, and C and D both are subclass of B like below

class A {}
class B extends A {}
class C extends B {}
class D extends B {}

Then

List<? extends A> la;
la = new ArrayList<B>();
la = new ArrayList<C>();
la = new ArrayList<D>();

List<? super B> lb;
lb = new ArrayList<A>(); //fine
lb = new ArrayList<C>(); //will not compile

public void someMethod(List<? extends B> lb) {
    B b = lb.get(0); // is fine
    lb.add(new C()); //will not compile as we do not know the type of the list, only that it is bounded above by B
}

public void otherMethod(List<? super B> lb) {
    B b = lb.get(0); // will not compile as we do not know whether the list is of type B, it may be a List<A> and only contain instances of A
    lb.add(new B()); // is fine, as we know that it will be a super type of A 
}

A bounded wildcard is like ? extends B where B is some type. That is, the type is unknown but a "bound" can be placed on it. In this case, it is bounded by some class, which is a subclass of B.

Josh Bloch also has a good explanation of when to use super and extends in this [google io video talk][1] where he mentions the Producer extends Consumer super mnemonic.

From the presentation slides:

>Suppose you want to add bulk methods to Stack<E> > > void pushAll(Collection<? extends E> src); > >– src is an E producer > > void popAll(Collection<? super E> dst); > >– dst is an E consumer > [1]: http://sites.google.com/site/io/effective-java-reloaded

There may be times when you'll want to restrict the kinds of types that are allowed to be passed to a type parameter. For example, a method that operates on numbers might only want to accept instances of Number or its subclasses. This is what bounded type parameters are for.

Collection<? extends MyObject> 

means that it can accept all object who have IS- A relationship with MyObject (i.e. any object which is a type of myObject or we can say any object of any subclass of MyObject) or a object of MyObject class.

For example:

class MyObject {}

class YourObject extends MyObject{}

class OurObject extends MyObject{}

Then,

Collection<? extends MyObject> myObject; 

will accept only MyObject or children of MyObject(i.e. any object of type OurObject or YourObject or MyObject, but not any object of superclass of MyObject).

In general,

> If a structure contains elements with a type of the form ? extends E, we can get elements out of the structure, but we cannot put > elements into the structure

List<Integer> ints = new ArrayList<Integer>();
ints.add(1);
ints.add(2);
List<? extends Number> nums = ints;
nums.add(3.14); // compile-time error
assert ints.toString().equals("[1, 2, 3.14]"); 

To put elements into the structure we need another kind of wildcard called Wildcards with super,

 List<Object> objs = Arrays.<Object>asList(2, 3.14, "four");
    List<Integer> ints = Arrays.asList(5, 6);
    Collections.copy(objs, ints);
    assert objs.toString().equals("[5, 6, four]");

    public static <T> void copy(List<? super T> dst, List<? extends T> src) {
          for (int i = 0; i < src.size(); i++) {
                dst.set(i, src.get(i));
         }
    }

Generic wildcards are created to make methods that operate on Collection more reusable.

For example, if a method has a parameter List<A>, we can only give List<A> to this method. It is a waste for this method's funtion under some circumstances：

1. If this method only reads objects from List<A>, then we should be allowed to give List<A-sub> to this method. (Because A-sub IS a A)
2. If this method only inserts objects to List<A>, then we should be allowed to give List<A-super> to this method. (Because A IS a A-super)

learn by example:

consider the sort() method in Collections class which use both extends and super:

    public static <T extends Comparable<? super T>> void sort(List<T> list){...}

so

why <T extends Comparable<...>>: becuase we need list items (T) to be a subclass of the Comparable interface.

why Comparable<? super T>: becuase we allow the Comparable type to be a Comparable of any super type of T.

Consider

interface Comparable<T>{
    public int compareTo(T o);
}

public static <T extends Comparable<? super T>> void sort(List<T> list){...}


public static <T extends Comparable<T>> void sort2(List<T> list){...}


class A implements Comparable<A>{
    @Override
    public int compareTo(A o) {
        ...
    }
}

class B extends A {
}

    List<A> listA = new ArrayList<>();
    List<B> listB = new ArrayList<>();

    sort(listA);  //ok
    sort(listB);  //ok
    
    sort2(listA); //ok
    sort2(listB); //Error