Why is there an extra <E> in this generic method?

Question

I learned java generics some time ago, but now I'm learning collections and found some code that I don't understand. Here is the code:

static <E> List<E> nCopies(int n, E value)

It is from class java.util.Collections.

My question is why there is:

<E> List<E>

and not only

List<E>

Obviously I am missing something, can someone clarify this for me?

Answer 1

You use the <E> to typify the method you are defining.

The most common example of generics is to have a typified class like this:

public class SomeClass<E> {
    ...
}

Then, when you are creating a new object of that class you define the type directly like this:

new SomeClass<String>();

That way any method in that class that refers to <E>, will treat <E> as a String, for that instance.

Now consider a static method (which is not bound to any particular instance of a class), in order to typify that method you have use another kind of typification which applies to methods, like this:

static <E> List<E> nCopies(int n, E value)

You use the <E> before the return type to say "this particular method will consider some E when it executes". What <E> will be is decided when you invoke the method:

nCopies(3, "a");

In this example <E> will be a String, so the return type will be a List<String>.

Finally, you can even mix them both:

public class SomeClass<E> {
    public <F> void doSomething(E e, F f) {
        ...
    }
}

In this case, if you have an instance of SomeClass, the E in the doSomething method will always be String (for that instance), but the F can be anything you want it to be.