Question

Can anyone explain what does the return type mean in the following code

public static <T> ArrayList<T> a()
{       
       return null;
} 

and

public static <String> ArrayList<Vector> a()
{       
       return null;
} 

Answer 1

public static <T> ArrayList<T> a() 

The first occurance of <T> introduces a type parameter which will be available within the method.

The actual return type is ArrayList<T>, where T is same as the one in the first.

You can read about it here – Generic Methods.

In the second one:

public static <String> ArrayList<Vector> a() {

Even though you have introduced a generic type parameter (i.e. String, which is not an actual type or argument like java.lang.String) you are not using it. And, also the method always returns an ArrayList<Vector> (ArrayList of Vectors).