What is the difference between and ?

Question

What is the difference between this method declaration:

public static  List process(List nums){

a

Answer 1

The first allows process of a List, a List, etc. The second doesn't.

Generics in Java are invariant. They're not covariant like arrays.

That is, in Java, Double[] is a subtype of Number[], but a List is NOT a subtype of List. A List, however, is a List.

There are good reasons for generics being invariant, but that's also why the extends and super type are often necessary for subtyping flexibility.

See also

• Java Tutorials/Generics/Subtyping
  • Explains why generics invariance is a good thing
• More fun with wildcards
  • Explains some uses of super and extends for bounded wildcards
• Java Generics: What is PECS?
  • This discusses the "Producer extends Consumer super" principle
  • Effective Java 2nd Edition, Item 28: Use bounded wildcards to increase API flexibility