Question

What is the difference between this method declaration:

public static <E extends Number> List<E> process(List<E> nums){

and

 public static List<Number> process(List<Number> nums){

Where would you use the former?

Answer 1

The first allows process of a List<Integer>, a List<Double>, 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<Double> is NOT a subtype of List<Number>. A List<Double>, however, is a List<? extends Number>.

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