Wildcards

Summary

Wildcards allow the bounding of the type parameters.

Upper-Bounded Wildcards

Upper-bounded wildcards are covariant.

This means that if , then A<? extends S> A<? extends T>. Similarly, for any type , then A<S> A<? extends S>

Lower-Bounded Wildcards

Lower-bounded wildcards are contravariant.

This means that if , then A<? super T> A<? super S>. Similarly, for any type , then A<S> A<? super S>

Unbounded Wildcards

The complex type A<?> is the supertype of all possible A<T>.

Differences between:

• A<?>: Complex type of some specific but unknown type
• A<Object>: Complex type of Object instances with type checking
• A: Complex type of Object instances without type checking.

PECS

Producer Extends, Consumer Super

• If the variable is a producer (it returns a variable of type ) it should be declared <? extends T>.
• If the variable is a consumer (it accepts a variable of type ) it should be declared <? super T>.