Search code examples
javagenericscastingwarningsunchecked

Unchecked cast warnings with parameterized types

I am reading about restrictions on generics from the Java Tutorials, specifically casting with parameterized types. I understand the examples that are presented. However, I am unsure about the following example:

List<? extends Number> l1 = new ArrayList<Integer>();
// unchecked cast warning
ArrayList<Number> l2 = (ArrayList<Number>) l1;
// no unchecked cast warning
ArrayList<? extends Number> l3 = (ArrayList<? extends Number>) l1;

I understand why there is a warning in the first case. Why is there no warning in the second case? Is it because the only operations that I can perform on l3 are safe (e.g., I cannot add (say) a Float to the list)?

UPDATE: Below is an excerpt from section 5.5.2 of the JLS that addresses similar questions.

A cast from a type S to a parameterized type (§4.5) T is unchecked unless at least one of the following is true:

  • S <: T
  • All of the type arguments (§4.5.1) of T are unbounded wildcards
  • T <: S and S has no subtype X other than T where the type arguments of X are not contained in the type arguments of T.
Solution

  • There's no warning because the cast doesn't change the generic part of the type. List<? extends Number> and ArrayList<? extends Number> have the same type argument. Instead, the cast is a checked cast from List to ArrayList.