wildcards in generic method of an interface

[HK]

Consider the following interface:


import java.util.List;
public interface Action<CDTA> {
void invoke(List<? extends CDTA> r);
}

Now I try to implement it like this:

private static class Xaction implements Action<Number> {
public void invoke(List<Number> l) {
// not yet implemented
}
}

The compiler, however, complains that Xaction does not
implement Action. I can only guess that type-safety
would break if the implementation was allowed. But how?

Can someone give an example that goes wrong if the
above is allowed?

Harald.

 

[Stefan Schulz]

Consider the following interface:


import java.util.List;
public interface Action<CDTA> {
void invoke(List<? extends CDTA> r);
}

Now I try to implement it like this:

private static class Xaction implements Action<Number> {
public void invoke(List<Number> l) {
// not yet implemented
}
}

The compiler, however, complains that Xaction does not
implement Action. I can only guess that type-safety
would break if the implementation was allowed. But how?

Can someone give an example that goes wrong if the
above is allowed?

Suppose someone calls your invoke method with a List<Double> Perfectly
fine with the declaration given in your interface. Now, a List<Double> is
not a List<Number>. A List<Number> can contain nothing but Integers, for
example. You can add Integers into a List<Number>, but not into a
List<Double>. Therefore, you must not "downcast" in contents types.