another java generic question

Question

I have the following class:

interface Able{/* ... */}
class A implements Able{/* ... */}

and I have

Map

        

Answer 1

From http://download.oracle.com/javase/tutorial/extra/generics/wildcards.html:

There is, as usual, a price to be paid for the flexibility of using wildcards. That price is that it is now illegal to write into [a wildcard-based container]. For instance, this is not allowed:

public void addRectangle(List shapes) {
    shapes.add(0, new Rectangle()); // Compile-time error!
}

You should be able to figure out why the code above is disallowed. The type of the second parameter to shapes.add() is ? extends Shape-- an unknown subtype of Shape. Since we don't know what type it is, we don't know if it is a supertype of Rectangle; it might or might not be such a supertype, so it isn't safe to pass a Rectangle there.