Lower bounded wildcard not checked against upper bounded type parameter

Question

I wonder why does this piece of code compile successfully?

Source code:

abstract class A
{
    public abstract  A

        

Answer 1

This compiler behavior was discussed on this Eclipse bug. Originally, the Eclipse compiler did error for the expression in your example, while javac did not. Although I haven't yet searched the JLS directly, the consensus seems to be that there is nothing in the spec requiring lower bounded wildcards to be checked against type parameter bounds. In this situation it's ultimately left to the caller to assign a type that satisfies the constraints (as surmised by Stephan Herrmann on that post).