incompatible types and fresh type-variable

Question

I get the following compilation message:

[javac]   ... error: incompatible types
[javac]         exceptionClassHolder = new Holder<>( (new Exception())         


        

Answer 1

Unfortunately, the existing answers don't explain what's going on here. First, the solution is to simply specify the type argument to Holder:

Holder> exceptionClassHolder;
exceptionClassHolder =
        new Holder>(new Exception().getClass());

The reason your version didn't work is because new Exception().getClass() returns a Class, where ? is a wildcard capture (referred to in the compiler error message as CAP#1). Since you use the "diamond operator" with new Holder<>, the compiler infers Class for T and so Holder> is the type of the created object.

However, this doesn't match your declared type of Holder>. It uses a nested wildcard, which doesn't capture: while CAP#1 extends Exception is some specific type extending Exception, the nested ? extends Exception represents literally any type extending Exception.

And while Class is a subtype of Class, Holder> is not a subtype of Holder> because generics aren't covariant, so the assignment fails.

By manually specifying Class for T, you help the compiler avoid this "trap".

See my similar answers on these posts:

• Java: Wildcard Types Mismatch Results in Compilation Error
• Bounded-wildcard related compiler error