Java – incompatible types and fresh type variables

I get the following compilation message:

[javac]   ... error: incompatible types
[javac]         exceptionClassHolder = new Holder<>( (new Exception()).getClass() );
[javac]                                ^
[javac]   required: Holder<Class<? extends Exception>>
[javac]   found:    Holder<Class<CAP#1>>
[javac]   where CAP#1 is a fresh type-variable:
[javac]     CAP#1 extends Exception from capture of ? extends Exception
[javac] 1 error

In my opinion, according to the information, everything should be right Cap #1 does extend exceptions So how should the above news be understood? Sscce is as follows (not released initially, because I want to know the error message itself in general):

class Holder<T> {
    public T t;
    public Holder(T t) {
       this.t = t;
    }
}

public class FooMain {
    public static void main(String args[]) throws Exception {
        Holder<Class<? extends Exception>> exceptionClassHolder;
        exceptionClassHolder = new Holder<>( (new Exception()).getClass() );
    }
}

Solution

Unfortunately, the existing answers do not explain what happened here First, the solution is to simply specify the type parameter of the holder:

Holder<Class<? extends Exception>> exceptionClassHolder;
exceptionClassHolder =
        new Holder<Class<? extends Exception>>(new Exception().getClass());

Your version doesn't work because of the new exception () Getclass() returns a class , where? Is a wildcard capture (called cap#1 in the compiler error message) Since you use the "Diamond operator" of the new "holder", the compiler infers that the class < cap#1 extends exception > for T and so on, and the holder < class < cap#1 extends exception > > is the type of object created

However, this is different from the holder type you declared > It uses a nested wildcard, which does not catch: while the exceptions of cap#1 extension are some specific types of extension exceptions, nested? An extension exception represents the type of any extension exception

The < cap#1 extension exception > is a subtype extension exception of class, and the holder < class < cap#1 extension exception > > is not the holder < class > allocation failed because genetics are't covariant

By manually specifying class for T, you can 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