Type mismatch for Class Generics

Question

I have the following code that won't compile and although there is a way to make it compile I want to understand why it isn't compiling. Can someone enlighten me as to specifically why I get the error message I will post at the end please?

public class Test {
    public static void main(String args[]) {
     Test t = new Test();
     t.testT(null);
    }

    public <T extends Test> void testT(Class<T> type) {
     Class<T> testType = type == null ? Test.class : type; //Error here
     System.out.println(testType);
    }
}

Type mismatch: cannot convert from Class<capture#1-of ? extends Test> to Class<T>

By casting Test.class to Class<T> this compiles with an Unchecked cast warning and runs perfectly.

Answer 1

Remove the conditional and the error is a little nicer...

public class Test {
    public static void main(String args[]) {
        Test t = new Test();
        t.testT(null);
    }

    public <T extends Test> void testT(Class<T> type) {
    Class<T> testClass = Test.class;
        System.out.println(testClass);
    }
}


Test.java:10: incompatible types
found   : java.lang.Class<Test>
required: java.lang.Class<T>
        Class<T> testClass = Test.class;

Answer 2

The reason is that Test.class is of the type Class<Test>. You cannot assign a reference of type Class<Test> to a variable of type Class<T> as they are not the same thing. This, however, works:

Class<? extends Test> testType = type == null ? Test.class : type;

The wildcard allows both Class<T> and Class<Test> references to be assigned to testType.

Answer 3

Suppose I extend Test:

public class SubTest extends Test {
  public static void main(String args[]) {
    Test t = new Test();
    t.testT(new SubTest());
  }
}

Now, when I invoked testT, the type parameter <T> is SubTest, which means the variable testType is a Class<SubTest>. Test.class is of type Class<Test>, which is not assignable to a variable of type Class<SubTest>.

Declaring the variable testType as a Class<? extends Test> is the right solution; casting to Class<T> is hiding a real problem.