java - Why calling method with generic return on a generic class is considered unsafe by javac?

Question

Consider the following code:

public class Main {
    public static class NormalClass {
        public Class<Integer> method() {
            return Integer.class;
        }
    }

    public static class GenericClass<T> {
        public Class<Integer> method() {
            return Integer.class;
        }
    }

    public static void main(String... args) {
        NormalClass safeInstance = new NormalClass();
        Class<Integer> safeValue = safeInstance.method();

        GenericClass unsafeInstance = new GenericClass();
        Class<Integer> unsafeValue = unsafeInstance.method();
    }
}

If I compile it with:

$ javac -Xlint:unchecked Main.java 

It returns:

Main.java:16: warning: [unchecked] unchecked conversion
        Class<Integer> unsafeValue = unsafeInstance.method();
                                                          ^
  required: Class<Integer>
  found:    Class
1 warning

Please note that only the generic method is considered unsafe, even if no generic type is referenced on the return type.

Is this a javac bug? Or there is a deeper reason for this I'm not taking into account?

Answer 1

Raw types were allowed to ensure compatibility with code written before generics were introduced. Raw types work by simply ignoring all type information from all method arguments and return types, even type information that is not related to the type parameter of the class. This can lead to strange results, as you have found. But it gets even stranger than this. For example, this compiles.

public class Main {

    public static class GenericClass<T> {
        public void foo(Class<Integer> clazz) {
        }
    }

    public static void main(String... args) {
        GenericClass unsafeInstance = new GenericClass();
        unsafeInstance.foo(String.class);
    }
}