Java Generic return type method derived to be object return type

Question

I was encountered this wired issue.

class Foo<R, C> {

    public C fooMethod() {
        Bar bar = new Bar();
        Map<String, String> m = new HashMap<>();
        return bar.barMethod(m);
    }
}

class Bar {
    public <T> T barMethod(Map<String, String> m) {
        Object barObj = null;
        ...
        return (T) barObj;
    }
}

If I declare m to be a raw type Map m = new Map() , I get a compile error:

incompatible types: Object cannot be converted to C, where C is a type-variable.

but if m is a typed object as declared above, then the compile error is gone.

I can't figure out why the parameter will make a different for compiler to figure out what the return type is. And to me the compiler should not think the generic type T in the Bar class example to be a Object type under any circumstances. Can some one point out where I'm wrong? Thanks in advance.

Answer 1

Map m = new HashMap<>();
return bar.barMethod(m); //error

The error is thrown because barMethod(Map<String, String> m) specifically requires a Map<String, String> . You can do a couple of things to make the compile error dissaper.

• Change barMethod parameter to a raw type.

   public <T> T barMethod(Map m) {...} 

  OR

• Cast return value to C in the method fooMethod

   return (C) bar.barMethod(m); 

  OR

• Change the return type of the method fooMethod to Object

   public Object fooMethod() 

Answer 2

I can't find any source in the spec but basically, because you've used the raw type Map in place of the generic Map<String, String> , Java converts all generic parameters (not just the Map ) to the type-erased Object . The method signature effectively becomes:

public Object barMethod(Map<Object, Object> m)

Because barMethod returns an Object , and Object is not necessarily a C (as returned by fooMethod ), the types are incompatible. That's why this results in a compiler error.