Multiple methods via reflection when implementing a method with generic return type

Question

When implementing a method with a generic return type and reflecting the declared methods on the implementing class, two methods are returned by the reflection API. Ex:

interface Referenceable<T> {
    T getReference();
}

class UUIDRef implements Referenceable<UUID> {
    public UUID getReference() {
      System.out.println("You called");
      return null;
    }
}

@Test
public void test() throws Exception {
    UUIDRef ref = new UUIDRef();
    for (Method m : UUIDRef.class.getDeclaredMethods()) {
      System.out.println(String.format("%s %s", m.getReturnType(), m.getName()));
      m.invoke(ref, null);
    }
}

This outputs:

class java.util.UUID getReference You called class java.lang.Object getReference You called

Why am I getting two methods as having been declared on UUIDRef? Is there any way to accurately learn which of the two is the most refined, having actually been declared on UUIDRef?

Have you tried adding the @Override annotation to the implemented method? — ra2085, Jun 11 '14 at 17:09
I'm actually compiling against 1.5 where an @Override isn't allowed here. — Jonathan, Jun 11 '14 at 17:11

score 2 · Accepted Answer · edited May 23 '17 at 10:26

2

To support the covariant return type, a bridge method must be created to be able to call the method in code that expects the declared return type.

In short, this bridge method is the method that would be called in the following scenario, where T is erased to Object:

public <T> T doSomething(Referenceable<T> obj) {
   return obj.getReference();
}

Use m.isBridge() to tell which is the bridge method.

edited May 23 '17 at 10:26

Community

1
1

answered Jun 11 '14 at 17:11

Mark Peters

80,126
17
159
190

1

Here is the reference: http://docs.oracle.com/javase/tutorial/java/generics/bridgeMethods.html – Ismail Badawi Jun 11 '14 at 17:11
That works. Thanks! (Waiting for timer to accept answer) – Jonathan Jun 11 '14 at 17:12

Multiple methods via reflection when implementing a method with generic return type

1 Answers1