Can unchecked warnings be avoided when overriding a method with raw type parameters?

Question

I am extending a class defined in a library which I cannot change:

public class Parent
{
    public void init(Map properties) { ... }
}

If I am defining a class 'Child' that extends Parent and I am using Java 6 with generics, what is the best way to override the init method without getting unchecked warnings?

public class Child extends Parent
{
    // warning: Map is a raw type. References to generic type Map<K,V> should be parameterized
    public void init(Map properties) { }
}

If I add generic parameters, I get:

   // error: The method init(Map<Object,Object>) of type Child has the same erasure as init(Map) of type Parent but does not override it
   public void init(Map<Object,Object>) { ... }
   // same error
   public void init(Map<? extends Object,? extends Object>) { ... }
   // same error
   public void init(Map<?,?>) { ... }

This error occurs regardless of whether I use a specific type, a bounded wildcard, or an unbounded wildcard. Is there a correct or idiomatic way to override a non-generic method without warnings, and without using @SuppressWarnings("unchecked")?

Answer 1

Yes, you have to declare the overriding method with the same signature as in the parent class, without adding any generics info.

I think your best bet is to add the @SuppressWarnings("unchecked") annotation to the raw-type parameter, not the method, so you won't squelch other generics warnings you might have in your own code.

Answer 2

Short answer: no way to do that.

Unsatisfying answer: disable the (specific) warnings in your IDE/build.xml.

If you cannot change the library, alas, you have to stick with non-generic methods.

The problem is that, despite after type erasure both init() have the same signature, they may in fact be different methods -- or the same(*). Compiler cannot tell should it do override or overload, so it's prohibited.

(*) Suppose the library developer meant init(Map<String,Integer>). Now you are implementing init(Map<String,String>). This is overloading, and two methods should exist in the vtable of Child class.

But what if the library developer meant init(Map<String,String>)? Then it's overriding, and your method should replace original init in Child class, and there would be only one method in the vtable of Child.

P.S. I hate how Generics implemented in Java :-(