Type safety: Unchecked cast from Object

Question

I try to cast an object to my Action class, but it results in a warning:

Type safety: Unchecked cast from Object to Action<ClientInterface>  Action<ClientInterface> action = null; try {  Object o = c.newInstance();  if (o instanceof Action<?>) {   action = (Action<ClientInterface>) o;  } else {   // TODO 2 Auto-generated catch block   throw new InstantiationException();  }  [...] 

Thank you for any help

Answer 1

Yes - this is a natural consequence of type erasure. If o is actually an instance of Action<String> that won't be caught by the cast - you'll only see the problem when you try to use it, passing in a ClientInterface instead of a string.

You can get rid of the warning using:

@SuppressWarnings("unchecked") 

as a function annotation, but you can't easily sort out the underlying problem :(

Answer 2

As usual, Jon Skeet is right.

To elaborate on the not-easily part of his answer:

Given

class ClientAction implements Action<ClientInterface> {} 

You can write:

Class<? extends Action<ClientInterface>> c = ClientAction.class; Action<ClientInterface> action = c.newInstance(); 

This eliminates both the cast and the warning, at the price of introducing a non-generic type so you can use .class to get a sufficiently accurately typed Class object.