A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

 Keep an optional delegate instance.
 Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:

 Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

class Delegator {
  var delegate;
 
  String operation() {
    if (delegate == null)
      return "default implementation";
    else
      return delegate.thing();
  }
}
 
class Delegate {
  String thing() => "delegate implementation";
}
 
main() {
  // Without a delegate:
  Delegator a = new Delegator();
  Expect.equals("default implementation",a.operation());
 
  // any object doesn't work unless we can check for existing methods
  // a.delegate=new Object();
  // Expect.equals("default implementation",a.operation());
 
  // With a delegate:
  Delegate d = new Delegate();
  a.delegate = d;
  Expect.equals("delegate implementation",a.operation());
}