Thoughts on Null Objects

The null object doesn't have to be used to represent an "invalid" object, it can represent the "lack of any real" object in a place where it's reasonable for that situation to exist. I'm a huge fan of it for this use, largely because I'm a Java programmer and I don't particularly like dealing with exceptions, especially runtime ones (except as a debugging tool).

One pattern I have is, when a base class would normally be abstract, to instead use instances of an unspecified base class as null objects. Then subclasses override mostly blank methods rather than implementing over a "virtual" layer.

E.g.

class Bar {
  protected static int[] NO_DATA = new int[0];

  protected int[] mydata;
  public void wibble() {}
  public boolean isTangible() {return false;}
  public final int getData() {return mydata;}
  // I know that last one isn't a terribly good thing to have in OO code, but it's useful for the example

  public final String toString() { return super.toString() + " (" + (isTangible ? "" : "in") + "tangible)"; } // will use the subclass' isTangible() - template/hook methods, whee

}

class FooBar extends Bar {
  public void wibble() { // do something!

  }
  public boolean isTangible() { return true; } // probably true in most subclasses... ?

}

class BarUser {
  private Bar mybar;
  public BarUser(Bar mybar) { this.mybar = mybar; }
  public void wibble() {
    mybar.wibble(); // can't throw NullPointerException if we're diligent about using the NullObject pattern.

    // If there's no real Bar in place, just returns silently, and probably with less overhead than a try-catch for NPE

    doSomeBarUserLevelWibbling();
  }
  public String toString() { return super.toString + " which uses " + mybar; }
}
 

[edit: For large blocks of code, use source instead of code tags]

[edited by - Magmai Kai Holmlor on January 12, 2004 11:02:27 AM]

Whenever you have a strategy pattern, you''ll probably want such ''do nothing'' objects. They''re not really ''null'' object though, maybe you should called them place-holders?