UnimplementedException: bad idea or not?

[samizdat@xxxxxxxxxxx (Chris Kelly)]

Is it a bad idea to throw an exception when a feature is not implemented,
but the caller of the method can't tell beforehand whether that method is
implemented?

For the sake of argument, assume you have:

  class UnimplementedException extends RuntimeException {
    ...
  }

  interface Interface {
    int getA();
    int getB();
  }

You want to use it like this:

  Interface i = getAnObjectSomePlace();
  System.out.println( "A=" + i.getA() + ", B=" + i.getB() );

And, you have two classes that implement Interface:

  class One {
    int getA() { return 1; }
    int getB() { return 1; }
  }

  class Two {
    int getA() { return 2; }
    int getB() { throw new UnimplementedException(); }
  }

Is there a better way to do this, such as by returning a numeric code
instead of throwing an exception? You don't want to bail just because
something is unimplemented, you just want to ignore it.

Because, the code above could become:

  Interface i = getAnObjectSomePlace();
  int  A,B;
  try {
    A = i.getA();
  }
  catch ( UnimplementedException e ) {
    A = -1;
  }

  try {
    B = i.getB();
  }
  catch ( UnimplementedException e ) {
    B = -1;
  }

  System.out.println( "A=" + A + ", B=" + B );



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