Avoiding instanceof in Java

You might be interested in this entry from Steve Yegge's Amazon blog: "when polymorphism fails". Essentially he's addressing cases like this, when polymorphism causes more trouble than it solves.

The issue is that to use polymorphism you have to make the logic of "handle" part of each 'switching' class - i.e. Integer etc. in this case. Clearly this is not practical. Sometimes it isn't even logically the right place to put the code. He recommends the 'instanceof' approach as being the lesser of several evils.

As with all cases where you are forced to write smelly code, keep it buttoned up in one method (or at most one class) so that the smell doesn't leak out.

As highlighted in the comments, the visitor pattern would be a good choice. But without direct control over the target/acceptor/visitee you can't implement that pattern. Here's one way the visitor pattern could possibly still be used here even though you have no direct control over the subclasses by using wrappers (taking Integer as an example):

public class IntegerWrapper {
    private Integer integer;
    public IntegerWrapper(Integer anInteger){
        integer = anInteger;
    }
    //Access the integer directly such as
    public Integer getInteger() { return integer; }
    //or method passthrough...
    public int intValue() { return integer.intValue(); }
    //then implement your visitor:
    public void accept(NumericVisitor visitor) {
        visitor.visit(this);
    }
}

Of course, wrapping a final class might be considered a smell of its own but maybe it's a good fit with your subclasses. Personally, I don't think instanceof is that bad a smell here, especially if it is confined to one method and I would happily use it (probably over my own suggestion above). As you say, its quite readable, typesafe and maintainable. As always, keep it simple.

Instead of a huge if, you can put the instances you handle in a map (key: class, value: handler).

If the lookup by key returns null, call a special handler method which tries to find a matching handler (for example by calling isInstance() on every key in the map).

When a handler is found, register it under the new key.

This makes the general case fast and simple and allows you to handle inheritance.

You can use reflection:

public final class Handler {
  public static void handle(Object o) {
    try {
      Method handler = Handler.class.getMethod("handle", o.getClass());
      handler.invoke(null, o);
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }
  public static void handle(Integer num) { /* ... */ }
  public static void handle(BigDecimal num) { /* ... */ }
  // to handle new types, just add more handle methods...
}

You can expand on the idea to generically handle subclasses and classes that implement certain interfaces.