CODEWITHSUNDEEP
javainstanceof
Luke Taylor
Luke Taylor

Reputation: 9599

Java: instanceof or custom getType

Assuming I have the following code:

public boolean doesElfLikeIt ( Monster mon )
 {
    if ( mon instanceof Orc ) { return false; }
    if ( mon instanceof Elf ) { return true; }

 }

Is this a good programming approach or should I rather go for something like this:

public boolean doesElfLikeIt ( Monster mon )
 {
    if ( mon.getType() == Orc.type ) { return false; }
    if ( mon.getType() == Elf.type ) { return true; }

 }

The reason why I'm asking this is because I hear a lot about how evil the instanceof comparison is, however I find it useful.

Upvotes: 2

Views: 2596

Answers (2)

Adam Burch
Adam Burch

Reputation: 53

Reverend Gonzo's proposed solution violates encapsolation and doesn't answer the question. Encapsulation is violated, as ALL Monsters must now know if they like Elves or not, coupling all Monsters to a specific sub-type (Elves) in a very indirect and deep-seated way. It doesn't answer the question, because it's entirely possible for Orcs to like Elves, but Elves to not like Orcs!

I feel your initial solution using instanceof is entirely fine. While SLaks brings up a good point, I would argue that racist Elves (Elves not liking Orcs, and ALL Orc relatives sound pretty racist ;) ) are an entirely legitimate design decision, and not an indication of programmer error.

To take a step back and address "How do I have Elves that like some Orcs?", I think the best answer would be "Model WHY they like and dislike Monsters in general, and Orcs in specific". As long as you're keying off a single data point (type), you're always going to have limited behavior.

Upvotes: 2

Reverend Gonzo
Reverend Gonzo

Reputation: 40851

Neither. What you really should be doing is something like:

class Monster {
  public abstract boolean likesElves();
}

class Orc extends Monster {
  public boolean likesElves() {
    return false;
  }
}

class Elf extends Monster {
  public boolean likesElves() {
    return true;
  }
}

Upvotes: 7

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