instanceof keyword usage

Question

Is using the instanceof keyword against the essence of object oriented programming? I mean is it a bad programming practice? I read somewhere that using instanceof keyword means that the design may not be that good. Any better workaround?

Answer 1

Generally speaking yes. It's best to keep all code that depends on being a specific class within that class, and using instanceof generally means that you've put some code outside that class.

Look at this very simple example:

public class Animal
{
}

public class Dog extends Animal
{
}

public class Cat extends Animal
{
}

public class SomeOtherClass
{
  public abstract String speak(Animal a)
  {
    String word = "";

    if (a instanceof Dog)
    {
      word = "woof";
    }
    else if (a instanceof Cat)
    {
      word = "miaow";
    }

    return word;
  }
}

Ideally, we'd like all of the behaviour that's specific to dogs to be contained in the Dog class, rather than spread around our program. We can change that by rewriting our program like this:

public abstract class Animal
{
  public String speak();
}

public class Dog extends Animal
{
  public String speak()
  {
    return "woof";
  }
}

public class Cat extends Animal
{
  public String speak()
  {
    return "miaow";
  }
}

public class SomeOtherClass
{
  public String speak(Animal a)
  {
    return a.speak();
  }
}

We've specified that an Animal has to have a speak method. Now SomeOtherClass doesn't need to know the particular details of each type of animal - it can hand that off to the subclass of Animal.

Answer 2

There are many good answers promoting virtual methods, but instanceof has its uses as well. Imagine that you iterate over List<Event>, to pick up all Urgent objects. You might do it using isUrgent() but I am not sure if it were necessarily more concise or readable. Also, isUrgent() would require making Event aware that its subclasses may possess the respective property, which might:

be regarded as something against modularity principles;

be even impossible, if Event belongs to some library that can not be modified.