Avoid using instanceOf in observer method

Question

I have the following system in Java:

public class Human {
     public void drown(Animal animal) {
          if (animal instanceOf Fish) {
              return;
          } else {
              animal.die();
          }
     }
}

public abstract class LandAnimal extends Animal{...}
public class Tiger extends LandAnimal{...}

public abstract class Fish extends Animal {...}
public class Trout extends Fish {...}

I have thought of adding a method

public abstract boolean drownable() {...}

in class Animal but I don't have access to the code of Animal class. As I know the use of instanceOf is considered bad OOP practice. How do I avoid the use of instanceOf in this case? Thanks.

Answer 1

The drown() method in Human should be (by the way, why do humans want to drown animals?):

 public void drown(Animal animal) {
      animal.drown();
 }

And each Animal will know what to do, for example:

// in class Tiger
public void drown() {
    die();
}

// in class Fish
public void drown() {
    // do nothing, fish can't drown
}

Answer 2

You would declare Animal.drown() and override it in Fish , containing the appropriate 'kill code' :). So you'd just need to call drown() on each animal and each instance will behave according to its type specific method implementation.

public class Human {
     public void drown(Animal animal) {
         animal.drown();
     }
}

Answer 3

Methods can be overloaded based on their arguments. You can have two different methods:

public void drown(Fish f) {
  return;
}

public void drown(LandAnimal a) {
  a.drown();
}

However, it should be noted that this makes determining whether some animal will drown the responsibility of the Human class, which, as other answers show, is arguable.