Question

Animal

public abstract class Animal {
 String name;

 public Animal(String name) {
  this.name = name;
 }

}

Lion

public class Lion extends Animal {

 public Lion(String name) {
  super(name);
  // TODO Auto-generated constructor stub
 }

 public void roar() {
  System.out.println("Roar");
 }
}

Deer

public class Deer extends Animal {

 public Deer(String name) {
  super(name);
 }

 public void runAway() {
  System.out.println("Running...");
 }

}

TestAnimals

public class TestAnimals {
 public static void main(String[] args) {
  Animal lion = new Lion("Geo");
  Animal deer1 = new Deer("D1");
  Animal deer2 = new Deer("D2");

  List<Animal> li = new ArrayList<Animal>();
  li.add(lion);
  li.add(deer1);
  li.add(deer2);
  for (Animal a : li) {
   if (a instanceof Lion) {
    Lion l = (Lion) a;
    l.roar();
   }
   if (a instanceof Deer) {
    Deer l = (Deer) a;
    l.runAway();
   }

  }
 }
}

Is there a better way to iterate through the list without having to cast ?In the above case it seem’s ok but if you have many extensions of the base class then we’ll need that many if block too.Is there a design pattern or principle to address this problem ?

Answer 1

An elegant way of avoiding instanceof without inventing some new artificial method in the base class (with a non-descriptive name such as performAction or doWhatYouAreSupposedToDo) is to use the visitor pattern. Here is an example:

Animal

import java.util.*;

abstract class Animal {
    String name;

    public Animal(String name) {
        this.name = name;
    }

    public abstract void accept(AnimalVisitor av);  // <-- Open up for visitors.

}

Lion and Deer

class Lion extends Animal {
    public Lion(String name) {
        super(name);
    }
    public void roar() {
        System.out.println("Roar");
    }

    public void accept(AnimalVisitor av) {
        av.visit(this);                            // <-- Accept and call visit.
    }
}


class Deer extends Animal {

    public Deer(String name) {
        super(name);
    }

    public void runAway() {
        System.out.println("Running...");
    }

    public void accept(AnimalVisitor av) {
        av.visit(this);                            // <-- Accept and call visit.
    }

}

Visitor

interface AnimalVisitor {
    void visit(Lion l);
    void visit(Deer d);
}

class ActionVisitor implements AnimalVisitor {

    public void visit(Deer d) {
        d.runAway();
    }

    public void visit(Lion l) {
        l.roar();
    }
}

TestAnimals

public class TestAnimals {
    public static void main(String[] args) {
        Animal lion = new Lion("Geo");
        Animal deer1 = new Deer("D1");
        Animal deer2 = new Deer("D2");

        List<Animal> li = new ArrayList<Animal>();
        li.add(lion);
        li.add(deer1);
        li.add(deer2);
        for (Animal a : li)
            a.accept(new ActionVisitor());         // <-- Accept / visit.
    }
}