Puzzle 47: Well, Dog My Cats!

This program uses a Counter class to keep track of how many times each kind of house pet makes a noise. What does the program print?

class Counter {

    private static int count;

    public static void increment() { count++; }

    public static int getCount()   { return count; }

}



class Dog extends Counter {

    public Dog() { }

    public void woof() { increment(); }

}



class Cat extends Counter {

    public Cat() { }

    public void meow() { increment(); }

}



public class Ruckus {

    public static void main(String[] args) {

        Dog dogs[] = { new Dog(), new Dog() };

        for (int i = 0; i < dogs.length; i++)

            dogs[i].woof();

        Cat cats[] = { new Cat(), new Cat(), new Cat() };

        for (int i = 0; i < cats.length; i++)

            cats[i].meow();

        System.out.print(Dog.getCount() + " woofs and ");

        System.out.println(Cat.getCount() + " meows");

    }

}

Solution 47: Well, Dog My Cats!

We have two dogs woofing and three cats meowing—a ruckus, to be sure—so the program should print 2 woofs and 3 meows, no? No: It prints 5 woofs and 5 meows. Where is all the extra noise coming from, and what can we do to stop it?

The sum of the number of woofs and meows printed by the program is 10, fully twice what it should be. The problem is that Dog and Cat inherit the count field from a common superclass, and count is a static field. A single copy of each static field is shared among its declaring class and all subclasses, so Dog and Cat use the same count field. Each call to woof or meow increments this field, so it is incremented five times. The program reads it twice, by calling Dog.getCount and Cat.getCount. In each case, 5 is returned and printed.

We cannot fix the problem by making count an instance field. That would create one counter per pet rather than one counter per kind of pet. To fix the program, we must correct a fundamental design error.

When designing one class to build on the behavior of another, you have two options: inheritance, in which one class extends the other; or composition, in which one class contains an instance of the other. Choose based on whether each instance of one class is an instance of the other class or has an instance of the other. In the first case, use inheritance; in the second, use composition. When in doubt, favor composition over inheritance [EJ Item 14].

Neither a dog nor a cat is a kind of counter, so it was wrong to use inheritance. Instead of extending Counter, Dog and Cat should each have a counter field. One counter is required for each kind of pet, rather than for each individual pet, so these fields must be static. We needn't bother with a Counter class; an int will do fine. Here is the redesigned program, which prints 2 woofs, 3 meows as expected:

class Dog {

    private static int woofCounter;

    public Dog() { }

    public static int woofCount() { return woofCounter; }

    public void woof() { woofCounter++; }

}



class Cat {

    private static int meowCounter;

    public Cat() { }

    public static int meowCount() { return meowCounter; }

    public void meow() { meowCounter++; }

}

The Ruckus class is unchanged with the exception of the two print statements, which are modified to use the new method names to access the counts:

System.out.print(Dog.woofCount() + " woofs, ");

System.out.println(Cat.meowCount() + " meows");

In summary, static fields are shared by their declaring class and any subclasses. If you need a separate copy of a field for each subclass, you must declare a separate static field in each subclass. If you need a separate copy for each instance, declare a nonstatic field in the base class. Also, favor composition over inheritance unless the derived class really is a kind of the base class.