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The Java Enum: A Singleton Pattern


The singleton pattern restricts the instantiation of a class to one object. In Java, to enforce this, the best approach is to use an enum. This great idea comes straight from the book Effective Java by Joshua Bloch. If you don't have it in your library, get it. One of the best Java books to date.

There are a few reasons why one would use an enum as a singleton in Java:

  1. Serialization for free.
  2. Guaranteed one instance (Cannot instantiate more then one enum even through reflection.)
  3. Thread safe

/**
 * Example of a Java Singleton.
 * It is suggested to use an enum as a singleton. The Class
 * cannot be instantiated more then once, specifically when
 * using reflection.
 * 
 * @author keaplogik
 */
public enum AnimalHelperSingleton {

    INSTANCE;

    private AnimalHelperSingleton(){

    }

    public Animal[] buildAnimalList(){
        final Animal[] animals = new Animal[10];

        animals[0] = new SimpleAnimal(Animal.AnimalClass.MAMMAL, 
                "Dog", true, Color.GRAY);
        animals[1] = new SimpleAnimal(Animal.AnimalClass.MAMMAL, 
                "Cat", true, Color.YELLOW);
        animals[2] = new SimpleAnimal(Animal.AnimalClass.AMPHIBIAN,
                "Frog", true, Color.GREEN);
        animals[3] = new SimpleAnimal(Animal.AnimalClass.BIRD,
                "Crow", true, Color.BLACK);
        animals[4] = new SimpleAnimal(Animal.AnimalClass.BIRD,
                "Cardinal", true, Color.RED);
        animals[5] = new SimpleAnimal(Animal.AnimalClass.ARTHROPOD,
                "Mantis", false, Color.GREEN);
        animals[6] = new SimpleAnimal(Animal.AnimalClass.ARTHROPOD,
                "Spider", false, Color.ORANGE);
        animals[7] = new SimpleAnimal(Animal.AnimalClass.MAMMAL, 
                "Tiger", true, Color.ORANGE);
        animals[8] = new SimpleAnimal(Animal.AnimalClass.MAMMAL, 
                "Bear", true, Color.BLACK);
        animals[9] = new SimpleAnimal(Animal.AnimalClass.BIRD, 
                "Owl", true, Color.BLACK);

        return animals;
    }

}
And in use:
//Call singleton to build the animal list.
Animal[] animals = AnimalHelperSingleton.INSTANCE.buildAnimalList();
Singletons have their use. Don't over use them, but when you need one, an enum makes the best approach. You can find additional code related to this post on github in my Java Design Concepts repo.
This post also relates to the Strategy Pattern blog post.