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Java Singleton – enum implementation

Singleton as enum is very simple to implement and very safe at the same time thanks to language guarantees for enum instantiation and serialization.

Properties of enum Singleton

  • works in all versions since Java 5
  • easy to implement
  • is serializable out of the box

Complete example

In this example we’ll create enum Singleton with dummy constructor – just to show when it is initialized. Java language specification guarantees that it will be initialized only once, as we want: 

package com.farenda.patterns.singleton;

public enum SingletonEnum {

    INSTANCE;

    SingletonEnum() {
        System.out.println("Singleton enum initialization!");
    }

    int sum(int x, int y) {
        System.out.printf("Called with x = %d, y = %d%n", x, y);
        return x+y;
    }

    public static void main(String[] args) {
        System.out.println("Is singleton: "
            + (SingletonEnum.INSTANCE == INSTANCE));
        System.out.println("Sum: " + INSTANCE.sum(3, 4));
    }
}

The above code produces the following output: 

Singleton enum initialization!
Is singleton: true
Called with x = 3, y = 4
Sum: 7

References:

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