Design Patterns - Singleton Pattern

13 Jul 2018

singleton-pattern

Intent

Ensure a class only have one instance, and provide a global point of access to it.

Several different client objects need to refer to the same thing, and you want to ensure that you do not hav emore than one of them

Clients create an instance of the Singleton solely through the getInstance static method

Add a private static method of the class that refers to the desired object (Initially Set to null)
Add a public static method that instantiates the class
Set the constructor’s status to be protected or private so that no one can directly instantiate the class and bypass the static constructor mechanism.
For case of multi-threading ensure synchronized keyword is added to the static method

	package com.art.head_first.chocolateboiler;

	class ChocolateBoiler {

	private static ChocolateBoiler uniqueInstance;

	private boolean empty;
	private boolean boiled;

	private ChocolateBoiler() {
	empty = true;
	boiled = false;
	}

	/*
	Static Singleton Method
	By Adding the synchronized keyword to getInstance(), we force every thread to wait its turn
	before it can enter the method. That is no two threads may enter the method at the same time.
	*/
	static synchronized ChocolateBoiler getInstance(){
	if (uniqueInstance == null){
	uniqueInstance = new ChocolateBoiler();
	}
	return uniqueInstance;
	}

	void fill(){
	if (empty){
	empty = false;
	boiled = false;
	System.out.println("Filling News Chocolates");
	}
	}

	void boil(){
	if (!empty && !boiled){
	boiled = true;
	System.out.println("Boiling Chocolates");
	}
	}

	void drain(){
	if (!empty && boiled){
	empty = true;
	System.out.println("Draining Chocolates");
	}
	}
	}

	package com.art.head_first.chocolateboiler;


	import org.junit.Test;

	public class ChocolateBoilerTest {

	@Test
	public void testChocolateBoiler(){
	ChocolateBoiler chocolateBoiler = ChocolateBoiler.getInstance();

	chocolateBoiler.fill();
	chocolateBoiler.boil();
	chocolateBoiler.drain();

	}
	}

The Singleton Pattern ensures you have at most one instance of a class in your application.
The Singleton Pattern also provides a global access point to that instance
Singleton Pattern is a convention for ensuring one and only one object is instantiated for a given class.
Make Private Constructor and expose class creation via static method.