Singleton pattern

Singleton pattern

A pattern that guarantees that there is only one instance.

The role of Singleton

The Singleton role has a static method to get a single instance. This method always returns the same instance.

package singleton;

public class Singleton {
	private static Singleton singleton = new Singleton();

    //Prohibit calling the constructor from outside the class by making the constructor private
	private Singleton() {
		System.out.println("Created an instance");
	}

    //Provide a static method that returns an instance
	public static Singleton getInstance() {
		return singleton;
	}
}

Caller

package singleton;

public class Main {

	public static void main(String[] args) {
		System.out.println("Start");
		Singleton obj1 = Singleton.getInstance();
		Singleton obj2 = Singleton.getInstance();
		if (obj1 == obj2) {
			System.out.println("Same instance");
		} else {
			System.out.println("Not the same instance");
		}
		System.out.println("End");
	}
}

// Start
//Created an instance
//Same instance
// End

sample

package singleton;

public class TicketMaker {
	private static TicketMaker ticketMaker = new TicketMaker();
	private int ticket = 1000;
	private TicketMaker() {
	}

	public static TicketMaker getInstance() {
		return ticketMaker;
	}

	//Grant syncronized to work properly even when called from multiple threads
	public synchronized int getNextTiketNumber() {
		return ticket++;
	}
}

A class with a limited number of instances

package singleton;

public class Triple {
	private static Triple[] triples = new Triple[] {
			new Triple(0),
			new Triple(1),
			new Triple(2),
	};

	private int id;

	private Triple(int id) {
		System.out.println("The instance" + id + " is created");
		this.id = id;
	}

	public static Triple getInstance(int id) {
		return triples[id];
	}

	public String toString() {
		return "[Triple id = " + id + "]";
	}

}

package singleton;

public class Main {

	public static void main(String[] args) {
		System.out.println("Start");
		for (int i = 0; i < 9; i++) {
			Triple triple = Triple.getInstance(i % 3);
			System.out.println(i + ":" + triple);

		}
		System.out.println("End");

	}
}
// Start
// The instance0 is created
// The instance1 is created
// The instance2 is created
// 0:[Triple id = 0]
// 1:[Triple id = 1]
// 2:[Triple id = 2]
// 3:[Triple id = 0]
// 4:[Triple id = 1]
// 5:[Triple id = 2]
// 6:[Triple id = 0]
// 7:[Triple id = 1]
// 8:[Triple id = 2]
// End