Singleton

Singleton Design Pattern:

• Creational Design Pattern
• Only one instance of the class should exist
• Other classes should be able to get Instance of Singleton class.
• Used in Logging, Cache, Session and Drivers.

Implementation:

• Constructor should be private
• Public method for returning instance
• Instance type - private static

Initialization Type:

• Eager Initialization
• Lazy Initialization
• Thread safe Method Initialization
• Thread safe block Initialization

1.Eager Initialization:

public class Singleton{

 private static Singleton singleton = new Singleton();

private Singleton() {}

public static Singleton getInstance(){

    return  singleton;

}

  public static void main(String[] args) {
    Singleton instance = Singleton.getInstance();
    Singleton instance1 = Singleton.getInstance();
    System.out.println(instance);
    System.out.println(instance1);
}}

2.Lazy Initialization:

public class Singleton{
 private static Singleton singleton;

private Singleton() {
}
public static Singleton getInstance(){
    if(singleton==null){
        singleton=new Singleton();
    }
    return  singleton;
}

  public static void main(String[] args) {
    Singleton instance = Singleton.getInstance();
    Singleton instance1 = Singleton.getInstance();
    System.out.println(instance);
    System.out.println(instance1);
}}

3.Thread Safe Method Initialization

public class Singleton{
 private static Singleton singleton;

private Singleton() {
}
public static synchronized Singleton getInstance(){
    if(singleton==null){
        singleton=new Singleton();
    }
    return  singleton;
}

   public static void main(String[] args) {
    Singleton instance = Singleton.getInstance();
    Singleton instance1 = Singleton.getInstance();
    System.out.println(instance);
    System.out.println(instance1);
}}

4.Thread Safe Block Initialization:

public class Singleton{
 private static Singleton singleton;

private Singleton() {
}
public static Singleton getInstance(){
    if(singleton==null){
       synchronized (Singleton.class) {
    if(singleton == null) {
      singleton = new Singleton();
    }
  }
    }
    return  singleton;
}

   public static void main(String[] args) {
    Singleton instance = Singleton.getInstance();
    Singleton instance1 = Singleton.getInstance();
    System.out.println(instance);
    System.out.println(instance1);
}}