Singleton Desgin Pattern

There are only two points in the definition of a singleton design pattern,

1. there should be only one instance allowed for a class and
2. we should allow global point of access to that single instance.

Sample code : singleton + early Initialization

public class Singleton {
  // Private constructor prevents instantiation from other classes
  private Singleton() {}
 
  /**
   * SingletonHolder is loaded on the first execution of Singleton.getInstance() 
   * or the first access to SingletonHolder.INSTANCE, not before.
   */
  private static class SingletonHolder { 
    private static final Singleton INSTANCE = new Singleton();
  }

  public static Singleton getInstance() {
    return SingletonHolder.INSTANCE;
  }
}

Sample code : singleton + lazy Initialization

public class Singleton {

  private static Singleton singleInstance;

    private Singleton() {}

  public static Singleton getSingleInstance() {

    if (singleInstance == null) {

      synchronized (Singleton.class) {

        if (singleInstance == null) {

          singleInstance = new Singleton();

        }

      }

    }

    return singleInstance;

  }