Singleton Pattern - Multiple Implementations by Allen Holub (holub.com/patterns)

SingeltonGoF.java

class Singelton {
  static Singleton instance = null;
  
  //private constructor
  private Singelton() {}
  
  public static Singleton getInstance() {
    if(instance == null) 
       instance = new Singleton();
    return instance;
  }
}

SingeltonWithWrapperBEST.java

class Singelton {
  
  private class InstanceContainer {
    private static final Singleton theInstance = new Singelton();
  }
  
  // you are free to define and access static variables without issue since the instance will only be defined in the inner class.
  static int MAX = 10;
  
  public static Singleton getInstance() {
    return InstanceContainer.theInstance;
  }
}

Singleton Pattern - Intro

Singleton Pattern should be:
* globally accessible
* there should only be one and only instance of that class

-> Can also be a class where all methods are static (like Utitlity class)

SingletonForMultiThreading

//Using Java's Synchronized keyword for handling issue in MultiThreading
class Singelton {
  private static Singleton instance = null;
  //above line can also be defined as
  //private static Singleton instance; // null by default
  
  //private constructor
  private Singelton() {}
  
  public synchronized static Singleton getInstance() {
    if(instance == null) 
       instance = new Singleton();
    return instance;
  }
}

SingletonUsingLock.java

//Using a custom Lock class for handling issue in MultiThreading
class Singelton {
  
  Lock busy;
  
  static Singleton instance = null;
  
  //private constructor
  private Singelton() {}
  
  public static Singleton getInstance() {
    busy.lock();
    if(instance == null) 
       instance = new Singleton();
    return instance;
    busy.unlock();
  }
}

SingletonWithoutIfCheck

//Initialize first to avoid issues in the if statement
class Singelton {
  private static Singleton instance = new Singelton();
  
  //private constructor
  private Singelton() {}
  
  public synchronized static Singleton getInstance() {
    return instance;
  }
}