SingletonPatternExample1 created by kenanhancer - https://repl.it/@kenanhancer/SingletonPatternExample1

Main.java

rclass Main {
  public static void main(String[] args) {
    
    SingletonV1 singletonv1 = SingletonV1.getInstance();
    
    singletonv1.doSomething();
    
    
    SingletonV2 singletonV2 = SingletonV2.getInstance();
    
    singletonV2.doSomething();
    
    
    SingletonV3 singletonV3 = SingletonV3.getInstance();
    
    singletonV3.doSomething();
    
    
    SingletonV4 singletonV4 = SingletonV4.getInstance();
    
    singletonV4.doSomething();
    
    
    SingletonV5 singletonV5 = SingletonV5.getInstance();
    
    singletonV5.doSomething();
    
    
    SingletonV6 singletonV6 = SingletonV6.getInstance();
    
    singletonV6.doSomething();
  }
}

SingletonV1.java

public class SingletonV1{
  
  private SingletonV1(){}
  
  private static class SingletonHolder{
    private static final SingletonV1 INSTANCE = new SingletonV1();
  }
  
  public static SingletonV1 getInstance(){
    return SingletonHolder.INSTANCE;
  }
  
  public void doSomething(){
    System.out.println("TEST");
  }
}

SingletonV2.java

public final class SingletonV2{
  
  private static volatile SingletonV2 _instance;
  
  private SingletonV2(){}
  
  public static SingletonV2 getInstance(){
    
    if(_instance == null){
      
      synchronized(SingletonV2.class){
        
        if(_instance == null){
          
          _instance = new SingletonV2();
        }
      }
    }
    
    return _instance;
  }
  
  public void doSomething(){
    System.out.println("TEST");
  }
}

SingletonV3.java

//Classic implementation of Singleton Design Pattern
public class SingletonV3{
  
  private static SingletonV3 _instance;
  
  //private constructor to force use of getInstance() to create a single instance.
  private SingletonV3(){}
  
  public static SingletonV3 getInstance(){
    
    if(_instance == null){
      
      _instance = new SingletonV3();
    }
    
    return _instance;
  }
  
  public void doSomething(){
    System.out.println("TEST");
  }
}

SingletonV4.java

//Thread synchronized implementation of Singleton Design Pattern
public class SingletonV4{
  
  private static SingletonV4 _instance;
  
  private SingletonV4(){}
  
  //Only one thread can execute this at a time.
  public static synchronized SingletonV4 getInstance(){
    
    if(_instance == null){
      _instance = new SingletonV4();
    }
    
    return _instance;
  }
  
  public void doSomething(){
    System.out.println("TEST");
  }
}

SingletonV5.java

//Eager instantiation implementation of Singleton Design Pattern
public class SingletonV5{
  
  private final static SingletonV5 _instance = new SingletonV5();
  
  private SingletonV5(){}
  
  public static SingletonV5 getInstance(){
    
    return _instance;
  }
  
  public void doSomething(){
    System.out.println("TEST");
  }
}

SingletonV6.java

//Double checked locking based implementation of Singleton Desgin Pattern
public final class SingletonV6{
  
  private static SingletonV6 _instance;
  
  private SingletonV6(){}
  
  public static SingletonV6 getInstance(){
    
    if(_instance == null){
      
      synchronized(SingletonV6.class){
        
        if(_instance == null){
          
          _instance = new SingletonV6();
        }
      }
    }
    
    return _instance;
  }
  
  public void doSomething(){
    
    System.out.println("TEST");
  }
  
}