面试单例回顾

之前面试了一家公司，主要在围绕单例展开的一个case：
[list=1]Q1： 写一个单例：

  public class Singleton{
      private static final Singleton instance = new Singleton();
      
      private Singleton(){
      }
      
      public static Singleton getInstance(){
            return instance;
      }

  }

[list=2]Q2:  写一个lazy load的单例：
   

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

  }

[list=3]Q3:  写一个线程安全的lazy load的单例：
   

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

  }

[list=4]Q4: 写一个轻量级的线程安全的lazyload的单例：
  

   public class Singleton{
      private static Singleton instance;
      private static AutomaticInteger count = new AutomaticInteger(0);
      private Singleton(){
      }
      
      public static Singleton getInstance(){
  
         if(count ==0 && instance == null){
             synchronized(Singleton.class){
                instance = new Singleton();
                count.increment();
             }
             
         }   
         return instance;
      }

  }
  

[/list]