単例模式

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public class SimpleTest {

   //   饿汉模式

   private SimpleTest(){

   }
   private static SimpleTest instance = new SimpleTest();

   public static SimpleTest getinstance(){

       return instance;
   }

}

public class SimpleTest1 {
//   懒汉模式
       private SimpleTest1(){

       }
       private static SimpleTest1 instance;


       public static SimpleTest1 getInstance(){
           if(instance == null){
               instance = new SimpleTest1();
           }
           return instance;
       }
   }

//线程安全
   private volatile static SimpleTest3 singleton;

   private SimpleTest3(){

   }

   public static SimpleTest3 getInstance() {
       if (singleton== null) {
           synchronized (SimpleTest3.class) {
               if (singleton== null) {
                   singleton= new SimpleTest3();
               }
           }
       }
       return singleton;
   }

public class SimpleTest4 {
//   静态内部类单例模式
   private SimpleTest4(){
   }

   public static SimpleTest4 getInstance(){
       return SingletonHolder.sInstance;
   }

   private static class SingletonHolder {
       private static final SimpleTest4 sInstance = new SimpleTest4();
   }

}

//枚举单例的优点就是简单，但是大部分应用开发很少用枚举，可读性并不是很高，不建议用。
public enum SimpleTest5 {

   INSTANCE;
     public void doSomeThing() {
     }

}

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