public class Singleton {
  private static Singleton uniqueInstance = null;
 
  private Singleton() {
    // Exists only to defeat instantiation.
  }
 
  public static Singleton getInstance() {
    if (uniqueInstance == null) {
      uniqueInstance = new Singleton();
    }
    return uniqueInstance;
  }
  // Other methods...
}

public class TestStream {
   private String name;
   public String getName() {
     return name;
   }
   public void setName(String name) {
     this.name = name;
   } 
   //该类只能有一个实例
   private TestStream(){}  //私有无参构造方法
   //该类必须自行创建
   //有2种方式
   /*private static final TestStream ts=new TestStream();*/
   private static TestStream ts1=null;
   //这个类必须自动向整个系统提供这个实例对象
   public static TestStream getTest(){
     if(ts1==null){
       ts1=new TestStream();
     }
    return ts1;
   }
   public void getInfo(){
     System.out.println("output message "+name);
   }
 }

 public class TestMain {
   public static void main(String [] args){
     TestStream s=TestStream.getTest();
     s.setName("张孝祥");
    System.out.println(s.getName());
     TestStream s1=TestStream.getTest();
     s1.setName("张孝祥");
     System.out.println(s1.getName());
     s.getInfo();
     s1.getInfo();
     if(s==s1){
       System.out.println("创建的是同一个实例");
    }else if(s!=s1){
       System.out.println("创建的不是同一个实例");
     }else{
       System.out.println("application error");
     }
   }
 }

//饿汉式单例类.在类初始化时，已经自行实例化 
 public class Singleton1 {
   //私有的默认构造子
   private Singleton1() {}
   //已经自行实例化 
   private static final Singleton1 single = new Singleton1();
   //静态工厂方法 
   public static Singleton1 getInstance() {
     return single;
   }
 }

 //懒汉式单例类.在第一次调用的时候实例化 
 public class Singleton2 {
   //私有的默认构造子
   private Singleton2() {}
   //注意，这里没有final  
   private static Singleton2 single=null;
   //静态工厂方法 
   public synchronized static Singleton2 getInstance() {
     if (single == null) { 
       single = new Singleton2();
     } 
    return single;
   }
 }

 import java.util.HashMap;
 import java.util.Map;
 //登记式单例类.
 //类似Spring里面的方法，将类名注册，下次从里面直接获取。
 public class Singleton3 {
   private static Map<String,Singleton3> map = new HashMap<String,Singleton3>();
   static{
     Singleton3 single = new Singleton3();
     map.put(single.getClass().getName(), single);
   }
   //保护的默认构造子
   protected Singleton3(){}
   //静态工厂方法,返还此类惟一的实例
   public static Singleton3 getInstance(String name) {
     if(name == null) {
       name = Singleton3.class.getName();
       System.out.println("name == null"+"--->name="+name);
     }
     if(map.get(name) == null) {
       try {
         map.put(name, (Singleton3) Class.forName(name).newInstance());
       } catch (InstantiationException e) {
         e.printStackTrace();
       } catch (IllegalAccessException e) {
         e.printStackTrace();
       } catch (ClassNotFoundException e) {
         e.printStackTrace();
       }
     }
     return map.get(name);
   }
   //一个示意性的商业方法
   public String about() {  
     return "Hello, I am RegSingleton.";  
   }  
   public static void main(String[] args) {
     Singleton3 single3 = Singleton3.getInstance(null);
    System.out.println(single3.about());
   }
 }