面试官：说说对单例模式的理解，最后的枚举实现我居然不知

public class SingleObject {
   //创建 SingleObject 的一个对象
   private static SingleObject instance = new SingleObject();
   //让构造函数为 private，这样该类就不会被实例化
   private SingleObject(){}
   //获取唯一可用的对象
   public static SingleObject getInstance(){
      return instance;
   }
   public void showMessage(){
      System.out.println("Hello World!");
   }
}

public class SingletonPatternDemo {
   public static void main(String[] args) {
      //不合法的构造函数
      //编译时错误：构造函数 SingleObject() 是不可见的
      //SingleObject object = new SingleObject();
      //获取唯一可用的对象
      SingleObject object = SingleObject.getInstance();
      //显示消息
      object.showMessage();
   }
}

Hello World!

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

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

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

public class Singleton {  
    private volatile static Singleton singleton;  
    private Singleton (){}  
    public static Singleton getSingleton() {  
        if (singleton == null) {  
            synchronized (Singleton.class) {  
                if (singleton == null) {  
                    singleton = new Singleton();  
                }  
            }  
        }  
        return singleton;  
    }  
}

public class Singleton {
    private static class SingletonHolder {
        private static final Singleton INSTANCE = new Singleton();
    }
    private Singleton (){}
    public static final Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }
}

public enum Singleton { 
    INSTANCE;  
    public void doMethod() {
    }  
}

public class Runtime {
    private static Runtime currentRuntime = new Runtime();
    /**
     * Returns the runtime object associated with the current Java application.
     * Most of the methods of class <code>Runtime</code> are instance 
     * methods and must be invoked with respect to the current runtime object. 
     * 
     * @return  the <code>Runtime</code> object associated with the current
     *          Java application.
     */
    public static Runtime getRuntime() { 
    return currentRuntime;
    }
    /** Don't let anyone else instantiate this class */
    private Runtime() {}
    ...
}