一个简单的单例模式的类

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

私有的构造方法不能别其他类实例化，只能在本类中使用new关键字实例化