单例模式

推荐使用方法：

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

　　线程不安全：（加上synchronized也不够好）：

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