小技巧：如何写单例

第一种：

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

第二种：

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

第三种：

    public enum A
    {
        instance;
        
        private A()
        {            
        }
        
        public static A getInstance()
        {
            return instance;
        }
    }