遞歸：方法自己調(diào)用自己

找規(guī)律
找出口

Demo{
        Demo demo = new Demo();  -- 有問(wèn)題，會(huì)導(dǎo)致棧溢出
    }

===>

Demo{
        Demo demo;
        public Demo(){
            demo = new Demo();
        }
    }

Demo{
       static Demo demo = new Demo(); // 沒(méi)問(wèn)題，只會(huì)初始化一次
   }

===>

Demo{
    
        static Demo demo;
        static{
            demo = new Demo();
        }
        
        public Demo(){
        
        }
    }