stack的empty()

public static void main(String[] args) {
        Stack stack=null;
        System.out.println("1、"+stack.empty());
        stack=new Stack();
        System.out.println("2、"+stack.empty());
    }

运行结果：stack没有指向对象

Exception in thread "main" java.lang.NullPointerException
at main.main(main.java:6)

注释掉System.out.println("1、"+stack.empty());运行结束是：true

总结：使用stack的empty()时，首先要给stack初始化

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