代码如下：

public void test(BSTNode<T> z)
    {
        BSTNode<T> y;
        if (z.left != null){
            y = z.parent;
            BSTNode<T> a;
            a = y.left;
            if (y != null){
                System.out.println("before a             "+a);
                System.out.println("before y.left        "+y.left);
                System.out.println("befere z.parent.left "+z.parent.left);
                y.left = null;
                //a = null;
                System.out.println("after a              "+a);
                System.out.println("after y.left         "+y.left);
                System.out.println("after z.parent.left  "+z.parent.left);
            }
        }
    }

打印结果如下：
before a threadSleep.BSTree$BSTNode@15db9742
before y.left threadSleep.BSTree$BSTNode@15db9742
befere z.parent.left threadSleep.BSTree$BSTNode@15db9742
after a threadSleep.BSTree$BSTNode@15db9742
after y.left null
after z.parent.left null

请问 “after a ”为什么不是null？

或者将代码：

y.left = null;

            //a = null;

改为

//y.left = null;

a = null;

结果为：

before a threadSleep.BSTree$BSTNode@15db9742

before y.left threadSleep.BSTree$BSTNode@15db9742

befere z.parent.left threadSleep.BSTree$BSTNode@15db9742

after a threadSleep.BSTree$BSTNode@15db9742

after y.left null

after z.parent.left null

运行环境：

jdk1.8