java - 变量名无法解析为变量 - 我找不到问题所在

Question

在第 110 行，它显示“return front3”，我收到此错误。我不知道为什么，我在 while 循环内创建了 Node front3 。

    public static Node add(Node poly1, Node poly2) {
        /** COMPLETE THIS METHOD **/
        // FOLLOWING LINE IS A PLACEHOLDER TO MAKE THIS METHOD COMPILE
        // CHANGE IT AS NEEDED FOR YOUR IMPLEMENTATION
        Node ptr1 = poly1;
        Node ptr2 = poly2;
        Node ptr3 = null;
        // Node front3;

        while (ptr1 != null && ptr2 != null) {
            if (ptr1.term.degree == ptr2.term.degree) {
                if (ptr3 == null) {
                    Node front3 = new Node(ptr1.term.coeff + ptr2.term.coeff,ptr1.term.degree,null);
                    ptr3 = front3;
                } else {
                    Node temp = new Node(ptr1.term.coeff + ptr2.term.coeff,ptr1.term.degree,null);
                    ptr3.next = temp;
                    ptr3 = temp;
                }
                ptr1 = ptr1.next;
                ptr2 = ptr2.next;
            } else if ( ptr1.term.degree > ptr2.term.degree) {
                if (ptr3 == null) {
                    Node front3 = new Node(ptr1.term.coeff,ptr1.term.degree,null);
                    ptr3 = front3;
                } else {
                    Node temp = new Node(ptr1.term.coeff, ptr1.term.degree , null);
                    ptr3.next = temp;
                    ptr3 = temp;
                }
                ptr1 = ptr1.next;
            } else if ( ptr1.term.degree < ptr2.term.degree ) {
                if (ptr3 == null) {
                    Node front3 = new Node(ptr2.term.coeff, ptr2.term.degree,null);
                    ptr3 = front3;
                } else {
                    Node temp = new Node(ptr2.term.coeff,ptr2.term.degree,null);
                    ptr3.next = temp;
                    ptr3 = temp;
                }
                ptr2 = ptr2.next;
            }
        }


        if (ptr3 == null) {
            return null;
        }

        return front3;
    }

然后我创建了一个不同的节点，Node front4，将其初始化为某些内容，然后我的程序运行了。这是在 while 循环之外完成的。

Answer 1

发生这种情况是因为对象仅存在于声明它们的 block 内。在您的情况下，您的 front3 将仅存在于您用于声明它的 if block 内:

if (ptr3 == null) {
    Node front3 = new Node(ptr2.term.coeff, ptr2.term.degree,null);
    ptr3 = front3; // Can use it here
}
// Cannot use it here

如果您确实需要返回 front3 对象，您应该在“方法级别”中声明它，就像您对 ptr 节点所做的那样。事实上，您已经在那里发表了评论。如果您只需应用以下更改，您就可以开始了:

当前:

// Node front3;

之后:

Node front3 = null; // Needs to initialize

并且您的 if 语句应更改为以下示例:

当前:

if (ptr3 == null) {
    Node front3 = new Node(ptr1.term.coeff,ptr1.term.degree,null);
    ptr3 = front3;
}

之后:

if (ptr3 == null) {
    front3 = new Node(ptr1.term.coeff,ptr1.term.degree,null); // No need for "Node", as it was already declared
    ptr3 = front3;
}

诗。我没有审查逻辑。这只是为了解释为什么您会收到“变量名称无法解析为变量”错误。