Java 实例变量不保留状态

Question

我有一个简单的堆栈实现，使用链接列表，但是在测试代码期间我注意到实例变量 first 不保留状态并在后续推送操作期间恢复为 NULL。虽然 N 保留其值，但变量 first 却没有。有人可以帮忙吗？

import java.util.NoSuchElementException;

public class Stack <T> {

    private Node first;
    private Node current;
    private int N;

    private class Node {
        T item;
        Node next;
    }

    public void push(T item) {
        Node oldFirst = first;

        Node first = new Node();
        first.item = item;
        first.next = oldFirst;

        N++;
    }

    public T pop()  throws NoSuchElementException{
        try {
            T item = first.item;
            first = first.next;
            N--;
            return item;
        } catch (java.lang.NullPointerException error) {
            throw new NoSuchElementException("Stack is empty.");
        }
    }

}

测试客户端：


import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class TestStack {

   /**
     * Test the LIFO property of the stack i.e. the item pushed last into the
     * stack is the one to be popped first from the stack.
     */
    @Test
    void testLIFOPropertyWithIntegerStack() {
        int[] testClient = new int[] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        Stack<Integer> stack = new Stack<> ();

        for (int item : testClient) {
            stack.push(item);
        }

        int index = testClient.length - 1;
        while (stack.size() != 0) {
            int item = stack.pop();
            assertEquals(item, testClient[index--]);
        }
    }

}

Answer 1

你的推送方式有问题。您的 Node first = new Node(); 将字段 first 隐藏在您的堆栈中。

所以，改变：

Node first = new Node();

到

// This is your object field
first = new Node();

并且测试将通过。请记住，您需要实施 size()，因为您的测试方法使用它。

public int size(){
    return N;
}

Java 实例变量不保留状态

Java Instance Variable does not retain state

java

stack

instance-variables