单元测试时出现 NullPointerException 错误

Question

When I unit test my pop and peek methods for my MyStack class, I encounter a NullPointerException relating to the getData method of my node class.

I cannot tell why and I am wondering if anyone has any ideas on how to fix it and make it so that there is not a NullPointerException . I have tried editing how the node works and how getData itself works but cannot find a solution and since cannot figure out the problem. Any help would be very much appreciated

import java.io.*; 
import java.util.*;

public class MyStack<E> implements StackInterface<E>
{
    public Node<E> head;
    public int nodeCount = 0;

    public static void main(String args[]) {
    }

    public E peek() {
        return head.getData();
    }

    public E pop() {
        if (nodeCount == 0) {
            throw new EmptyStackException();
        }
        E item = head.getData();
        head = head.getNext();
        nodeCount--;
        return item;
    }

    public boolean empty() {
        if (head == null && nodeCount == 0) {
            return true;
        } else {
            return false;
        }
    }
    
    public void push(E data) {
        Node<E> head = new Node<E>(data);
        nodeCount++;
    }

    public int search(Object o) {
        int count = 0;
        Node<E> current = new Node<E>(head.getData());
        while (current.getData() != o) {
            current.getNext();
            count++;
        }
        return count;
    }
}

public class Node<E> 
{
    public E data;
    public Node<E> next;
    // getters and setters  
    public Node(E data) 
    { 
        this.data = data; 
        this.next = null; 
    } 
    public E getData() {
        return this.data;
    }
    public void setData(E data) {
        this.data = data;
    }
    public Node<E> getNext() {
        return next;
    }
    public void setNext(Node<E> next) {
        this.next = next;
    }
}

Answer 1

One problem is in your push method. There, you are not assigning the new head to the member variable defined at class-level. An updated push method could look like this:

public void push(E data) {
    Node<E> newHead = new Node<>(data);
    newHead.setNext(head);
    head = newHead;
    nodeCount++;
}

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In peek you should check if the stack is empty before trying to access getData() :

public E peek() {
    if (empty()) {
        throw new EmptyStackException();
    }
    return head.getData();
}

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Another NullPointerException happens in the search method where head.getData() is null for an empty stack. Furthermore, this method does not report the correct position of an item on the stack. I won't go into details in this answer as you have already asked a separate question .

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I highly encourage to look into how to use a debugger to step through your code. Thereby, you can execute your program line by line and see where it is deviating from what you expect. Debugging is an essential skill as a programmer. Here are three resources:

• IntelliJ IDEA Tutorial: Debug your first Java application
• Eclipse Beginner's Guide to Quick Start Debugging
• Java Debugging with Eclipse - Tutorial