问题描述
我已经将链接列表实现为Java。我创建了所有内容，但是我很难删除具有特定数据的特定节点。它抛出一个 NullPointerException 。我相信，我得到一个 NullPointerException ，因为下一个节点为空。如果有人能指出我正确的方向，这将是伟大的。
I have implemented a Linked List into Java. I have created everything, but I am having difficulty removing a specific node with specific data. It is throwing a NullPointerException. I believe, I am getting a NullPointerException because the next node is null. If someone could please point me in the right direction that would be great.
输入
anything
one
two
three
例外：
Exception in thread "main" java.lang.NullPointerException
at LinkedList.remove(LinkedList.java:28)
at Main.main(Main.java:29)
课程：
链接列表类
Classes:Linked list class
public class LinkedList {
// fields
private Node head;
private Node last;
private int size = 0;
// constructor, used when the class is first called
public LinkedList() {
head = last = new Node(null);
}
// add method
public void add(String s) {
last.setNext(new Node(s));
last = last.getNext();
size++;
}
// remove method, if it returns false then the specified index element doens not exist
// otherwise will return true
public boolean remove(String data) {
Node current = head;
last = null;
while(current != null) {
if(current.getData().equals(data)) {
current = current.getNext();
if(last == null) {
last = current;
}else {
last.getNext().setNext(current);
size--;
return true;
}
}else {
last = current;
current = current.getNext();
}
}
return false;
}
//will return the size of the list - will return -1 if list is empty
public int size() {
return size;
}
// will check if the list is empty or not
public boolean isEmpty() {
return true;
}
// @param (index) will get the data at specified index
public String getData(int index) {
if(index <= 0) {
return null;
}
Node current = head.getNext();
for(int i = 1;i < index;i++) {
if(current.getNext() == null) {
return null;
}
current = current.getNext();
}
return current.getData();
}
//@param will check if the arguement passed is in the list
// will return true if the list contains arg otherwise false
public boolean contains(String s) {
for(int i = 1;i<=size();i++) {
if(getData(i).equals(s)) {
return true;
}
}
return false;
}
//@return contents of the list - recursively
public String toString() {
Node current = head.getNext();
String output = "[";
while(current != null) {
output += current.getData()+",";
current = current.getNext();
}
return output+"]";
}
//@return first node
public Node getHead() {
return head;
}
// @return (recursively) list
public void print(Node n) {
if(n == null) {
return;
}else {
System.out.println(n.getData());
print(n.getNext());
}
}
}
主要
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class Main {
static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
public static void main(String[] args) throws IOException{
LinkedList list = new LinkedList(); // declaring main linked list
LinkedList b_List = new LinkedList(); // declaring the backup list
String input = null;
// getting input from user, will stop when user has entered 'fin'
while(!(input = br.readLine()).equals("fin")) {
list.add(input); // adding to main list
b_List.add(input);
}
list.print(list.getHead().getNext());
System.out.println("Input Complete.");
if(list.size() == 1) {
System.out.println("You have entered only one name. He/She is the survior");
}else {
System.out.println("Enter the name(s) would like to remove: ");
while(b_List.size() != 1) {
String toRemove = br.readLine();
b_List.remove(toRemove);
}
}
System.out.println("The contestants were: ");
list.print(list.getHead().getNext());
}
}
节点
public class Node {
// Fields
private String data;
private Node next;
// constructor
public Node(String data) {
this(data,null);
}
// constructor two with Node parameter
public Node(String data, Node node) {
this.data = data;
next = node;
}
/**
* Methods below return information about fields within class
* */
// @return the data
public String getData() {
return data;
}
// @param String data to this.data
public void setData(String data) {
this.data = data;
}
// @return next
public Node getNext() {
return next;
}
// @param Node next set to this.next
public void setNext(Node next) {
this.next = next;
}
}
推荐答案
首先，你的头只是一个先行标记，所以你不应该从它开始删除检查。
First of all, your head is just a before-first marker so you shouldn't start the remove check from it.
第二，你的<$ c $如果节点数据 null
第三 - 你的实现是c> remove 方法失败无论如何由于 last.getNext（）。setNext（当前）而被打破 - 它不会将上一个节点与下一个节点链接，它会将当前链接到下一个节点（即无效）
Third - your implementation is broken anyway because of last.getNext().setNext(current) - it won't link previous node with next, it will link current to next (i.e. will do nothing)
第四 - 由于最后的神秘操作，它仍然无法移除第一个元素......
Fourth - it still fails to remove first element because of mysterious operations with last...
正确执行删除将是这样的：
public boolean remove(String data){
Node current = head.getNext();
Node previous = null;
while (current != null) {
String dataOld = current.getData();
if ((dataOld == null && data == null) || (dataOld != null && dataOld.equals(data))) {
Node afterRemoved = current.getNext();
if (previous == null) {
head.setNext(afterRemoved);
} else {
previous.setNext(afterRemoved);
}
if (afterRemoved.getNext() == null) {
last = afterRemoved;
}
size--;
return true;
} else {
previous = current;
current = current.getNext();
}
}
return false;
}
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