Java 迭代中出现空指针异常，无法搜索节点。delete和replace方法是有问题的方法
您触发自己的NPE：Java 迭代中出现空指针异常，无法搜索节点。delete和replace方法是有问题的方法,java,Java,您触发自己的NPE： import javax.swing.*; import java.io.*; class MyDatabase implements Database { Node head = null, tail = null, rover = null; String ako; File myFile = new File("sample.dat"); Node n = new Node(); Node current; Node p; Node x = new Node();
import javax.swing.*;
import java.io.*;
class MyDatabase implements Database {
Node head = null, tail = null, rover = null;
String ako;
File myFile = new File("sample.dat");
Node n = new Node();
Node current; Node p;
Node x = new Node();
public void insert(Node myNewNode) {
if (head == null){
head = myNewNode;
head.next = null;
}
else {
tail = head;
while(tail.next != null)
tail = tail.next;
tail.next = myNewNode;
myNewNode.next = null;
}
current = head;
}
public boolean delete(Node nodeToDelete) {
//the delete and replace methods are the ones that have problems
current = head;
p = head;
head = null;
//here, no matter what you enter, this if statement is never executed. Yes, never. even if they are equal.
if(nodeToDelete.title == head.title) {
head = head.next;
return true;
}
else{
while(current != nodeToDelete)
current = current.next;//Null Pointer exception here
while(p.next != nodeToDelete)
p = p.next;//Null Pointer exception here
current = current.next;
p = current;
}
current = head;//this is for listIterator purposes.
return true;
}
public boolean replace(Node nodeToReplace, Node myNewNode) {
//the delete and replace methods are the ones that have problems
//here i tested if the head.title and nodeToReplace.title have values
//the println correctly prints the value that I input
current = head;
String s = head.title;// for example i entered "max"
String s1 = nodeToReplace.title;// i also entered "max"
System.out.println(s);//prints out "max"
System.out.println(s1);// prints out "max"
if(s == s1) { // if statement is not executed. Note: i entered the same string.
myNewNode.next = head.next;
head = myNewNode;
}
else {
while(current != null) {
String s2 = current.title;
if(s2 == s1) {
current = new Node(myNewNode);
}
}
}
current = head;
return true;
}
public Node search(Node nodeToSearch) {
current = head;
while(current != null) {
if(current == nodeToSearch) {
Node p = new Node(current);
current = head;
return p;
}
}
return null;
}
public boolean saveToFile(String filename) throws Exception {
Node p = new Node();
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(myFile));
out.writeObject(p);
out.close();
return true;
}
public boolean loadFromFile(String filename) throws Exception {
ObjectInputStream in = new ObjectInputStream(new FileInputStream(myFile));
head = (Node) in.readObject();
return true;
}
public Node listIterator() {
try{
if(current == head) {
rover = current;
current = current.next;
return rover;
}
else {
rover = current;
current = current.next;
Node p = new Node(rover);
return p;
}
}
catch(NullPointerException e) {
current = head;
return null;
}
}
public Node listIterator2() {
try{
if(current == head) {
rover = current;
current = current.next;
return rover;
}
else {
rover = current;
current = current.next;
return rover;
}
}
catch(NullPointerException e) {
current = head;
return null;
}
}
public boolean equals(Database db) {
Node p;
while(rover != null) {
p = head;
while(p != null) {
if(rover != p)
return false;
p = p.next;
}
rover = rover.next;
}
return true;
}
public String whoIAm() {
ako = "Michael Glenn R. Roquim Jr. !";
return ako;
}
}
您触发自己的NPE：
import javax.swing.*;
import java.io.*;
class MyDatabase implements Database {
Node head = null, tail = null, rover = null;
String ako;
File myFile = new File("sample.dat");
Node n = new Node();
Node current; Node p;
Node x = new Node();
public void insert(Node myNewNode) {
if (head == null){
head = myNewNode;
head.next = null;
}
else {
tail = head;
while(tail.next != null)
tail = tail.next;
tail.next = myNewNode;
myNewNode.next = null;
}
current = head;
}
public boolean delete(Node nodeToDelete) {
//the delete and replace methods are the ones that have problems
current = head;
p = head;
head = null;
//here, no matter what you enter, this if statement is never executed. Yes, never. even if they are equal.
if(nodeToDelete.title == head.title) {
head = head.next;
return true;
}
else{
while(current != nodeToDelete)
current = current.next;//Null Pointer exception here
while(p.next != nodeToDelete)
p = p.next;//Null Pointer exception here
current = current.next;
p = current;
}
current = head;//this is for listIterator purposes.
return true;
}
public boolean replace(Node nodeToReplace, Node myNewNode) {
//the delete and replace methods are the ones that have problems
//here i tested if the head.title and nodeToReplace.title have values
//the println correctly prints the value that I input
current = head;
String s = head.title;// for example i entered "max"
String s1 = nodeToReplace.title;// i also entered "max"
System.out.println(s);//prints out "max"
System.out.println(s1);// prints out "max"
if(s == s1) { // if statement is not executed. Note: i entered the same string.
myNewNode.next = head.next;
head = myNewNode;
}
else {
while(current != null) {
String s2 = current.title;
if(s2 == s1) {
current = new Node(myNewNode);
}
}
}
current = head;
return true;
}
public Node search(Node nodeToSearch) {
current = head;
while(current != null) {
if(current == nodeToSearch) {
Node p = new Node(current);
current = head;
return p;
}
}
return null;
}
public boolean saveToFile(String filename) throws Exception {
Node p = new Node();
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(myFile));
out.writeObject(p);
out.close();
return true;
}
public boolean loadFromFile(String filename) throws Exception {
ObjectInputStream in = new ObjectInputStream(new FileInputStream(myFile));
head = (Node) in.readObject();
return true;
}
public Node listIterator() {
try{
if(current == head) {
rover = current;
current = current.next;
return rover;
}
else {
rover = current;
current = current.next;
Node p = new Node(rover);
return p;
}
}
catch(NullPointerException e) {
current = head;
return null;
}
}
public Node listIterator2() {
try{
if(current == head) {
rover = current;
current = current.next;
return rover;
}
else {
rover = current;
current = current.next;
return rover;
}
}
catch(NullPointerException e) {
current = head;
return null;
}
}
public boolean equals(Database db) {
Node p;
while(rover != null) {
p = head;
while(p != null) {
if(rover != p)
return false;
p = p.next;
}
rover = rover.next;
}
return true;
}
public String whoIAm() {
ako = "Michael Glenn R. Roquim Jr. !";
return ako;
}
}
此外：
head = null;
//here, no matter what you enter, this if statement is never executed. Yes, never. even if they are equal.
if(nodeToDelete.title == head.title) {
equals（）还有一件事：如果元素不在列表中，您似乎总是抛出NPE（删除时）（您将找不到该元素，到达列表的末尾（为null），然后尝试处理其中的实例变量） 您将head设置为null，然后立即尝试访问它。。。（head始终为空，因此head.title将抛出NPE）
此外：
head = null;
//here, no matter what you enter, this if statement is never executed. Yes, never. even if they are equal.
if(nodeToDelete.title == head.title) {
equals（）还有一件事：如果您的元素不在列表中，您似乎总是会抛出NPE（删除时）（您将找不到元素，到达列表末尾为null，然后尝试解决其中的实例变量）.我喜欢将代码简化为基本内容，因此这里有一个工作版本，它只按照方法的建议执行
if(s == s1) { // if statement is not executed. Note: i entered the same string.
import java.io.*；
导入java.util.ArrayList；
导入java.util.List；
导入java.util.ListIterator；
类MyDatabase{
私有最终列表标题=新ArrayList（）；
公开作废插入（书名）{
标题。添加（标题）；
}
公共布尔删除（书名）{
返回标题。删除（标题）；
}
公共布尔替换（书籍标题1，书籍标题2）{
int pos=标题索引（标题1）；
如果（位置>=0）{
标题集（位置，标题2）；
返回true；
}
返回false；
}
公共图书搜索（字符串标题）{
用于（书名）{
如果（书名等于（书名））
还书；
}
返回null；
}
public void saveToFile（字符串文件名）引发IOException{
ObjectOutputStream out=新的ObjectOutputStream（新文件输出流（文件名））；
out.书面对象（标题）；
out.close（）；
}
public void loadFromFile（字符串文件名）引发IOException，ClassNotFoundException{
ObjectInputStream in=新ObjectInputStream（新文件输入流（文件名））；
标题。清除（）；
titles.addAll（（列表）在.readObject（）中）；
in.close（）；
}
公共ListIterator ListIterator（）{
返回titles.listIterator（）；
}
公共布尔等于（对象o）{
返回o MyDatabase的instanceof&（（MyDatabase）o）.titles.equals（titles）；
}
公共字符串whoIAm（）{
返回“小迈克尔·格伦·R·罗奎姆！”；
}
}
课堂用书{
最后的字符串标题；
最后一年；
书籍（字符串标题，整数年）{
this.title=标题；
今年=年；
}
@凌驾
公共布尔等于（对象o）{
如果（this==o）返回true；
如果（o==null | | getClass（）！=o.getClass（））返回false；
簿册=（簿册）o；
如果（年！=账面年）返回false；
如果（！title.equals（book.title））返回false；
返回true；
}
}
if(s == s1) { // if statement is not executed. Note: i entered the same string.
import java.io.*；
导入java.util.ArrayList；
导入java.util.List；
导入java.util.ListIterator；
类MyDatabase{
私有最终列表标题=新ArrayList（）；
公开作废插入（书名）{
标题。添加（标题）；
}
公共布尔删除（书名）{
返回标题。删除（标题）；
}
公共布尔替换（书籍标题1，书籍标题2）{
int pos=标题索引（标题1）；
如果（位置>=0）{
标题集（位置，标题2）；
返回true；
}
返回false；
}
公共图书搜索（字符串标题）{
用于（书名）{
如果（书名等于（书名））
还书；
}
返回null；
}
public void saveToFile（字符串文件名）引发IOException{
ObjectOutputStream out=新的ObjectOutputStream（新文件输出流（文件名））；
out.书面对象（标题）；
out.close（）；
}
public void loadFromFile（字符串文件名）引发IOException，ClassNotFoundException{
ObjectInputStream in=新ObjectInputStream（新文件输入流（文件名））；
标题。清除（）；
titles.addAll（（列表）在.readObject（）中）；
in.close（）；
}
公共ListIterator ListIterator（）{
返回titles.listIterator（）；
}
公共布尔等于（对象o）{
返回o MyDatabase的instanceof&（（MyDatabase）o）.titles.equals（titles）；
}
公共字符串whoIAm（）{
返回“小迈克尔·格伦·R·罗奎姆！”；
}
}
课堂用书{
最后的字符串标题；
最后一年；
书籍（字符串标题，整数年）{
this.title=标题；
今年=年；
}
@凌驾
公共布尔等于（对象o）{
如果（this==o）返回true；
如果（o==null | | getClass（）！=o.getClass（））返回false；
簿册=（簿册）o；
如果（年！=账面年）返回false；
如果（！title.equals（book.title））返回false；
返回true；
}
}