Java-线程中的异常;AWT-EventQueue-0“;java.lang.NullPointerException
我的完整代码将被编译,但当我运行它时,我会得到下面的错误,并使用NullPointerException声明。我还包括了错误所指的代码部分,但我不知道如何使链表在第一个条目时不具有空值。请注意-错误行83指的是:if(scores.isEmpty())大多数错误都是未知来源的,我不知道如何追踪 错误:Java-线程中的异常;AWT-EventQueue-0“;java.lang.NullPointerException,java,swing,nullpointerexception,actionlistener,actionevent,Java,Swing,Nullpointerexception,Actionlistener,Actionevent,我的完整代码将被编译,但当我运行它时,我会得到下面的错误,并使用NullPointerException声明。我还包括了错误所指的代码部分,但我不知道如何使链表在第一个条目时不具有空值。请注意-错误行83指的是:if(scores.isEmpty())大多数错误都是未知来源的,我不知道如何追踪 错误: *Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException *at TopTenList$enterB
*Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
*at TopTenList$enterButtonListener.actionPerformed(TopTenList.java:83)
*at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
*at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
*at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
*at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
*at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
*at java.awt.Component.processMouseEvent(Unknown Source)
*at javax.swing.JComponent.processMouseEvent(Unknown Source)
*at java.awt.Component.processEvent(Unknown Source)
*at java.awt.Container.processEvent(Unknown Source)
*at java.awt.Component.dispatchEventImpl(Unknown Source)
*at java.awt.Container.dispatchEventImpl(Unknown Source)
*at java.awt.Component.dispatchEvent(Unknown Source)
*at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
*at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
*at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
*at java.awt.Container.dispatchEventImpl(Unknown Source)
*at java.awt.Window.dispatchEventImpl(Unknown Source)
*at java.awt.Component.dispatchEvent(Unknown Source)
*at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
*at java.awt.EventQueue.access$200(Unknown Source)
*at java.awt.EventQueue$3.run(Unknown Source)
*at java.awt.EventQueue$3.run(Unknown Source)
*at java.security.AccessController.doPrivileged(Native Method)
*at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
*at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
*at java.awt.EventQueue$4.run(Unknown Source)
*at java.awt.EventQueue$4.run(Unknown Source)
*at java.security.AccessController.doPrivileged(Native Method)
*at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
*at java.awt.EventQueue.dispatchEvent(Unknown Source)
*at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
*at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
*at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
*at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
*at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
*at java.awt.EventDispatchThread.run(Unknown Source)
代码:
私有类enterButtonListener实现ActionListener
{
已执行的公共无效操作(操作事件e)
{
字符串名称=”;
整型得分=0;
字符串newScore=name+“”+score.toString();
if(scores.isEmpty())
{
添加(新闻核心);
返回;
}
对于(int i=0;i10)
{
分数。删除(10);
}
}
}
}
这是我的完整代码:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.JList;
import java.util.*;
import java.util.Scanner;
import java.util.LinkedList;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class TopTenList extends JFrame
{
private TopTenList tt;
private JTextArea listView;
private JTextField name;
private JTextField score;
private LinkedList<String> scores;
private JButton enterButton;
// This is the code for the GUI Window
public TopTenList()
{
listView = new JTextArea();
name = new JTextField();
score = new JTextField();
// Put the textArea in the center of the frame
add(listView);
listView.setEditable(false);
listView.setBackground(Color.WHITE);
//Create panel and label for the Name and score text fields
JPanel namePanel = new JPanel(new GridLayout(2,2));
namePanel.add(new JLabel ("Enter User Name: "));
namePanel.add(name);
namePanel.add(new JLabel ("Enter New Score: "));
namePanel.add(score);
add(namePanel, BorderLayout.NORTH);
//Create Enter score button
enterButton = new JButton ("Enter");
add(enterButton, BorderLayout.SOUTH);
//Add action listener to the button
enterButton.addActionListener(new enterButtonListener());
// Set up the frame
setTitle("Top Ten Scoreholders"); // Window Title
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // Behavior on close
pack();
setVisible(true); // Display the window
}
// Create the Linked List
public void TopTenList()
{
scores = new LinkedList<String>();
}
// Populate the list
private class enterButtonListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
String name="";
Integer score=0;
String newScore = name + " "+score.toString();
if(scores.isEmpty())
{
scores.add(newScore);
return;
}
for (int i=0; i<=scores.size(); i++)
{
if(i==scores.size())
{
scores.add(newScore);
break;
}
if (isOnList(newScore, scores.get(i)))
{
scores.add(i,newScore);
break;
}
// Shrink the list to the top ten scores
while (scores.size()>10)
{
scores.remove(10);
}
}
}
}
// method to evaluate placement on score list
public boolean isOnList (String first, String second)
{
Integer firstScore = Integer.parseInt(first.substring(first.lastIndexOf(' ')+1));
Integer secondScore = Integer.parseInt(second.substring(second.lastIndexOf(' ')+1));
return firstScore > secondScore;
}
// make the list for display
public String toString()
{
String scoreList = "";
for (int i = 0; i <scores.size(); i++)
{
scoreList = scoreList + scores.get(i)+"\n";
}
return scoreList;
}
public static void main(String [ ] args)
{
new TopTenList();
}
}
import java.awt.*;
导入java.awt.event.*;
导入javax.swing.*;
导入javax.swing.JList;
导入java.util.*;
导入java.util.Scanner;
导入java.util.LinkedList;
导入java.awt.event.ActionListener;
导入java.awt.event.ActionEvent;
公共类TopTenList扩展了JFrame
{
私人TopTenList tt;
私有区域列表视图;
私有JTextField名称;
私人JTextField评分;
私人LinkedList分数;
私有JButton enterButton;
//这是GUI窗口的代码
公共TopTenList()
{
listView=新的JTextArea();
name=新的JTextField();
分数=新的JTextField();
//将文本区域放在框架的中心
添加(列表视图);
listView.setEditable(false);
listView.setBackground(颜色:白色);
//为“名称”和“分数”文本字段创建面板和标签
JPanel名称面板=新JPanel(新网格布局(2,2));
添加(新的JLabel(“输入用户名:”);
名称面板。添加(名称);
添加(新JLabel(“输入新分数”);
名称面板。添加(分数);
添加(名称面板,边框布局。北);
//创建输入分数按钮
enterButton=新的JButton(“回车”);
添加(enterButton,BorderLayout.SOUTH);
//将操作侦听器添加到按钮
addActionListener(新的enterButtonListener());
//架起架子
setTitle(“前十名得分者”);//窗口标题
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);//关闭时的行为
包装();
setVisible(true);//显示窗口
}
//创建链接列表
公共无效TopTenList()
{
分数=新的LinkedList();
}
//填充列表
私有类enterButtonListener实现ActionListener
{
已执行的公共无效操作(操作事件e)
{
字符串名称=”;
整型得分=0;
字符串newScore=name+“”+score.toString();
if(scores.isEmpty())
{
添加(新闻核心);
返回;
}
对于(int i=0;i10)
{
分数。删除(10);
}
}
}
}
//评估分数表上的位置的方法
公共布尔值isOnList(字符串第一,字符串第二)
{
整数firstScore=Integer.parseInt(first.substring(first.lastIndexOf(“”)+1));
Integer secondScore=Integer.parseInt(second.substring(second.lastIndexOf(“”)+1));
返回firstScore>secondScore;
}
//列出要显示的列表
公共字符串toString()
{
字符串scoreList=“”;
对于(inti=0;i我认为,要解决空指针异常,必须初始化LinkedList,在上面的某个地方声明它
LinkedList<Type> scores = new LinkedList<Type>();
LinkedList分数=新建LinkedList();
(Type是存储在LinkedList中的数据类型)
像这样声明LinkedList将创建一个新的空LinkedList对象,该对象可以存储所需的数据,并且随着向其中添加元素,其大小将逐渐增大
您可以在此链接上阅读有关LinkedList的更多信息:这不是构造函数,而是伪构造函数:
public void TopTenList()
{
scores = new LinkedList<String>();
}
// no void return type here, so this is a constructor
public TopTenList()
{
scores = new LinkedList<String>();
}
public void TopTenList()
{
分数=新的LinkedList();
}
为什么是“pseudo”?因为构造函数没有声明返回类型,不是void,不是nothing。因为这个错误,您认为分数在TopTenList构造函数中初始化,但事实上不是,因为同样没有TopTenList构造函数
这是一个构造函数:
public void TopTenList()
{
scores = new LinkedList<String>();
}
// no void return type here, so this is a constructor
public TopTenList()
{
scores = new LinkedList<String>();
}
//这里没有void返回类型,所以这是一个构造函数
公共TopTenList()
{
分数=新的LinkedList();
}
使用此选项,您的LinkedList分数将被初始化。或者您可以在声明的位置初始化它:
private LinkedList<String> scores = new LinkedList<String();
private LinkedList scores=new LinkedListscores
为空。请在您尚未向我们展示的代码中查看您的代码,以了解原因。可能存在重复的