我正在为我的编程模块开发一个学生项目，我们必须在其中构建一个项目，以包含许多不同的Java功能，主要是GUI设计和高级数据结构，因此我正在开发一个用于Java学生培训的系统，他们将在系统中注册，使用Java中的不同教程并进行测试。我现在停留在其中一个教程中，因为我认为这是一个好主意，而不是让多个页面通过教程，当用户按下“下一步”按钮时，教程中的图像和内容将在该页面上更新或“返回键。因此，图像和文本文件的所有名称都将存储在链接列表中（因此顺序正确），教程将从索引1（开始）开始，因此将显示图像1和内容1，如用户点击下一步索引将增加1，图像和内容将更新。 我目前收到
的错误消息java.lang.NullPointerException at gui.Tutorial1page1.imageSelect(Tutorial1page1.java:66)
at gui.Tutorial1page1.<init>(Tutorial1page1.java:69)
at gui.Tutorial1page1$1.run(Tutorial1page1.java:39)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(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.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)
任何关于从哪里开始寻找或甚至不可能的建议都将非常感激。也许应该提一下整个事情只是在我添加链表之前阅读图片。谢谢： - ）
package gui;
import java.awt.EventQueue;
import java.awt.TextArea;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.Color;
import javax.swing.JButton;
import java.awt.Font;
import javax.swing.BorderFactory;
import javax.swing.JLabel;
import javax.swing.ImageIcon;
import javax.swing.JProgressBar;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.FileReader;
import java.io.IOException;
import java.util.LinkedList;
import java.util.Scanner;
public class Tutorial1page1 extends JFrame {
/**
*
*/
private static final long serialVersionUID = 1L;
private JPanel contentPane;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
Tutorial1page1 frame = new Tutorial1page1();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
//declare the linkedlist for the images
private LinkedList<String> imageContent;
//declare int for the index of what image to select
//this will increase by one when user hits next button & decrease for back button
int index = 1;
//store the names of the files in order, add last will add them to end of list
public void Images()
{
imageContent = new LinkedList<String>();
imageContent.addLast("\"IfStatement.png\"");
imageContent.addLast("\"IfElseStatement.png\"");
}
//select image
public String imageSelect(int index)
{
return imageContent.get(index);
}
String image = imageSelect(index);
/**
* Create the frame.
* @throws IOException
*/
public Tutorial1page1() throws IOException {
//create, format and locate jframe
setResizable(false);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);//exit program when framed closed
setBounds(300, 75, 800, 600);
contentPane = new JPanel();
contentPane.setBackground(Color.WHITE);
setContentPane(contentPane);
contentPane.setLayout(null);
//create, format and position button for going back through pages of tutorial
//should this be disabled on first page??
JButton btnBack = new JButton(" < Back ");
btnBack.setForeground(Color.WHITE);
btnBack.setFont(new Font("Tahoma", Font.PLAIN, 20));
btnBack.setBackground(Color.BLUE);
btnBack.setBounds(150,500,120,30);
contentPane.add(btnBack);
//create, format and position button for quitting the tutorial
JButton btnQuit = new JButton("Quit");
btnQuit.setForeground(Color.WHITE);
btnQuit.setFont(new Font("Tahoma", Font.PLAIN, 20));
btnQuit.setBackground(Color.BLUE);
btnQuit.setBounds(350,500,120,30);
contentPane.add(btnQuit);
//add button listener for Next button
btnQuit.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
dispose();//close current screen
//open main screen
try {
Main frame = new Main();
//open last page of tutorial, in this case main module page
frame.setVisible(true);
//open in centre of screen
frame.setLocationRelativeTo(null);
} catch (Exception e) {
e.printStackTrace();
}
}
});
//create, format and position button for going forward through pages of tutorial
JButton btnNext = new JButton("Next > ");
btnNext.setForeground(Color.WHITE);
btnNext.setFont(new Font("Tahoma", Font.PLAIN, 20));
btnNext.setBackground(Color.BLUE);
btnNext.setBounds(550,500,120,30);
contentPane.add(btnNext);
//add button listener for Next button
btnNext.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
dispose();//close current screen
//open main screen
try {
Tutorial1page2 frame = new Tutorial1page2(); //open next page of tutorial
frame.setVisible(true);
//open in centre of screen
frame.setLocationRelativeTo(null);
} catch (Exception e) {
e.printStackTrace();
}
}
});
//create, format and position help button on each page of tutorial
//uses label appearance method to look like linked text
JButton btnHelp = new JButton("Help");
btnHelp.setForeground(Color.WHITE);
btnHelp.setBackground(Color.BLUE);
btnHelp.setFont(new Font("Tahoma", Font.BOLD, 15));
btnHelp.setBounds(740,0,50,50);
contentPane.add(btnHelp);
Login.labelAppearance(btnHelp);
//import and position resized icon
BufferedImage iconresized = ImageIO.read(this.getClass().getResource("/gui/iconresized.png"));
JLabel picLabel = new JLabel(new ImageIcon(iconresized));
contentPane.add(picLabel);
picLabel.setBounds(5,10,50,80);
//create, format and position label for the Tutorial description
//this will remain the same on all pages of this tutorial
JLabel lblTutorial1 = new JLabel("TUTORIAL 1 - Composition : If, switch, while & for statements");
lblTutorial1.setFont(new Font("Gisha", Font.BOLD, 22));
lblTutorial1.setForeground(Color.BLUE);
lblTutorial1.setBounds(60, 40, 675, 44);
lblTutorial1.setBorder(BorderFactory.createLineBorder(Color.BLUE));
contentPane.add(lblTutorial1);
//create, format and position label for the title of this page
//this changes on each page of each tutorial
JLabel lblIntroductionT1 = new JLabel("Introduction - Select Statements");
lblIntroductionT1.setFont(new Font("Gisha", Font.ITALIC, 15));
lblIntroductionT1.setForeground(Color.RED);
lblIntroductionT1.setBounds(20, 125, 300, 26);
contentPane.add(lblIntroductionT1);
//create new text field for displaying notes on the page
//this will wrap text to the text field horizontally, but allow a vertical scroll bar
//if the text goes on too long to fit the field vertically
TextArea Content1 = new TextArea("", 4, 30, TextArea.SCROLLBARS_VERTICAL_ONLY);
Content1.setBounds(20, 155, 400, 300);
contentPane.add(Content1);
//add(textField, BorderLayout.NORTH);
Content1.setEditable(false);
//read in text file for content
FileReader readtextfile = new FileReader("src/gui/Tutorial1content.txt");
//open scanner
Scanner FileReaderScan = new Scanner(readtextfile);
//create sting Tut1content to store what is read in
String Tut1content ="";
//while there is another line in text file add it to string
while(FileReaderScan.hasNextLine()){
String temp = FileReaderScan.nextLine() + "\n";
Tut1content = Tut1content + temp;
}
//add contents of string to the text area
Content1.append(Tut1content);
//close scanner
FileReaderScan.close();
//import and position the diagram to go with text
//both should be parallel to each other
//read in image
BufferedImage IfState = ImageIO.read(this.getClass().getResource(imageSelect(index)));
//set image in JLabel
JLabel picLabel2 = new JLabel(new ImageIcon(IfState));
//add border to picture
picLabel2.setBorder(BorderFactory.createLineBorder(Color.black));
//add to frame
contentPane.add(picLabel2);
//set bounds, location
picLabel2.setBounds(450,155,300,300);
//create and position progress bar so user can track where they are in tutorial
JProgressBar tut1progressBar = new JProgressBar(0,5);
tut1progressBar.setValue(1); //page 1 of 5
tut1progressBar.setBounds(220, 550, 380, 14);
contentPane.add(tut1progressBar);
//add label to progress bar to show page number currently on
JLabel lblProgress1 = new JLabel("Page 1/5");
lblProgress1.setFont(new Font("Gisha", Font.ITALIC, 10));
lblProgress1.setForeground(Color.RED);
lblProgress1.setBounds(690, 540, 80, 26);
contentPane.add(lblProgress1);
}
}
答案 0 :(得分：0)
如果我弄错了，请纠正我，但是在你的代码中没有任何地方我看到你调用Images方法，这是实例化LinkedList<T>并填充它的方法。由于您在.get引用上致电null，因此您获得NullPointerException。
另外请注意，请遵循代码中的Java命名约定（因此方法名称和变量应位于 camelCase 中）。