Easy Program在多个Action监听器时抛出NullPointerException错误。

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英文:

Easy Program throws NullPointerException when multiple Action Listeners

问题

我已尝试在3个单独的文件中使用多个动作监听器，但当我想要显示第三个文件时，它会抛出空指针异常，但会抛出此错误。我只是一个初学者，也许我在连接类到另一个类时失败了。这可能是主要问题。提前感谢您的帮助。

源代码：

Start.java

  1. package com.raising.darkness;
  3. import javax.swing.*;
  4. import java.awt.*;
  5. import java.awt.event.ActionEvent;
  6. import java.awt.event.ActionListener;
  8. public class Start {
  10.     // 省略了一些代码...
  12.     public class StartHandler implements ActionListener {
  14.         @Override
  15.         public void actionPerformed(ActionEvent ex) {
  16.             heroClasses.createClassChoose();
  17.         }
  18.     }
  19. }
  21. HeroClasses.java
  23. ```java
  24. package com.raising.darkness;
  26. import javax.swing.*;
  27. import java.awt.*;
  28. import java.awt.event.ActionEvent;
  29. import java.awt.event.ActionListener;
  31. public class HeroClasses {
  32.     // 省略了一些代码...
  34.     public class ClassHandler implements ActionListener {
  35.         @Override
  36.         public void actionPerformed(ActionEvent ex) {
  37.             choosenClass = ex.getActionCommand();
  38.             averagePanel.mainPanel();
  39.         }
  40.     }
  41. }
  43. AveragePanel.java
  45. ```java
  46. package com.raising.darkness;
  48. import javax.swing.*;
  49. import java.awt.*;
  51. public class AveragePanel {
  53.     // 省略了一些代码...
  54. }

请注意，我已经省略了一些源代码中的部分，只包含了与问题相关的部分。希望这有助于您理解问题的所在。

英文:

I have tried using multiple action listeners in 3 seperate files but it throws NullPointer exception when I want to display a "3-rd file" but it throws this error.I am just a begginer and maybe I failed with connecting class to another.That may be the main problem.Thanks for helping in advance.

Source :

Start.java

  1.     package com.raising.darkness;
  2.     
  3.     import javax.swing.*;
  4.     import java.awt.*;
  5.     import java.awt.event.ActionEvent;
  6.     import java.awt.event.ActionListener;
  7.     
  8.     public class Start {
  9.     
  10.         JFrame frame;
  11.         Container container;
  12.         JPanel startTitle;
  13.         JLabel startText;
  14.         Font startTitleFont= new Font("Dialog", Font.BOLD, 60);
  15.         Font startTitleButton = new Font("Dialog", Font.PLAIN, 40);
  16.         Font chooseTitleFont = new Font("Dialog", Font.PLAIN, 59);
  17.         JPanel startButtonPanel;
  18.         JButton startButton;
  19.     
  20.     
  21.         Color backgroundFrame = new Color(180,180,180);
  22.         Color buttonColor = new Color(144,144,144);
  23.     
  24.     
  25.     
  26.         StartHandler startHandler = new StartHandler();
  27.     
  28.     
  29.         AveragePanel averagePanel = new AveragePanel(this);
  30.         HeroClasses heroClasses = new HeroClasses(this);
  31.     
  32.     
  33.         public static void main(String[] args) { new Start(); }
  34.         public Start(){
  35.             frame=new JFrame();
  36.             frame.setSize(1024,800);
  37.             frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  38.             frame.setTitle("Raising Darkness");
  39.             frame.getContentPane().setBackground(backgroundFrame);
  40.             frame.setLayout(null);
  41.     
  42.             container=frame.getContentPane();
  43.     
  44.              //Title Text
  45.             startTitle = new JPanel();
  46.             startTitle.setBounds(162,100,700,150);
  47.             startTitle.setBackground(backgroundFrame);
  48.     
  49.             startText = new JLabel("RAISING DARKNESS");
  50.     
  51.             startText.setFont(startTitleFont);
  52.     
  53.             startTitle.add(startText);
  54.             container.add(startTitle);
  55.     
  56.     
  57.             startButtonPanel = new JPanel();
  58.             startButtonPanel.setBounds(387,600,200,70);
  59.             startButtonPanel.setBackground(backgroundFrame);
  60.             startButton = new JButton();
  61.             startButton.setText("START");
  62.             startButton.setBackground(buttonColor);
  63.     
  64.             startButton.setFont(startTitleButton);
  65.             startButton.setFocusPainted(false);
  66.             startButton.addActionListener(startHandler);
  67.     
  68.             startButtonPanel.add(startButton);
  69.             container.add(startButtonPanel);
  70.     
  71.     
  72.             frame.setVisible(true);
  73.     
  74.         }
  75.     
  76.     
  77.         public class StartHandler implements ActionListener{
  78.     
  79.             @Override
  80.             public void actionPerformed(ActionEvent ex) {
  81.     
  82.                 heroClasses.createClassChoose();
  83.             }
  84.         }
  85.     
  86.     
  87.     }

HeroClasses.java

  1.     package com.raising.darkness;
  2.     
  3.     import javax.swing.*;
  4.     import java.awt.*;
  5.     import java.awt.event.ActionEvent;
  6.     import java.awt.event.ActionListener;
  7.     
  8.     public class HeroClasses{
  9.         Start start;
  10.         Color normalChooseColor = new Color(144,144,144);
  11.         JPanel chooseTitlePanel, chooseButtonsPanel;
  12.         JLabel chooseTitleLabel;
  13.         JButton chooseMageButton, chooseWarriorButton, chooseArcherButton;
  14.         String choosenClass;
  15.     
  16.         Color chooseClassColor = new Color(100,100,100);
  17.     
  18.         ClassHandler classHandler = new ClassHandler();
  19.     
  20.         AveragePanel averagePanel = new AveragePanel(this);
  21.     
  22.     
  23.         public HeroClasses(Start start){
  24.                 this.start = start;
  25.             }
  26.     
  27.         public void createClassChoose(){
  28.             start.startButtonPanel.setVisible(false);
  29.             start.startTitle.setVisible(false);
  30.             start.frame.getContentPane().setBackground(normalChooseColor);
  31.     
  32.     
  33.             chooseTitlePanel = new JPanel();
  34.             chooseTitlePanel.setBounds(162,200,700,100);
  35.             chooseTitlePanel.setBackground(normalChooseColor);
  36.     
  37.             chooseTitleLabel= new JLabel();
  38.             chooseTitleLabel.setText("Choose Class!");
  39.             chooseTitleLabel.setFont(start.chooseTitleFont);
  40.             chooseTitlePanel.add(chooseTitleLabel);
  41.             start.container.add(chooseTitlePanel);
  42.     
  43.             chooseButtonsPanel = new JPanel();
  44.             chooseButtonsPanel.setBounds(162,500,700,100);
  45.             chooseButtonsPanel.setLayout(new GridLayout(1,3,50,0));
  46.             chooseButtonsPanel.setBackground(normalChooseColor);
  47.     
  48.             //CLASSES
  49.     
  50.             chooseMageButton = new JButton("Mage");
  51.             chooseMageButton.setBackground(chooseClassColor);
  52.             chooseMageButton.setFont(start.startTitleButton);
  53.             chooseMageButton.addActionListener(classHandler);
  54.             chooseMageButton.setActionCommand("Mage");
  55.             chooseMageButton.setFocusPainted(false);
  56.             chooseButtonsPanel.add(chooseMageButton);
  57.     
  58.             chooseWarriorButton = new JButton("Warrior");
  59.             chooseWarriorButton.setBackground(chooseClassColor);
  60.             chooseWarriorButton.setFont(start.startTitleButton);
  61.             chooseWarriorButton.addActionListener(classHandler);
  62.             chooseWarriorButton.setActionCommand("Warrior");
  63.             chooseWarriorButton.setFocusPainted(false);
  64.             chooseButtonsPanel.add(chooseWarriorButton);
  65.     
  66.             chooseArcherButton = new JButton("Archer");
  67.             chooseArcherButton.setBackground(chooseClassColor);
  68.             chooseArcherButton.setFont(start.startTitleButton);
  69.             chooseArcherButton.addActionListener(classHandler);
  70.             chooseArcherButton.setActionCommand("Archer");
  71.             chooseArcherButton.setFocusPainted(false);
  72.             chooseButtonsPanel.add(chooseArcherButton);
  73.     
  74.     
  75.             start.container.add(chooseButtonsPanel);
  76.     
  77.     
  78.     
  79.     
  80.         }
  81.     
  82.     
  83.     
  84.     public class ClassHandler implements ActionListener{
  85.         @Override
  86.         public void actionPerformed(ActionEvent ex) {
  87.             choosenClass = ex.getActionCommand();
  88.             averagePanel.mainPanel();
  89.     
  90.     
  91.         }
  92.     }
  93.     
  94.     }

AveragePanel.java

  1.     package com.raising.darkness;
  2.     
  3.     import javax.swing.*;
  4.     import java.awt.*;
  5.     
  6.     public class AveragePanel {
  7.     
  8.         Start start;
  9.         HeroClasses heroClasses;
  10.         String actualClass;
  11.         JPanel picturePanel, statsPanel, storyPanel, choicesPanel;
  12.         JLabel pictureLabel;
  13.         ImageIcon imageIcon;
  14.     
  15.     
  16.         public AveragePanel(HeroClasses heroClasses){this.heroClasses = heroClasses;}
  17.         public AveragePanel(Start start){this.start = start;}
  18.     
  19.     
  20.     
  21.     
  22.         public void mainPanel(){
  23.             heroClasses.chooseButtonsPanel.setVisible(false);
  24.             heroClasses.chooseTitlePanel.setVisible(false);
  25.     
  26.              actualClass = heroClasses.choosenClass ;
  27.             System.out.println("Current Class is :" +actualClass);
  28.     
  29.             picturePanel  = new JPanel();
  30.             picturePanel.setBounds(50,50,500,350);
  31.             picturePanel.setBackground(Color.WHITE);
  32.                 start.container.add(picturePanel);
  33.     
  34.     
  35.     
  36.         }
  37.     
  38.     
  39.     }

答案1

得分: 0

以下是您要翻译的内容：

  1. 在AveragePanel类中有2个构造函数。您使用了public AveragePanel(HeroClasses heroClasses){this.heroClasses = heroClasses;}，所以您的字段start为空，您有两个选择。
    1. 更改mainPanel方法，并从heroClasses获取start面板：heroClasses.start.container.add(picturePanel); 方法将如下所示：
    1. public void mainPanel(){
    2.     heroClasses.chooseButtonsPanel.setVisible(false);
    3.     heroClasses.chooseTitlePanel.setVisible(false);
    5.     actualClass = heroClasses.choosenClass;
    6.     System.out.println("Current Class is :" + actualClass);
    8.     picturePanel = new JPanel();
    9.     picturePanel.setBounds(50, 50, 500, 350);
    10.     picturePanel.setBackground(Color.WHITE);
    11.     heroClasses.start.container.add(picturePanel);
    12. }
    1. 更好的解决方案是更改构造函数，并使用heroClasses设置start面板。然后您可以同时使用两个构造函数，方法mainPanel()将按相同的方式工作。以下是构造函数：
    1. public AveragePanel(HeroClasses heroClasses){
    2.     this.heroClasses = heroClasses;
    3.     this.start = heroClasses.start;
    4. }
    5. public AveragePanel(Start start){this.start = start;}

    方法mainPanel()没有更改。

最佳实践是在构造函数中初始化字段。在您的情况下，您需要在HeroClasses类中进行更改。通过构造函数初始化averagePanel字段：

  1. public class HeroClasses{
  2.     // 其他字段...
  3.     AveragePanel averagePanel;
  5.     public HeroClasses(Start start){
  6.         this.start = start;
  7.         this.averagePanel = new AveragePanel(this);
  8.     }
  10.     // 其他方法...
  11. }

这是您的翻译，请使用这些翻译来替代原始文本中的相应部分。

英文:

You have 2 constructors in AveragePanel class. You use public AveragePanel(HeroClasses heroClasses){this.heroClasses = heroClasses;}, so your field start is null, you have two options.

  1. Change method main panel, and get start panel from heroClasses: heroClasses.start.container.add(picturePanel); Method will be looks like:

    1.      public void mainPanel(){
    2.      heroClasses.chooseButtonsPanel.setVisible(false);
    3.      heroClasses.chooseTitlePanel.setVisible(false);
    5.       actualClass = heroClasses.choosenClass ;
    6.      System.out.println("Current Class is :" +actualClass);
    8.      picturePanel  = new JPanel();
    9.      picturePanel.setBounds(50,50,500,350);
    10.      picturePanel.setBackground(Color.WHITE);
    11.      heroClasses.start.container.add(picturePanel);
    13.  }

  2. Better solution is change constructor, and set start panel using heroClasses. Then you can use both constructors and method mainPanel() will works this same.
    Below constructors:

    1.      public AveragePanel(HeroClasses heroClasses){
    2.  	    this.heroClasses = heroClasses;
    3.  	    this.start = heroClasses.start;
    4.  	}
    5.      public AveragePanel(Start start){this.start = start;}

method mainPanel() with no changes.

The best practice is initializion field in constructor. In your case, you have to change that in your HeroClasses class. Initialize field averagePanel by contructor:

  1. import javax.swing.*;
  2. import java.awt.*;
  3. import java.awt.event.ActionEvent;
  4. import java.awt.event.ActionListener;
  6. public class HeroClasses{
  7.     Start start;
  8.     Color normalChooseColor = new Color(144,144,144);
  9.     JPanel chooseTitlePanel, chooseButtonsPanel;
  10.     JLabel chooseTitleLabel;
  11.     JButton chooseMageButton, chooseWarriorButton, chooseArcherButton;
  12.     String choosenClass;
  14.     Color chooseClassColor = new Color(100,100,100);
  16.     ClassHandler classHandler = new ClassHandler();
  18.     AveragePanel averagePanel;
  21.     public HeroClasses(Start start){
  22.             this.start = start;
  23.             this.averagePanel = new AveragePanel(this);
  24.         }
  26.     public void createClassChoose(){
  27.         start.startButtonPanel.setVisible(false);
  28.         start.startTitle.setVisible(false);
  29.         start.frame.getContentPane().setBackground(normalChooseColor);
  32.         chooseTitlePanel = new JPanel();
  33.         chooseTitlePanel.setBounds(162,200,700,100);
  34.         chooseTitlePanel.setBackground(normalChooseColor);
  36.         chooseTitleLabel= new JLabel();
  37.         chooseTitleLabel.setText("Choose Class!");
  38.         chooseTitleLabel.setFont(start.chooseTitleFont);
  39.         chooseTitlePanel.add(chooseTitleLabel);
  40.         start.container.add(chooseTitlePanel);
  42.         chooseButtonsPanel = new JPanel();
  43.         chooseButtonsPanel.setBounds(162,500,700,100);
  44.         chooseButtonsPanel.setLayout(new GridLayout(1,3,50,0));
  45.         chooseButtonsPanel.setBackground(normalChooseColor);
  47.         //CLASSES
  49.         chooseMageButton = new JButton("Mage");
  50.         chooseMageButton.setBackground(chooseClassColor);
  51.         chooseMageButton.setFont(start.startTitleButton);
  52.         chooseMageButton.addActionListener(classHandler);
  53.         chooseMageButton.setActionCommand("Mage");
  54.         chooseMageButton.setFocusPainted(false);
  55.         chooseButtonsPanel.add(chooseMageButton);
  57.         chooseWarriorButton = new JButton("Warrior");
  58.         chooseWarriorButton.setBackground(chooseClassColor);
  59.         chooseWarriorButton.setFont(start.startTitleButton);
  60.         chooseWarriorButton.addActionListener(classHandler);
  61.         chooseWarriorButton.setActionCommand("Warrior");
  62.         chooseWarriorButton.setFocusPainted(false);
  63.         chooseButtonsPanel.add(chooseWarriorButton);
  65.         chooseArcherButton = new JButton("Archer");
  66.         chooseArcherButton.setBackground(chooseClassColor);
  67.         chooseArcherButton.setFont(start.startTitleButton);
  68.         chooseArcherButton.addActionListener(classHandler);
  69.         chooseArcherButton.setActionCommand("Archer");
  70.         chooseArcherButton.setFocusPainted(false);
  71.         chooseButtonsPanel.add(chooseArcherButton);
  74.         start.container.add(chooseButtonsPanel);
  79.     }
  83. public class ClassHandler implements ActionListener{
  84.     @Override
  85.     public void actionPerformed(ActionEvent ex) {
  86.         choosenClass = ex.getActionCommand();
  87.         averagePanel.mainPanel();
  90.     }
  91. }
  93. }

huangapple
  • 本文由 发表于 2020年8月9日 05:00:53
  • 转载请务必保留本文链接：https://go.coder-hub.com/63320189.html
  • java
  • nullpointerexception
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