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带有 getText() 的 NullPointerException

发布于 2024-12-31 23:14:49 字数 5231 浏览 5 评论 0原文

我试图从文本字段获取文本，但每次运行该程序时都会收到 NullPointerException。我在互联网上发现的类似问题涉及将文本字段重新声明为局部变量，但我看不到我在这里这样做。

错误是：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

在指定的行。任何帮助将不胜感激

编辑：

这个简短的程序演示了这个问题

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Test{

public static final Test application = new Test();
private JFrame jFrame = null;
private JPanel jContentPane = null, buttonsPanel = null, cardPanel = null, manualPanel = null, uploadPanel = null,
        defaultPanel = null;
private JPanel[] mainPanel = {getDefaultPanel(), getManualPanel(), getUploadPanel()};
private JButton manualButton = null, uploadButton = null, manualAssignButton = null;
private JTextField manualEntryField = null;
private JLabel manualLabel = null;

final static String DEFAULTPANEL = "Default";
final static String MANUALPANEL = "Manual";
final static String UPLOADPANEL = "Upload";

/**
 * Main method
 * @param args
 */
public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            application.getJFrame().setVisible(true);
        }
    });
}

/**
 * Get the main application frame
 * @return JFrame
 */
private JFrame getJFrame() {
    if (jFrame == null) {
        jFrame = new JFrame();
        jFrame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
        jFrame.setResizable(false);
        // Add the main content pane
        jFrame.setContentPane(getJContentPanel());
        jFrame.addWindowListener(new java.awt.event.WindowAdapter() {   
            public void windowClosing(java.awt.event.WindowEvent e) {    
                closeApplication();
            }
        });
    }
    jFrame.pack();
    jFrame.setLocationRelativeTo(null);
    return jFrame;
}

/**
 * Get main content
 * @return JPanel
 */
private JPanel getJContentPanel(){
    if (jContentPane == null) {
        jContentPane = new JPanel();
        jContentPane.setPreferredSize(new Dimension(500, 200));
        jContentPane.setLayout(new BorderLayout());
        jContentPane.add(getButtonsPanel(), BorderLayout.NORTH);
        jContentPane.add(getCardPanel(), BorderLayout.CENTER);
    }
    return jContentPane;
}

private JPanel getButtonsPanel() {
    if(buttonsPanel == null){
        buttonsPanel = new JPanel();
        buttonsPanel.setPreferredSize(new Dimension(500, 50));
        buttonsPanel.setBorder(BorderFactory.createLineBorder(Color.gray));

        manualButton = new JButton("Enter manually");
        manualButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                CardLayout cl = (CardLayout)(cardPanel.getLayout());
                cl.show(cardPanel, MANUALPANEL);
            }
        });

        uploadButton = new JButton("Upload file");
        uploadButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                CardLayout cl = (CardLayout)(cardPanel.getLayout());
                cl.show(cardPanel, UPLOADPANEL);
            }
        });

        buttonsPanel.add(manualButton);
        buttonsPanel.add(uploadButton);
    }
    return buttonsPanel;
}

private JPanel getCardPanel() {
    if(cardPanel == null){
        cardPanel = new JPanel(new CardLayout());
        cardPanel.setPreferredSize(new Dimension(500, 200));
        cardPanel.setBorder(BorderFactory.createLineBorder(Color.gray));
        cardPanel.add(mainPanel[0], DEFAULTPANEL);
        cardPanel.add(mainPanel[1], MANUALPANEL);
        cardPanel.add(mainPanel[2], UPLOADPANEL);
    }
    return cardPanel;
}

private JPanel getDefaultPanel() {
    if(defaultPanel == null){
        defaultPanel = new JPanel();
    }
    return defaultPanel;
}

private JPanel getManualPanel() {
    if(manualPanel == null){
        manualPanel = new JPanel();
        manualPanel.setLayout(new FlowLayout());

        manualEntryField = new JTextField(10);
        manualEntryField.setText("Enter code");

        manualAssignButton = new JButton("Assign");
        manualAssignButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
             // Here is the exception
            System.out.println(manualEntryField.getText().trim());
            }
        });

        manualLabel = new JLabel("Please enter a code and press assign");

        manualPanel.add(manualEntryField);
        manualPanel.add(manualAssignButton);
        manualPanel.add(manualLabel);

    }
    return manualPanel;
}

private JPanel getUploadPanel() {
    if(uploadPanel == null){
        uploadPanel = new JPanel();
    }
    return uploadPanel;
}

private void closeApplication() {
    System.exit(0);
}
}
原文

I'm trying to get text from a text field but every time I run the program I get a NullPointerException. Similar problems I've found on the internet involve the text field being redeclared as a local variable, but I can't see that I am doing that here.

The error is :

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

at the line specified. Any help would be much appreciated

EDIT:

This short program demonstrates the problem

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Test{

public static final Test application = new Test();
private JFrame jFrame = null;
private JPanel jContentPane = null, buttonsPanel = null, cardPanel = null, manualPanel = null, uploadPanel = null,
        defaultPanel = null;
private JPanel[] mainPanel = {getDefaultPanel(), getManualPanel(), getUploadPanel()};
private JButton manualButton = null, uploadButton = null, manualAssignButton = null;
private JTextField manualEntryField = null;
private JLabel manualLabel = null;

final static String DEFAULTPANEL = "Default";
final static String MANUALPANEL = "Manual";
final static String UPLOADPANEL = "Upload";

/**
 * Main method
 * @param args
 */
public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            application.getJFrame().setVisible(true);
        }
    });
}

/**
 * Get the main application frame
 * @return JFrame
 */
private JFrame getJFrame() {
    if (jFrame == null) {
        jFrame = new JFrame();
        jFrame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
        jFrame.setResizable(false);
        // Add the main content pane
        jFrame.setContentPane(getJContentPanel());
        jFrame.addWindowListener(new java.awt.event.WindowAdapter() {   
            public void windowClosing(java.awt.event.WindowEvent e) {    
                closeApplication();
            }
        });
    }
    jFrame.pack();
    jFrame.setLocationRelativeTo(null);
    return jFrame;
}

/**
 * Get main content
 * @return JPanel
 */
private JPanel getJContentPanel(){
    if (jContentPane == null) {
        jContentPane = new JPanel();
        jContentPane.setPreferredSize(new Dimension(500, 200));
        jContentPane.setLayout(new BorderLayout());
        jContentPane.add(getButtonsPanel(), BorderLayout.NORTH);
        jContentPane.add(getCardPanel(), BorderLayout.CENTER);
    }
    return jContentPane;
}

private JPanel getButtonsPanel() {
    if(buttonsPanel == null){
        buttonsPanel = new JPanel();
        buttonsPanel.setPreferredSize(new Dimension(500, 50));
        buttonsPanel.setBorder(BorderFactory.createLineBorder(Color.gray));

        manualButton = new JButton("Enter manually");
        manualButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                CardLayout cl = (CardLayout)(cardPanel.getLayout());
                cl.show(cardPanel, MANUALPANEL);
            }
        });

        uploadButton = new JButton("Upload file");
        uploadButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                CardLayout cl = (CardLayout)(cardPanel.getLayout());
                cl.show(cardPanel, UPLOADPANEL);
            }
        });

        buttonsPanel.add(manualButton);
        buttonsPanel.add(uploadButton);
    }
    return buttonsPanel;
}

private JPanel getCardPanel() {
    if(cardPanel == null){
        cardPanel = new JPanel(new CardLayout());
        cardPanel.setPreferredSize(new Dimension(500, 200));
        cardPanel.setBorder(BorderFactory.createLineBorder(Color.gray));
        cardPanel.add(mainPanel[0], DEFAULTPANEL);
        cardPanel.add(mainPanel[1], MANUALPANEL);
        cardPanel.add(mainPanel[2], UPLOADPANEL);
    }
    return cardPanel;
}

private JPanel getDefaultPanel() {
    if(defaultPanel == null){
        defaultPanel = new JPanel();
    }
    return defaultPanel;
}

private JPanel getManualPanel() {
    if(manualPanel == null){
        manualPanel = new JPanel();
        manualPanel.setLayout(new FlowLayout());

        manualEntryField = new JTextField(10);
        manualEntryField.setText("Enter code");

        manualAssignButton = new JButton("Assign");
        manualAssignButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
             // Here is the exception
            System.out.println(manualEntryField.getText().trim());
            }
        });

        manualLabel = new JLabel("Please enter a code and press assign");

        manualPanel.add(manualEntryField);
        manualPanel.add(manualAssignButton);
        manualPanel.add(manualLabel);

    }
    return manualPanel;
}

private JPanel getUploadPanel() {
    if(uploadPanel == null){
        uploadPanel = new JPanel();
    }
    return uploadPanel;
}

private void closeApplication() {
    System.exit(0);
}
}

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评论（4）

逆光飞翔i 2025-01-07 23:14:49

查看您的 SSCCE 后，我通过注释掉实例化的冗余 Test 并在构造函数中创建面板数组，成功解决了 NPE。 EG

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Test{

//public static final Test application = new Test();
private JFrame jFrame = null;
private JPanel jContentPane = null, buttonsPanel = null, cardPanel = null, manualPanel = null, uploadPanel = null,
        defaultPanel = null;
private JPanel[] mainPanel = new JPanel[3];
private JButton manualButton = null, uploadButton = null, manualAssignButton = null;
private JTextField manualEntryField = null;
private JLabel manualLabel = null;

final static String DEFAULTPANEL = "Default";
final static String MANUALPANEL = "Manual";
final static String UPLOADPANEL = "Upload";

Test() {
    mainPanel[0] = getDefaultPanel();
    mainPanel[1] = getManualPanel();
    mainPanel[2] = getUploadPanel();
}

/**
 * Main method
 * @param args
 */
public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            new Test().getJFrame().setVisible(true);
        }
    });
}

/**
 * Get the main application frame
 * @return JFrame
 */
private JFrame getJFrame() {
    if (jFrame == null) {
        jFrame = new JFrame();
        jFrame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
        jFrame.setResizable(false);
        // Add the main content pane
        jFrame.setContentPane(getJContentPanel());
        jFrame.addWindowListener(new java.awt.event.WindowAdapter() {
            public void windowClosing(java.awt.event.WindowEvent e) {
                closeApplication();
            }
        });
    }
    jFrame.pack();
    jFrame.setLocationRelativeTo(null);
    return jFrame;
}

/**
 * Get main content
 * @return JPanel
 */
private JPanel getJContentPanel(){
    if (jContentPane == null) {
        jContentPane = new JPanel();
        jContentPane.setPreferredSize(new Dimension(500, 200));
        jContentPane.setLayout(new BorderLayout());
        jContentPane.add(getButtonsPanel(), BorderLayout.NORTH);
        jContentPane.add(getCardPanel(), BorderLayout.CENTER);
    }
    return jContentPane;
}

private JPanel getButtonsPanel() {
    if(buttonsPanel == null){
        buttonsPanel = new JPanel();
        buttonsPanel.setPreferredSize(new Dimension(500, 50));
        buttonsPanel.setBorder(BorderFactory.createLineBorder(Color.gray));

        manualButton = new JButton("Enter manually");
        manualButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                CardLayout cl = (CardLayout)(cardPanel.getLayout());
                cl.show(cardPanel, MANUALPANEL);
            }
        });

        uploadButton = new JButton("Upload file");
        uploadButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                CardLayout cl = (CardLayout)(cardPanel.getLayout());
                cl.show(cardPanel, UPLOADPANEL);
            }
        });

        buttonsPanel.add(manualButton);
        buttonsPanel.add(uploadButton);
    }
    return buttonsPanel;
}

private JPanel getCardPanel() {
    if(cardPanel == null){
        cardPanel = new JPanel(new CardLayout());
        cardPanel.setPreferredSize(new Dimension(500, 200));
        cardPanel.setBorder(BorderFactory.createLineBorder(Color.gray));
        cardPanel.add(mainPanel[0], DEFAULTPANEL);
        cardPanel.add(mainPanel[1], MANUALPANEL);
        cardPanel.add(mainPanel[2], UPLOADPANEL);
    }
    return cardPanel;
}

private JPanel getDefaultPanel() {
    if(defaultPanel == null){
        defaultPanel = new JPanel();
    }
    return defaultPanel;
}

private JPanel getManualPanel() {
    if(manualPanel == null){
        manualPanel = new JPanel();
        manualPanel.setLayout(new FlowLayout());

        manualEntryField = new JTextField(10);
        System.out.println("creating text field");
        manualEntryField.setText("Enter code");

        manualAssignButton = new JButton("Assign");
        manualAssignButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
             // Here is the exception
            System.out.println(manualEntryField);
            System.out.println(manualEntryField.getText());
            System.out.println(manualEntryField.getText().trim());
            }
        });

        manualLabel = new JLabel("Please enter a code and press assign");

        manualPanel.add(manualEntryField);
        manualPanel.add(manualAssignButton);
        manualPanel.add(manualLabel);

    }
    return manualPanel;
}

private JPanel getUploadPanel() {
    if(uploadPanel == null){
        uploadPanel = new JPanel();
    }
    return uploadPanel;
}

private void closeApplication() {
    System.exit(0);
}
}

注意：我无法完全解释为什么这些更改解决了 NPE，我只是重构“可疑”代码。

After looking at your SSCCE, I managed to solve the NPE by commenting out the redundant Test that was instantiated, and creating the panel array in the constructor. E.G.

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Test{

//public static final Test application = new Test();
private JFrame jFrame = null;
private JPanel jContentPane = null, buttonsPanel = null, cardPanel = null, manualPanel = null, uploadPanel = null,
        defaultPanel = null;
private JPanel[] mainPanel = new JPanel[3];
private JButton manualButton = null, uploadButton = null, manualAssignButton = null;
private JTextField manualEntryField = null;
private JLabel manualLabel = null;

final static String DEFAULTPANEL = "Default";
final static String MANUALPANEL = "Manual";
final static String UPLOADPANEL = "Upload";

Test() {
    mainPanel[0] = getDefaultPanel();
    mainPanel[1] = getManualPanel();
    mainPanel[2] = getUploadPanel();
}

/**
 * Main method
 * @param args
 */
public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            new Test().getJFrame().setVisible(true);
        }
    });
}

/**
 * Get the main application frame
 * @return JFrame
 */
private JFrame getJFrame() {
    if (jFrame == null) {
        jFrame = new JFrame();
        jFrame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
        jFrame.setResizable(false);
        // Add the main content pane
        jFrame.setContentPane(getJContentPanel());
        jFrame.addWindowListener(new java.awt.event.WindowAdapter() {
            public void windowClosing(java.awt.event.WindowEvent e) {
                closeApplication();
            }
        });
    }
    jFrame.pack();
    jFrame.setLocationRelativeTo(null);
    return jFrame;
}

/**
 * Get main content
 * @return JPanel
 */
private JPanel getJContentPanel(){
    if (jContentPane == null) {
        jContentPane = new JPanel();
        jContentPane.setPreferredSize(new Dimension(500, 200));
        jContentPane.setLayout(new BorderLayout());
        jContentPane.add(getButtonsPanel(), BorderLayout.NORTH);
        jContentPane.add(getCardPanel(), BorderLayout.CENTER);
    }
    return jContentPane;
}

private JPanel getButtonsPanel() {
    if(buttonsPanel == null){
        buttonsPanel = new JPanel();
        buttonsPanel.setPreferredSize(new Dimension(500, 50));
        buttonsPanel.setBorder(BorderFactory.createLineBorder(Color.gray));

        manualButton = new JButton("Enter manually");
        manualButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                CardLayout cl = (CardLayout)(cardPanel.getLayout());
                cl.show(cardPanel, MANUALPANEL);
            }
        });

        uploadButton = new JButton("Upload file");
        uploadButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                CardLayout cl = (CardLayout)(cardPanel.getLayout());
                cl.show(cardPanel, UPLOADPANEL);
            }
        });

        buttonsPanel.add(manualButton);
        buttonsPanel.add(uploadButton);
    }
    return buttonsPanel;
}

private JPanel getCardPanel() {
    if(cardPanel == null){
        cardPanel = new JPanel(new CardLayout());
        cardPanel.setPreferredSize(new Dimension(500, 200));
        cardPanel.setBorder(BorderFactory.createLineBorder(Color.gray));
        cardPanel.add(mainPanel[0], DEFAULTPANEL);
        cardPanel.add(mainPanel[1], MANUALPANEL);
        cardPanel.add(mainPanel[2], UPLOADPANEL);
    }
    return cardPanel;
}

private JPanel getDefaultPanel() {
    if(defaultPanel == null){
        defaultPanel = new JPanel();
    }
    return defaultPanel;
}

private JPanel getManualPanel() {
    if(manualPanel == null){
        manualPanel = new JPanel();
        manualPanel.setLayout(new FlowLayout());

        manualEntryField = new JTextField(10);
        System.out.println("creating text field");
        manualEntryField.setText("Enter code");

        manualAssignButton = new JButton("Assign");
        manualAssignButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
             // Here is the exception
            System.out.println(manualEntryField);
            System.out.println(manualEntryField.getText());
            System.out.println(manualEntryField.getText().trim());
            }
        });

        manualLabel = new JLabel("Please enter a code and press assign");

        manualPanel.add(manualEntryField);
        manualPanel.add(manualAssignButton);
        manualPanel.add(manualLabel);

    }
    return manualPanel;
}

private JPanel getUploadPanel() {
    if(uploadPanel == null){
        uploadPanel = new JPanel();
    }
    return uploadPanel;
}

private void closeApplication() {
    System.exit(0);
}
}

Note: I cannot quite explain why the changes solved the NPE, I was just refactoring 'suspicious' code.

回复 原文
把时间冻结 2025-01-07 23:14:49

我确实解决了与 NullPointerException 相关的查询：

问题是您声明类变量：

JTextField manualEntryField = null;

只需编写：

private JTextField manualEntryField;

只需删除那个 null 的东西，反对它，其余的代码就可以了。无法说出为什么会发生，但可能是其他人可以。

这是您的代码，仅进行了更改，请立即尝试：

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Test{

public static final Test application = new Test();
    private JFrame jFrame = null;
    private JPanel jContentPane = null, buttonsPanel = null, cardPanel = null, manualPanel = null, uploadPanel = null, defaultPanel = null;
    private JPanel[] mainPanel = {getDefaultPanel(), getManualPanel(), getUploadPanel()};
    private JButton manualButton = null, uploadButton = null, manualAssignButton = null;
    private JTextField manualEntryField;
    private JLabel manualLabel = null;

    final static String DEFAULTPANEL = "Default";
    final static String MANUALPANEL = "Manual";
    final static String UPLOADPANEL = "Upload";

    /**
     * Main method
     * @param args
     */
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                application.getJFrame().setVisible(true);
            }
        });
    }

    /**
     * Get the main application frame
     * @return JFrame
     */
    private JFrame getJFrame() {
        if (jFrame == null) {
            jFrame = new JFrame();
            jFrame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
            jFrame.setResizable(false);
            // Add the main content pane
            jFrame.setContentPane(getJContentPanel());
            jFrame.addWindowListener(new java.awt.event.WindowAdapter() {   
                public void windowClosing(java.awt.event.WindowEvent e) {    
                    closeApplication();
                }
            });
        }
        jFrame.pack();
        jFrame.setLocationRelativeTo(null);
        return jFrame;
    }

    /**
     * Get main content
     * @return JPanel
     */
    private JPanel getJContentPanel(){
        if (jContentPane == null) {
            jContentPane = new JPanel();
            jContentPane.setPreferredSize(new Dimension(500, 200));
            jContentPane.setLayout(new BorderLayout());
            jContentPane.add(getButtonsPanel(), BorderLayout.NORTH);
            jContentPane.add(getCardPanel(), BorderLayout.CENTER);
        }
        return jContentPane;
    }

    private JPanel getButtonsPanel() {
        if(buttonsPanel == null){
            buttonsPanel = new JPanel();
            buttonsPanel.setPreferredSize(new Dimension(500, 50));
            buttonsPanel.setBorder(BorderFactory.createLineBorder(Color.gray));

            manualButton = new JButton("Enter manually");
            manualButton.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    CardLayout cl = (CardLayout)(cardPanel.getLayout());
                    cl.show(cardPanel, MANUALPANEL);
                }
            });

            uploadButton = new JButton("Upload file");
            uploadButton.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    CardLayout cl = (CardLayout)(cardPanel.getLayout());
                    cl.show(cardPanel, UPLOADPANEL);
                }
            });

            buttonsPanel.add(manualButton);
            buttonsPanel.add(uploadButton);
        }
        return buttonsPanel;
    }

    private JPanel getCardPanel() {
        if(cardPanel == null){
            cardPanel = new JPanel(new CardLayout());
            cardPanel.setPreferredSize(new Dimension(500, 200));
            cardPanel.setBorder(BorderFactory.createLineBorder(Color.gray));
            cardPanel.add(mainPanel[0], DEFAULTPANEL);
            cardPanel.add(mainPanel[1], MANUALPANEL);
            cardPanel.add(mainPanel[2], UPLOADPANEL);
        }
        return cardPanel;
    }

    private JPanel getDefaultPanel() {
        if(defaultPanel == null){
            defaultPanel = new JPanel();
        }
        return defaultPanel;
    }

    private JPanel getManualPanel() {
        if(manualPanel == null){
            manualPanel = new JPanel();
            manualPanel.setLayout(new FlowLayout());

            manualEntryField = new JTextField(10);
            manualEntryField.setText("Enter code");

            manualAssignButton = new JButton("Assign");
            manualAssignButton.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                 // Here is the exception
                    System.out.println(manualEntryField.getText().trim());
                }
            });

            manualLabel = new JLabel("Please enter a code and press assign");

            manualPanel.add(manualEntryField);
            manualPanel.add(manualAssignButton);
            manualPanel.add(manualLabel);

        }
        return manualPanel;
    }

    private JPanel getUploadPanel() {
        if(uploadPanel == null){
            uploadPanel = new JPanel();
        }
        return uploadPanel;
    }

    private void closeApplication() {
        System.exit(0);
    }
}

问候

I did solved your query related to NullPointerException :

The problem being that you declaring your class variable :

JTextField manualEntryField = null;

Just write :

private JTextField manualEntryField;

Just remove that null thing, against it and the rest of the code is fine. Cannot say as to why it happened, but might be someone else can.

Here is your code with only that change, do try this out now :

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Test{

public static final Test application = new Test();
    private JFrame jFrame = null;
    private JPanel jContentPane = null, buttonsPanel = null, cardPanel = null, manualPanel = null, uploadPanel = null, defaultPanel = null;
    private JPanel[] mainPanel = {getDefaultPanel(), getManualPanel(), getUploadPanel()};
    private JButton manualButton = null, uploadButton = null, manualAssignButton = null;
    private JTextField manualEntryField;
    private JLabel manualLabel = null;

    final static String DEFAULTPANEL = "Default";
    final static String MANUALPANEL = "Manual";
    final static String UPLOADPANEL = "Upload";

    /**
     * Main method
     * @param args
     */
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                application.getJFrame().setVisible(true);
            }
        });
    }

    /**
     * Get the main application frame
     * @return JFrame
     */
    private JFrame getJFrame() {
        if (jFrame == null) {
            jFrame = new JFrame();
            jFrame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
            jFrame.setResizable(false);
            // Add the main content pane
            jFrame.setContentPane(getJContentPanel());
            jFrame.addWindowListener(new java.awt.event.WindowAdapter() {   
                public void windowClosing(java.awt.event.WindowEvent e) {    
                    closeApplication();
                }
            });
        }
        jFrame.pack();
        jFrame.setLocationRelativeTo(null);
        return jFrame;
    }

    /**
     * Get main content
     * @return JPanel
     */
    private JPanel getJContentPanel(){
        if (jContentPane == null) {
            jContentPane = new JPanel();
            jContentPane.setPreferredSize(new Dimension(500, 200));
            jContentPane.setLayout(new BorderLayout());
            jContentPane.add(getButtonsPanel(), BorderLayout.NORTH);
            jContentPane.add(getCardPanel(), BorderLayout.CENTER);
        }
        return jContentPane;
    }

    private JPanel getButtonsPanel() {
        if(buttonsPanel == null){
            buttonsPanel = new JPanel();
            buttonsPanel.setPreferredSize(new Dimension(500, 50));
            buttonsPanel.setBorder(BorderFactory.createLineBorder(Color.gray));

            manualButton = new JButton("Enter manually");
            manualButton.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    CardLayout cl = (CardLayout)(cardPanel.getLayout());
                    cl.show(cardPanel, MANUALPANEL);
                }
            });

            uploadButton = new JButton("Upload file");
            uploadButton.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    CardLayout cl = (CardLayout)(cardPanel.getLayout());
                    cl.show(cardPanel, UPLOADPANEL);
                }
            });

            buttonsPanel.add(manualButton);
            buttonsPanel.add(uploadButton);
        }
        return buttonsPanel;
    }

    private JPanel getCardPanel() {
        if(cardPanel == null){
            cardPanel = new JPanel(new CardLayout());
            cardPanel.setPreferredSize(new Dimension(500, 200));
            cardPanel.setBorder(BorderFactory.createLineBorder(Color.gray));
            cardPanel.add(mainPanel[0], DEFAULTPANEL);
            cardPanel.add(mainPanel[1], MANUALPANEL);
            cardPanel.add(mainPanel[2], UPLOADPANEL);
        }
        return cardPanel;
    }

    private JPanel getDefaultPanel() {
        if(defaultPanel == null){
            defaultPanel = new JPanel();
        }
        return defaultPanel;
    }

    private JPanel getManualPanel() {
        if(manualPanel == null){
            manualPanel = new JPanel();
            manualPanel.setLayout(new FlowLayout());

            manualEntryField = new JTextField(10);
            manualEntryField.setText("Enter code");

            manualAssignButton = new JButton("Assign");
            manualAssignButton.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                 // Here is the exception
                    System.out.println(manualEntryField.getText().trim());
                }
            });

            manualLabel = new JLabel("Please enter a code and press assign");

            manualPanel.add(manualEntryField);
            manualPanel.add(manualAssignButton);
            manualPanel.add(manualLabel);

        }
        return manualPanel;
    }

    private JPanel getUploadPanel() {
        if(uploadPanel == null){
            uploadPanel = new JPanel();
        }
        return uploadPanel;
    }

    private void closeApplication() {
        System.exit(0);
    }
}

Regards

回复 原文
撩发小公举 2025-01-07 23:14:49

如果textBox为空，则此异常是正常的。试试这个
if(getText().size()<1) 条件或使用 try-catch，当捕获异常时设置值非大小字符串“”。

If textBox is empty, it's normal for this exception. Try this
if(getText().size()<1) condition or use try-catch and when you catch an exception set the value non-size string "".

回复 原文
只有一腔孤勇 2025-01-07 23:14:49

当文本框为空时您是否单击按钮？如果是这样，那么当您调用trim()函数时，您可能会遇到该异常，因此在调用trim()之前，请先查看getText()返回的内容

Are you clicking the button when your text box is empty ? If so, then u can end up with that exception when u r calling the trim() function , so see what getText() returns , before calling the trim()

回复 原文
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