如何在 Java 中更新使用 NetBeans 图形 GUI 设计器创建的文本字段

Question

几十年来我一直在编写嵌入式 C 代码，我希望通过学习 Java 来更新我的技能。

我还需要一个项目的图形测试工具（读取串口、处理数据、根据 GUI 按钮选择将结果写入串口等）

I'm trying to use NetBeans graphical GUI designer to create the GUI and then add Java code to implement the logic but I'm still learning Java so I'm not sure how to access the text field in my GUI from the Java code. 具体来说，我想要一个屏幕上的文本框，它每秒更新一次当前时间（这最终将成为一个健康检查路由，寻找错过的命令超时等）。

我遇到的问题是如何从我的 healthCheck() 例程中访问名为 timeDisplayField 的文本字段 object。 每秒都会调用一次 healthCheck() 例程，但是当我取消注释任何尝试到达 timeDisplayField object 时，我会收到编译错误或 NullPointer 运行时错误。

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */

package myPackage;

import java.util.Calendar;
import java.util.Timer;
import java.util.TimerTask;
import java.util.Date;

/**
 *
 * @author SGrunza
 */


public class mainWindow extends javax.swing.JFrame {

    /**
     * Creates new form mainWindow
     */



        public void healthCheck()
{
    Calendar    now = Calendar.getInstance();
    int         hour = now.get( Calendar.HOUR_OF_DAY );
    int         minute = now.get( Calendar.MINUTE );
    int         second = now.get( Calendar.SECOND );

    String      timeString = new String();

    //System.out.printf("%02d:%02d:%02d\r\n", hour, minute, second );

    timeString = timeString.format( "%02d:%02d:%02d\r\n", hour, minute, second );
    timeDisplayField.setText( timeString );
}

    public mainWindow() 
    {
        Timer       healthTimer = new Timer();

        healthTimer.scheduleAtFixedRate( new TimerTask()
                {
                    @Override 
                    public void run()
                {
                    //System.out.println( "healthTask " + new Date() +
                    //        "Thread name: " + Thread.currentThread().getName() );
                    healthCheck();
                }
                },
                0,  // delay until first run
                1000 ); // delay for each subsequent run


        initComponents();
    }

    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">//GEN-BEGIN:initComponents
    private void initComponents() {

        interfaceTypeGroup = new javax.swing.ButtonGroup();
        jPanel1 = new javax.swing.JPanel();
        nfcButton = new javax.swing.JRadioButton();
        smipButton = new javax.swing.JRadioButton();
        serialButton = new javax.swing.JRadioButton();
        appStatusField = new javax.swing.JTextField();
        msgField = new javax.swing.JTextField();
        cardReaderStatusField = new javax.swing.JTextField();
        timeDisplayField = new javax.swing.JTextField();
        activateButton = new javax.swing.JButton();
        readButton = new javax.swing.JButton();
        deactivateButton = new javax.swing.JButton();
        checkSleepButton = new javax.swing.JButton();
        getInfoButton = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        interfaceTypeGroup.add(nfcButton);
        nfcButton.setSelected(true);
        nfcButton.setText("NFC");

        interfaceTypeGroup.add(smipButton);
        smipButton.setForeground(new java.awt.Color(153, 153, 153));
        smipButton.setText("SMIP");

        interfaceTypeGroup.add(serialButton);
        serialButton.setForeground(new java.awt.Color(153, 153, 153));
        serialButton.setText("Serial");

        appStatusField.setText("appStatus");

        msgField.setText("msgWindow");

        cardReaderStatusField.setText("cardReader_status");

        timeDisplayField.setText("timeDisplay");

        activateButton.setLabel("Activate");

        readButton.setText("Read Sensor");

        deactivateButton.setText("Deactivate");

        checkSleepButton.setText("Check Sleep");

        getInfoButton.setText("Get Info");

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(msgField)
                    .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING, false)
                        .addComponent(activateButton, javax.swing.GroupLayout.Alignment.LEADING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(deactivateButton, javax.swing.GroupLayout.Alignment.LEADING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(readButton, javax.swing.GroupLayout.Alignment.LEADING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addComponent(getInfoButton, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addContainerGap())
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addComponent(checkSleepButton, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addGap(12, 12, 12))))
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addGap(12, 12, 12)
                        .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(nfcButton)
                            .addComponent(smipButton)
                            .addComponent(serialButton))
                        .addGap(18, 18, 18)
                        .addComponent(cardReaderStatusField))
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addGap(12, 12, 12)
                        .addComponent(appStatusField, javax.swing.GroupLayout.PREFERRED_SIZE, 203, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 564, Short.MAX_VALUE)
                        .addComponent(timeDisplayField, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)))
                .addGap(12, 12, 12))
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addComponent(nfcButton)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(smipButton)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(serialButton))
                    .addComponent(cardReaderStatusField))
                .addGap(18, 18, 18)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(timeDisplayField, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(appStatusField, javax.swing.GroupLayout.PREFERRED_SIZE, 63, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addGap(18, 18, 18)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(activateButton)
                    .addComponent(getInfoButton))
                .addGap(18, 18, 18)
                .addComponent(readButton)
                .addGap(18, 18, 18)
                .addComponent(deactivateButton)
                .addGap(37, 37, 37)
                .addComponent(checkSleepButton)
                .addGap(18, 18, 18)
                .addComponent(msgField, javax.swing.GroupLayout.DEFAULT_SIZE, 217, Short.MAX_VALUE)
                .addContainerGap())
        );

        timeDisplayField.getAccessibleContext().setAccessibleName("timeDisplayField");

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        );

        pack();
    }// </editor-fold>//GEN-END:initComponents

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(mainWindow.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(mainWindow.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(mainWindow.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(mainWindow.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                new mainWindow().setVisible(true);
            }
        });
    }


    // Variables declaration - do not modify//GEN-BEGIN:variables
    public javax.swing.JButton activateButton;
    public javax.swing.JTextField appStatusField;
    public javax.swing.JTextField cardReaderStatusField;
    public javax.swing.JButton checkSleepButton;
    public javax.swing.JButton deactivateButton;
    public javax.swing.JButton getInfoButton;
    public javax.swing.ButtonGroup interfaceTypeGroup;
    public javax.swing.JPanel jPanel1;
    public javax.swing.JTextField msgField;
    public javax.swing.JRadioButton nfcButton;
    public javax.swing.JButton readButton;
    public javax.swing.JRadioButton serialButton;
    public javax.swing.JRadioButton smipButton;
    public javax.swing.JTextField timeDisplayField;
    // End of variables declaration//GEN-END:variables
}

我对 Java 还很陌生，不知道在哪里寻求帮助。 我怀疑部分问题在于使用 NetBeans GUI 构建器，但我真的不想手动构建所有 GUI。

将其归结为一个简单的问题：如何更改 healthCheck() 以更新屏幕上的文本字段？

Answer 1

答案在什么是 NullPointerException 以及如何修复它。 您可以在堆栈跟踪（错误）中看到它：

Exception in thread "Timer-0" java.lang.NullPointerException
    at rae.mainWindow.healthCheck(mainWindow.java:24)
    at rae.mainWindow$1.run(mainWindow.java:35)
    at java.util.TimerThread.mainLoop(Timer.java:555)
    at java.util.TimerThread.run(Timer.java:505)

在第 24 行，你有这一行：

timeDisplayField.setText(timeString);

timeString不是null这意味着timeDisplayField是null 。 这是因为您首先启动计时器，然后调用创建文本字段的initComponents方法：

public mainWindow() {
    Timer healthTimer = new Timer();

    healthTimer.scheduleAtFixedRate(new TimerTask() {
        @Override
        public void run() {
            // System.out.println( "healthTask " + new Date() +
            // "Thread name: " + Thread.currentThread().getName() );
            healthCheck();
        }
    }, 0, // delay until first run
            1000); // delay for each subsequent run

    initComponents(); //here
}

您可以通过在时间启动之前调用 initComponents 来修复它：

public mainWindow() {
    Timer healthTimer = new Timer();

    initComponents(); //Before timer

    healthTimer.scheduleAtFixedRate(new TimerTask() {
        @Override
        public void run() {
            // System.out.println( "healthTask " + new Date() +
            // "Thread name: " + Thread.currentThread().getName() );
            healthCheck();
        }
    }, 0, // delay until first run
            1000); // delay for each subsequent run

}

但是，不建议在 Swing 环境中使用java.util.Timer 。 考虑将其更改为为此类情况创建的Swing 计时器。 在这篇文章中可以找到一个很好的用法示例。