Java變量不會更改JLabel的文本（NullPointerException），但會顯示在JOptionDialog框中嗎？

Question

我正在嘗試為自動售貨機模擬器做一個硬幣計數器，並且試圖將硬幣數量放入JLabel中。 我是Java的新手，但是在其他類別中我有不同的JLabel可以更改。 但是，這似乎只是拒絕更改！ 奇怪的是，正確的值顯示在JOptionPane消息中，但不會更改JLabel。 這肯定意味着正在使用其他類的正確值。 （coins200Lbl是公開的，如果有區別的話）。

（忽略無數的未命名變量，我想我會先嘗試做一個工作，然后再正確命名其余的:)）

可能是我錯過的一個愚蠢的錯誤，但是我已經嘗試了好幾天，但都沒有成功。

提前致謝！

package vendingmachine;

import javax.swing.*;

public class adminMenu extends javax.swing.JFrame {
int coins;
int[] coinCount;

public adminMenu() {
    initComponents();

}
public adminMenu(int[] c) {
    coinCount = c;
    int amount200p = coinCount[7]/200;
    JOptionPane.showMessageDialog(null, amount200p);
    coin200.setText("£2: " + Integer.toString(amount200p) + " coins");
}



public int calcCoins() {
    return coins; 
}


@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">                          
private void initComponents() {

    jLabel6 = new javax.swing.JLabel();
    jLabel8 = new javax.swing.JLabel();
    jLabel13 = new javax.swing.JLabel();
    jButton1 = new javax.swing.JButton();
    jButton2 = new javax.swing.JButton();
    fundsLbl = new javax.swing.JLabel();
    jLabel3 = new javax.swing.JLabel();
    jLabel4 = new javax.swing.JLabel();
    jLabel5 = new javax.swing.JLabel();
    jLabel7 = new javax.swing.JLabel();
    jLabel9 = new javax.swing.JLabel();
    jLabel10 = new javax.swing.JLabel();
    jLabel11 = new javax.swing.JLabel();
    homeBtn = new javax.swing.JButton();
    coin200 = new javax.swing.JLabel();

    jLabel6.setText("£2:");

    jLabel8.setText("£2:");

    jLabel13.setText("0");

    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

    jButton1.setText("Withdraw Funds");
    jButton1.addActionListener(new java.awt.event.ActionListener() {
        public void actionPerformed(java.awt.event.ActionEvent evt) {
            jButton1ActionPerformed(evt);
        }
    });

    jButton2.setText("Insert Funds");

    fundsLbl.setFont(new java.awt.Font("Tahoma", 0, 14)); // NOI18N
    fundsLbl.setText("Current funds:");

    jLabel3.setText("£1:");

    jLabel4.setText("50p:");

    jLabel5.setText("20p:");

    jLabel7.setText("10p:");

    jLabel9.setText("5p:");

    jLabel10.setText("2p:");

    jLabel11.setText("1p:");

    homeBtn.setText("Home");
    homeBtn.addActionListener(new java.awt.event.ActionListener() {
        public void actionPerformed(java.awt.event.ActionEvent evt) {
            homeBtnActionPerformed(evt);
        }
    });

    coin200.setText("£2:");

    javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
    getContentPane().setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addGap(22, 22, 22)
            .addComponent(fundsLbl)
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(layout.createSequentialGroup()
                    .addGap(44, 44, 44)
                    .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
                        .addComponent(jLabel3)
                        .addComponent(jLabel4)
                        .addComponent(jLabel5))
                    .addGap(112, 112, 112))
                .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                    .addComponent(coin200, javax.swing.GroupLayout.PREFERRED_SIZE, 101, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addGap(27, 27, 27)))
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addComponent(jLabel7)
                .addComponent(jLabel9, javax.swing.GroupLayout.Alignment.TRAILING)
                .addComponent(jLabel10, javax.swing.GroupLayout.Alignment.TRAILING)
                .addComponent(jLabel11, javax.swing.GroupLayout.Alignment.TRAILING))
            .addContainerGap(137, Short.MAX_VALUE))
        .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
            .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
            .addComponent(jButton2)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
            .addComponent(jButton1)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
            .addComponent(homeBtn)
            .addGap(81, 81, 81))
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addContainerGap()
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(layout.createSequentialGroup()
                    .addComponent(jLabel7)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                    .addComponent(jLabel9)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                    .addComponent(jLabel10)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                    .addComponent(jLabel11)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                .addGroup(layout.createSequentialGroup()
                    .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                        .addComponent(fundsLbl)
                        .addComponent(coin200))
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                    .addComponent(jLabel3)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                    .addComponent(jLabel4)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                    .addComponent(jLabel5)
                    .addGap(18, 18, 18)))
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                .addComponent(jButton1)
                .addComponent(jButton2)
                .addComponent(homeBtn))
            .addContainerGap())
    );

    pack();
}// </editor-fold>                        


private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    // TODO add your handling code here:
}                                        

private void homeBtnActionPerformed(java.awt.event.ActionEvent evt) {                                        
    this.setVisible(false);
    new VendingMachineGUI().setVisible(true);
}                                       

public static void main(String args[]) {
    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(adminMenu.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (InstantiationException ex) {
        java.util.logging.Logger.getLogger(adminMenu.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (IllegalAccessException ex) {
        java.util.logging.Logger.getLogger(adminMenu.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (javax.swing.UnsupportedLookAndFeelException ex) {
        java.util.logging.Logger.getLogger(adminMenu.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    }
    //</editor-fold>

    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new adminMenu().setVisible(true);
        }
    });
}

// Variables declaration - do not modify                     
public javax.swing.JLabel coin200;
private javax.swing.JLabel fundsLbl;
private javax.swing.JButton homeBtn;
private javax.swing.JButton jButton1;
private javax.swing.JButton jButton2;
private javax.swing.JLabel jLabel10;
private javax.swing.JLabel jLabel11;
private javax.swing.JLabel jLabel13;
private javax.swing.JLabel jLabel3;
private javax.swing.JLabel jLabel4;
private javax.swing.JLabel jLabel5;
private javax.swing.JLabel jLabel6;
private javax.swing.JLabel jLabel7;
private javax.swing.JLabel jLabel8;
private javax.swing.JLabel jLabel9;
// End of variables declaration                   

}

NullPointerException是：

run:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at vendingmachine.adminMenu.<init>(adminMenu.java:17)
at vendingmachine.VendingMachineGUI.adminBtnActionPerformed(VendingMachineGUI.java:551)
at vendingmachine.VendingMachineGUI.access$1800(VendingMachineGUI.java:5)
at vendingmachine.VendingMachineGUI$19.actionPerformed(VendingMachineGUI.java:213)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6525)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6290)
at java.awt.Container.processEvent(Container.java:2234)
at java.awt.Component.dispatchEventImpl(Component.java:4881)
at java.awt.Container.dispatchEventImpl(Container.java:2292)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
at java.awt.Container.dispatchEventImpl(Container.java:2278)
at java.awt.Window.dispatchEventImpl(Window.java:2750)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:751)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:702)
at java.awt.EventQueue$3.run(EventQueue.java:696)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:724)
at java.awt.EventQueue$4.run(EventQueue.java:722)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:721)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

建立成功（總時間：11秒）`

Answer 1

好的，詹姆斯，真正的問題是JLabel coin200沒有初始化，但是您試圖調用JLabel的setText()方法。 因此，您嘗試不調用setText()方法（ null ）。 解決方法是，在addMenu(int [] c)方法的開頭調用initComponents() addMenu(int [] c)方法。 像這樣

public adminMenu(int[] c) {

    initComponents();
    coinCount = c;
    int amount200p = coinCount[7]/200;
    JOptionPane.showMessageDialog(null, amount200p);
    coin200.setText("£2: " + Integer.toString(amount200p) + " coins");
}