Java变量不会更改JLabel的文本（NullPointerException），但会显示在JOptionDialog框中吗？

Question

I'm trying to do a coin counter for a Vending Machine simulator, and I am trying to put the number of coins into a JLabel. I'm relatively new to Java, but i've got different JLabels in other classes to change; however this one seems to just refuse to change! The strange thing is that the correct value shows up in the JOptionPane message, but doesn't change the JLabel. This would surely mean that the correct value from the other class is being used. (The coins200Lbl is public - if that makes any difference).

(Ignore the countless unnamed variables, I thought i'd try and make one work first before naming the rest properly :) )

May be a stupid mistake that i've missed, but i've been trying to figure it out for days with no success.

Thanks in advance!

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                   

}

The NullPointerException is:

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)

BUILD SUCCESSFUL (total time: 11 seconds) `

Answer 1

Okay James, the real problem is the JLabel coin200 is not initialized, but you'd attempted to call the setText() method of the JLabel. So you are attempting to call the setText() method of nothing ( null ). The solution is, call the initComponents() method at the beginning of the addMenu(int [] c) method. Just like this

public adminMenu(int[] c) {

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