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我正在尝试为自动售货机模拟器做一个硬币计数器，并且我正试图将硬币的数量放入JLabel中。我对Java比较陌生，但我在其他类中有不同的JLabel可以更改;但是，这似乎只是拒绝改变！奇怪的是，正确的值出现在JOptionPane消息中，但不会更改JLabel。这肯定意味着正在使用来自其他课程的正确值。 （coin200Lbl是公开的 - 如果这有什么区别的话）。Java变量不会更改JLabel（NullPointerException）的文本，但会显示在JOptionDialog框中？
（忽略的无数无名的变量，我想我会尝试和命名其余部分之前做一个工作的首位正确:)）
可能是我已经错过了一个愚蠢的错误，但我一直在试图找出没有成功的日子。
在此先感谢！
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
}
的空指针异常是：
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)
生成成功（总时间：11秒）， '
其余的代码在哪里？ NPE在17号线上，并且你没有向我们展示17条线路。 :-) – Amber 2015-03-31 16:37:14
编辑了这篇文章，我错过了复制和粘贴中的前几行。第17行是：coin200.setText（“£2：”+ Integer.toString（amount200p）+“coins”）;.干杯！ – 2015-03-31 17:02:02