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我试图创建一个函数，让我可以一口气创建一组 JPanel 对象。但是，当我尝试处理由用户输入字段创建的数据时，我得到一个NullPointerException. 如果我创建标签/字段并将它们添加到我的buildPanel方法中的面板中，一切都很好。使用这种newSet方法，我得到了错误。对象正确显示在窗口中，但我的听众似乎无法从中获取信息。我也尝试不将面板传递给函数，但这没有区别。我可以用这种方式注册文本字段吗？

我假设它不会将 应用于textField对象daysF，而是创建一个名为“文本”的标签，该标签在函数完成后并不真正存在。

private void newSet(int textLength, String labelText, 
                   JTextField text, JLabel label, JPanel panel){
    label = new JLabel(labelText);
    text = new JTextField(textLength);
    panel.add(label);
    panel.add(text);
}

private void buildPanel(){
    panel = new JPanel();       
    newSet(3, "Days gone: ", daysF, daysL, panel);
}

这是侦听器，它将文本输入转换为双精度，以便稍后进行一些计算。

private class CalcButtonListener implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        double days = fieldToDouble(daysF);
}

我NullPointerException来了，带着field.getText()。

线程“AWT-EventQueue-0”中的异常 java.lang.NullPointerException

private double fieldToDouble(JTextField field){
    String str = field.getText();
    if (str.equals("")) str = "0";
    double amt = Double.parseDouble(str);
    return amt;
}

这也是堆栈跟踪。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at TravelExpenses.fieldToDouble(TravelExpenses.java:109)
at TravelExpenses.access$1(TravelExpenses.java:108)
at TravelExpenses$CalcButtonListener.actionPerformed(TravelExpenses.java:120)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
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