我现在才使用 Java 大约一个月，所以我还没有完全掌握它。我被分配为课堂制作一个带有 GUI 的计算器程序。除了用户可以输入多个小数点之外，我一切正常。所以我试图解决这个问题（我没有），现在程序将运行没有任何错误，但每当我点击一个按钮，如 1、2、3，或像 +、- 等运算符时，我得到以下错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at inlab05.InLab05$event.actionPerformed(InLab05.java:190)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
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:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
这是我认为错误所在的代码部分：
public class event implements ActionListener {
public void actionPerformed(ActionEvent a) {
String text = a.getActionCommand();
if (text.equals("1")) {
result.setText(result.getText() + "1");
} else if (text.equals("2")) {
result.setText(result.getText() + "2");
} else if (text.equals("3")) {
result.setText(result.getText() + "3");
} else if (text.equals("4")) {
result.setText(result.getText() + "4");
} else if (text.equals("5")) {
result.setText(result.getText() + "5");
} else if (text.equals("6")) {
result.setText(result.getText() + "6");
} else if (text.equals("7")) {
result.setText(result.getText() + "7");
} else if (text.equals("8")) {
result.setText(result.getText() + "8");
} else if (text.equals("9")) {
result.setText(result.getText() + "9");
} else if (text.equals("0")) {
result.setText(result.getText() + "0");
} else if (text.equals(".")) {
result.setText(result.getText() + ".");
}
String str = result.getText();
textBox = Double.parseDouble(str);
if (a.getSource()
== textAdd) {
op = 1;
firstInput = textBox;
result.setText("");
}
if (a.getSource()
== textSubtract) {
op = 2;
firstInput = textBox;
result.setText("");
}
if (a.getSource()
== textMultiply) {
op = 3;
firstInput = textBox;
result.setText("");
}
if (a.getSource()
== textDivide) {
op = 4;
firstInput = textBox;
result.setText("");
}
if (a.getSource()
== textPercent) {
op = 5;
firstInput = textBox;
result.setText("");
}
if (a.getSource()
== textSqrt) {
op = 6;
firstInput = textBox;
answer = Math.sqrt(textBox);
str = Double.toString(answer);
result.setText(str);
}
if (a.getSource()
== textSign) {
double neg;
op = 7;
neg = 0 - textBox;
str = Double.toString(neg);
result.setText(str);
}
if (a.getSource()
== textEqual) {
if (op == 1) {
answer = firstInput + textBox;
str = Double.toString(answer);
result.setText(str);
} else if (op == 2) {
answer = firstInput - textBox;
str = Double.toString(answer);
result.setText(str);
} else if (op == 3) {
answer = firstInput * textBox;
str = Double.toString(answer);
result.setText(str);
} else if (op == 4) {
answer = firstInput / textBox;
str = Double.toString(answer);
result.setText(str);
} else if (op == 5) {
answer = firstInput % textBox;
str = Double.toString(answer);
result.setText(str);
}
}
}
}
其余的代码是 GUI 和什么不是。
有谁知道这可能是什么？我不记得在尝试修复小数点问题时更改了我所知道的任何内容，并且我将代码更改回上次工作时的状态
错误说它发生在第 190 行，即result.setText(result.getText() + "1");
当然，如果我单击不同的按钮，行号会改变，这只是用户按下数字 1 按钮的示例
很抱歉这么长的帖子希望你们能帮忙:)