java - Java 新手。按任意按钮时计算器程序出现奇怪的错误

Question

我现在才使用 Java 大约一个月，所以我还没有完全掌握它。我被分配为类制作一个带有 GUI 的计算器程序。除了用户可以输入多个小数点之外，我一切都正常。所以我尝试解决这个问题(我没有)，现在程序将运行而不会出现任何错误，但是每当我单击按钮(例如 1、2、3)或运算符(例如 +、- 等)时，我得到出现以下错误:

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 等等。

有人知道这可能是什么吗？当我尝试解决小数点问题时，我不记得更改过任何我所知道的内容，并且我将代码更改回上次工作时的状态

错误表明它发生在第 190 行，即 result.setText(result.getText() + "1");

当然，如果我单击不同的按钮，行号将会改变，这只是用户按下数字 1 按钮的示例

抱歉，这篇文章太长了，希望大家能帮忙:)

Answer 1

如果使用任何类型的 IDE(Eclipse、NetBeans...)，您甚至可以单击InLab05.java:190 部分

at inlab05.InLab05$event.actionPerformed(InLab05.java:190)

并且会直接带你到 NullPointerException 发生的地方。