java - java制作计算器时出现NumberFormatException等问题

Question

所以我正在制作一个 Java 计算器，并在单击功能按钮时遇到一些错误(尽管它编译得很好)。我的 CMD 中第一行的错误是 NumberFormat 异常。经过一番搜索后，我发现在我的代码中的某个地方，我试图从空字符串中获取 double 值(基本上我想做的是从标签文本中解析 double 值)。我猜是这部分:

// if statement that puts the labels text into the first or second number
    if(firstNumber)
    {
        number1 = Double.parseDouble(label.getText().trim());
    } else {
        number2 = Double.parseDouble(label.getText().trim());
    }

基本上问题是当我按下/* + 或 - 按钮时，我得到一个 NumberFormatExeption: For input string "/"等。我不太清楚如何修复这个错误(我仍然(在某种程度上)是Java的初学者)。在错误行的更深处(有很多)是很多我不理解的错误，例如 EventDispatchThread、EventQueue 和许多其他错误。我也找不到关于我的经验水平的解释，所以我在这里寻求帮助。

数字按钮都工作正常。

运行时:[ http://gyazo.com/71cb4dde449ccf7ece44017388a71a0f]

输入数字:[ http://gyazo.com/5c7ab6c54ac6da180845c66866d66f8f]

所有其他按钮都会在我的 CMD 中出现错误。

这是我的代码(某些部分的间距可能会困惑):

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
// import for Nimbus look
import javax.swing.UIManager.*;

public class Calculator extends JFrame implements ActionListener
{
    private JPanel bottom = new JPanel(); private JPanel top = new JPanel();
    private JLabel label = new JLabel(" ");
    private JButton[] buttons = new JButton[16];
    // booleans for calculator functions
    boolean add = false, substract = false, devide = false, multiply = false, firstNumber = true;
    // numbers that will be calculated
    double number1, number2;

    public Calculator()
    {
        setLayout(new BoxLayout(getContentPane(), BoxLayout.PAGE_AXIS));

        // bottom panel
        bottom.setPreferredSize(new Dimension(100,100));
        bottom.setLayout(new BorderLayout());

        //add bottom panel to frame
        add(bottom);

        // top panel
        top.setPreferredSize(new Dimension(300,400));
        top.setLayout(new GridLayout(4,4,3,3));
        top.setBackground(Color.BLACK);
        //dont add top panel to frame: you want top to be on bottom

        // add top panel to bottom panel
        bottom.add(top);

        // label
        label.setFont(new Font("Courier", Font.PLAIN, 20));
        label.setBackground(Color.BLACK);
        label.setForeground(Color.WHITE);
        label.setHorizontalAlignment(SwingConstants.RIGHT); // text is right-aligned
        label.setOpaque(true);

        // add the label to the bottom panel
        bottom.add(label, BorderLayout.NORTH);

        // creating buttons
        for(int i = 0; i < buttons.length; i++)
        {
            buttons[i] = new JButton("789/456*123+c0=-".substring(i, i+1));
            buttons[i].addActionListener(this);
           // add them to the top panel
           top.add(buttons[i]);
        }

        // Nimbus look
        try {
            for (LookAndFeelInfo info : UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (Exception e) {
           // If Nimbus is not available, you can set the GUI to another look and feel.
           //set to default somehow o.o
        }

        // frame setters
        setTitle("Calculator");
        setSize(400,400);
        setVisible(true);
        setResizable(false);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public void Clear()
    {
        label.setText(" ");
    }

    @Override
    public void actionPerformed(ActionEvent e)
    {
        //if's for all function buttons
        if(e.getSource() == buttons[3])
        {
           // devide
           devide = true;

           substract = false;
           add = false;
           multiply = false;

           firstNumber = false;
           Clear();

           label.setText("/");
        }

        if(e.getSource() == buttons[7])
        {
           // multiply
           multiply = true;

           substract = false;
           devide = false;
           add = false;

           firstNumber = false;
           Clear();

           label.setText("*");
        }

        if(e.getSource() == buttons[11])
        {
           // add
           add = true;

           substract = false;
           devide = false;
           multiply = false;

           firstNumber = false;
           Clear();

           label.setText("+");
        }

        if(e.getSource() == buttons[12])
        {
           // clear
           label.setText("0");
           number1 = 0.00;
           number2 = 0.00;

           add = false;
           substract = false;
           devide = false;
           multiply = false;

           firstNumber = true;
           Clear();
        }

        if(e.getSource() == buttons[15])
        {
           // substract
           substract = true;

           add = false;
           devide = false;
           multiply = false;

           firstNumber = false;
           Clear();

           label.setText("-");
        }

        // for loops that add the numbers on the buttons to the label
        for(int i = 0; i < 3; i++)
        {
           if(e.getSource() == buttons[i])
           {
               if(label.getText() == "0")
               {
                   label.setText("");
                   label.setText(label.getText() + buttons[i].getText());
               } else {
                   label.setText(label.getText() + buttons[i].getText());
               }
           }
        }

        for(int i = 4; i < 7; i++)
        {
           if(e.getSource() == buttons[i])
           {
               if(label.getText() == "0")
               {
                   label.setText("");
                   label.setText(label.getText() + buttons[i].getText());
               } else {
                   label.setText(label.getText() + buttons[i].getText());
               }
           }
        }

        for(int i = 8; i < 11; i++)
        {
           if(e.getSource() == buttons[i])
           {
               if(label.getText() == "0")
               {
                label.setText("");
                label.setText(label.getText() + buttons[i].getText());
               } else {
                label.setText(label.getText() + buttons[i].getText());
               }
           }
        }

        for(int i = 13; i < 14; i++)
        {
           if(e.getSource() == buttons[i])
           {
             if(label.getText() == "0")
             {
                label.setText("");
                label.setText(label.getText() + buttons[i].getText());
             } else {
                label.setText(label.getText() + buttons[i].getText());
             }
         }
     }

     // if statement that puts the labels text into the first or second number
     if(firstNumber)
     {
        number1 = Double.parseDouble(label.getText().trim());
     } else {
        number2 = Double.parseDouble(label.getText().trim());
     }

     // calculation
     if(e.getSource() == buttons[14])
     {
        // calculate
        if(devide){number1 = ((double)(number1) / (double)(number2)); }
        if(multiply){number1 = ((double)(number1) * (double)(number2)); }
        if(add){number1 = ((double)(number1) + (double)(number2)); }
        if(substract){number1 = ((double)(number1) - (double)(number2)); }
        label.setText(Double.toString(number1));
     }

 }

 public static void main(String[] args)
 {
    new Calculator();
 }
}

最后，计算器的计算结果是不正确的。我也无法理解造成这种情况的原因。请记住，我是 Java 初学者，这是我在 stackoverflow 上的第一个问题。预先感谢您帮助我，无论谁愿意:)

更新:我通过按如下方式放置代码来修复错误:

if(e.getSource() == buttons[15])
    {
        // substract
        substract = true;

        add = false;
        devide = false;
        multiply = false;

        firstNumber = false;
        isNumberKey = false;

        if(isNumberKey)
        {
            if(firstNumber)
            {
                label.setText(label.getText().replace("/",""));
                label.setText(label.getText().replace("*",""));
                label.setText(label.getText().replace("+",""));
                label.setText(label.getText().replace("-",""));
                number1 = Double.parseDouble(label.getText().trim());
            } else {
                label.setText(label.getText().replace("/",""));
                label.setText(label.getText().replace("*",""));
                label.setText(label.getText().replace("+",""));
                label.setText(label.getText().replace("-",""));
                number2 = Double.parseDouble(label.getText().trim());
            }
        }

        Clear();

        label.setText("-");
    }

我现在需要做的就是修正计算...感谢大家的帮助!

Answer 1

无法完全深入研究您的逻辑，但这里有一些提示:

首先，您不必猜测错误出在哪里。如果出现像这里这样的未捕获异常，产生该异常的线程会将其堆栈跟踪打印到控制台。它看起来像这样:

Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: "/"
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1222)
at java.lang.Double.parseDouble(Double.java:510)
at mypackage.Calculator.actionPerformed(Calculator.java:229)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6297)

从这里您可以看到，异常发生在名为 AWT-EventQueue-0 的线程中，该线程是 swing 的标准事件调度线程(所有 GUI 应用程序都在单线程中处理事件)。在堆栈跟踪中搜索的另一件事是查找代码中的哪一行抛出了它。这是一行:

at mypackage.Calculator.actionPerformed(Calculator.java:229)

在源代码中，它是行:

number2 = Double.parseDouble(label.getText().trim());

，所以你猜对了。您可以做的是在那里添加另一个 boolean 值，仅当点击数字按钮时才会设置该 boolean 值，然后将该部分代码更改为如下所示:

if( isNumber){
  if(firstNumber)
    {
        number1 =     Double.parseDouble(label.getText().trim());
    } else {
        number2 = Double.parseDouble(label.getText().trim());
    }
}

除此之外，不要使用多个 boolean 标志进行操作，而是使用 Java 枚举类型

enum Operation {devide, substract, add, multiply};

您的代码将更具可读性并且具有“java”风格。良好的起点，继续吧!