Question

所以我的任务是使用堆栈数据结构设计一个简单的GUI计算器来执行加法，减法，乘法和除法。计算器窗口应至少有两个面板 - 一个用于显示，另一个用于按钮（0 - 9，。，+， - ，X，/，=，C）。输入表达式后，将显示其后缀或前缀，然后显示结果。

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;
import java.util.Stack;

public class JCalculator implements ActionListener {
JFrame jfrm;
JFrame jfrm2;
JTextField txt;
JLabel results;
String str = "";
Stack operands = new Stack();
char[] a = new char[0];
int used = 0;



JCalculator(){
    jfrm = new JFrame("JCalc");
    jfrm.getContentPane().setLayout(new GridLayout(0,1));
    jfrm.setSize(210,210);

   results = new JLabel("",SwingConstants.RIGHT);
   jfrm.getContentPane().add(results);
   jfrm.setLocation(400,300);
   jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

   JButton[] calbut= new JButton[14];
   String []but = {"7","8","9","/","4","5","6","*","1","2","3","-","0","C"
   };
   JButton equal = new JButton("=");
   JButton add = new JButton("+");

   JPanel jbutton = new JPanel();
   jbutton.setLayout(new GridLayout(4,3));
   for(int i=0; i< but.length;i++){
       jbutton.add(calbut[i] = new JButton(but[i]));

       calbut[i].addActionListener(this);
   }
   jbutton.add(equal);
   jbutton.add(add);

   equal.addActionListener(this);
   add.addActionListener(this);

   jfrm.getContentPane().add(jbutton);


    jfrm.setVisible(true);
}
public void actionPerformed(ActionEvent ae){
    char[] a = new char[0];
    int used = 0;
    if(used == a.length){
        char[] newa= new char[a.length + 1];
        for(int i = 0; i <used; i++) newa[i]= a[i];
        a= newa;
    }

    if(ae.getActionCommand().equals("1")){
        str = results.getText();
        results.setText(str + "1");
        operands.push(1);
    }
    if(ae.getActionCommand().equals("2")){
        str = results.getText();
        results.setText(str + "2");
        operands.push(2);
    }
    if(ae.getActionCommand().equals("3")){
        str = results.getText();
        results.setText(str + "3");
        operands.push(3);
    }
    if(ae.getActionCommand().equals("4")){
        str = results.getText();
        results.setText(str + "4");
        operands.push(4);
    }
    if(ae.getActionCommand().equals("5")){
        str = results.getText();
        results.setText(str + "5");
        operands.push(5);
    }
    if(ae.getActionCommand().equals("6")){
        str = results.getText();
        results.setText(str + "6");
        operands.push(6);
    }
    if(ae.getActionCommand().equals("7")){
        str = results.getText();
        results.setText(str + "7");
        operands.push(7);
    }
    if(ae.getActionCommand().equals("8")){
        str = results.getText();
        results.setText(str + "8");
        operands.push(8);
    }
    if(ae.getActionCommand().equals("9")){
        str = results.getText();
        results.setText(str + "9");
        operands.push(9);
    }
    if (ae.getActionCommand().equals("0")) {
        str = results.getText();
        results.setText(str + "0");
        operands.push(0);
    }
    if(ae.getActionCommand().equals("+")){
        str = results.getText();
        double operand = Double.parseDouble(str);
        operands.push(operand);
        results.setText("");
        a[used]= '+';
        used++;

    }
    if(ae.getActionCommand().equals("-")){
        str = results.getText();
        double operand = Double.parseDouble(str);
        operands.push(operand);
        results.setText("");
        a[used]= '+';
        used++;
    }
    if(ae.getActionCommand().equals("/")){
        str = results.getText();
        double operand = Double.parseDouble(str);
        operands.push(operand);
        results.setText("");
        a[used]= '+';
        used++;
    }
    if(ae.getActionCommand().equals("*")){
        str = results.getText();
        double operand = Double.parseDouble(str);
        operands.push(operand);
        results.setText("");
        a[used]= '*';
        used++;
    }
    if(ae.getActionCommand().equals("=")){

        for(int i = 0; i< used; i++){
           performBinaryOp(a[i]);
        }

        str = String.valueOf(operands.pop());
        results.setText(str);
    }


}
 public void performBinaryOp(char nextOperation) {
     double leftOperand, rightOperand;
    Double result = new Double(0);
    rightOperand = (Double) operands.pop();
    leftOperand = (Double) operands.pop();
    switch (nextOperation) {
        case '+':
            result = new Double(leftOperand + rightOperand);
            break;
        case '-':
            result = new Double(leftOperand - rightOperand);
            break;
        case '*':
            result = new Double(leftOperand * rightOperand);
            break;
        case '/':
            result = new Double(leftOperand / rightOperand);
            break;
    }
    operands.push(result);
}

public static void main(String[] args)
{
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            new JCalculator();                
        }            
    });        
}
}

我遇到的错误是按下等号按钮，例如按2 + 3 =它返回第一个值2.0然后我收到错误说：

Exception in thread "AWT-EventQueue-0" java.util.EmptyStackException
at java.util.Stack.peek(Unknown Source)
at java.util.Stack.pop(Unknown Source)
at JCalculator.actionPerformed(JCalculator.java:152)
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$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.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)

我很确定我的问题是我有一个空堆栈，但我不确定代码错在哪里或如何解决它，所以任何帮助都将非常感激。

Answer 1

我建议您将Swing和UI放在一边，直到计算器正常工作为止。创建一个单独的Calculator类，可以接收文本字符串并对其进行评估。完成后，再添加一个UI。

是的，你有一个堆栈问题，就像异常告诉你的那样：

at JCalculator.actionPerformed(JCalculator.java:152)

在文本编辑器中打开JCalculator，打开行号显示，然后转到第152行。这就是问题所在。

我不愿意阅读您发布的代码，但我会说使用调试器进行快速调整应该很快就能清除它。

Answer 2

您的actionPerformed()方法shadows一些类变量;请注意，您已重新声明char [] a和int used，因此您的代码几乎肯定不会按预期运行。我没有遵循这个合乎逻辑的结论。

public class JCalculator implements ActionListener {
JFrame jfrm;
JFrame jfrm2;
JTextField txt;
JLabel results;
String str = "";
Stack operands = new Stack();
char[] a = new char[0];
int used = 0;
/* ... */
public void actionPerformed(ActionEvent ae){
    char[] a = new char[0];
    int used = 0;
    if(used == a.length){
        char[] newa= new char[a.length + 1];
        for(int i = 0; i <used; i++) newa[i]= a[i];
        a= newa;
    }

这段代码让我觉得你不接受两位数的数字：

if(ae.getActionCommand().equals("1")){
    str = results.getText();
    results.setText(str + "1");
    operands.push(1);
}

operands.push(1)可能需要推送正在构建的数字，而不仅仅是最近的数字。（当然，如果您打算只支持一位数的数字，这看起来还不错。）

我建议尝试使用单个函数替换这十个小if语句。值得花些时间在这里找出一个函数，它将来会大大简化这段代码的维护，并且技能至关重要。（我想建议您先考虑你的整齐JButton初始化 - 并尝试在替换函数中使用数组。它可能不是最好，但它应该没问题。）

if(ae.getActionCommand().equals("/")){
    str = results.getText();
    double operand = Double.parseDouble(str);
    operands.push(operand);
    results.setText("");
    a[used]= '+';
    used++;
}

您会注意到a[used]被分配了错误符号。修复变量阴影问题后，此将成为问题。尝试找出用另一个函数替换这四个if块的方法 - 再次，当你向计算器添加新的操作符时，它会更难以犯这样的错误，它会修复运算符中的错误 - 应用代码要容易得多，因为您只需要在一个位置修复错误。

我不能强烈强调将代码分解为“最小的合理”函数是多么有用。您可以比大型函数更容易地测试小函数，并且将来您的代码将更容易阅读。

Answer 3

万一你需要有关计算器的帮助，这是我在Java中的第一个项目

https://github.com/orefalo/ExpressionEvaluator

Java计算器堆栈

3 个答案: