试图为简单的计算器编写代码。无法让程序解决用户输入的等式

Question

目前，当用户按下计算器上的按钮时，它会在上面的文本框中显示值，但我无法通过它来解决方程式。按下的每个按钮都显示在TextField中，但是当我按下＆＃39; =＆＃39; （等于）我希望它能解决这个问题。你能帮我解决这个问题吗？

//Michael Moradi
//Period C1
//June 2, 2015
//Final Project Semester 2

import javax.swing.*; //imports all that is needed for the code

import java.awt.*;
import java.awt.event.*;

public class Calculator extends JFrame implements ActionListener {

    public JButton button1, button2, button3, button4, button5, button6, button7, button8, button9, button10, button11, button12, button13, button14, button15, button16;
    public JTextArea text; //makes these accessible throughout the code

    public Calculator()
    {
        setSize(350,300); //sets size to 300 by 300
        setResizable(false); //does not let user change the size of the window
        setDefaultCloseOperation(EXIT_ON_CLOSE);//makes app close when I press the x on the top left

        Container contentPane = getContentPane();//gets the contentPane
        contentPane.setBackground(Color.CYAN);//sets background color to white         

        contentPane.setLayout(new FlowLayout());//makes the contentPane read from left to right

        text = new JTextArea(1, 25);
        contentPane.add(text);
        text.setEditable(false);

        button1 = new JButton ("7");
        contentPane.add(button1);
        button1.addActionListener(this);

        button2 = new JButton ("8");
        contentPane.add(button2);
        button2.addActionListener(this);

        button3 = new JButton ("9");
        contentPane.add(button3);
        button3.addActionListener(this);

        button4 = new JButton ("÷");
        contentPane.add(button4);
        button4.addActionListener(this);

        button5 = new JButton ("4");
        contentPane.add(button5);
        button5.addActionListener(this);

        button6 = new JButton ("5");
        contentPane.add(button6);
        button6.addActionListener(this);

        button7 = new JButton ("6");
        contentPane.add(button7);
        button7.addActionListener(this);

        button8 = new JButton ("x");
        contentPane.add(button8);
        button8.addActionListener(this);

        button9 = new JButton ("1");
        contentPane.add(button9);
        button9.addActionListener(this);

        button10 = new JButton ("2");
        contentPane.add(button10);
        button10.addActionListener(this);

        button11 = new JButton ("3");
        contentPane.add(button11);
        button11.addActionListener(this);

        button12 = new JButton ("-");
        contentPane.add(button12);
        button12.addActionListener(this);

        button13 = new JButton ("0");
        contentPane.add(button13);
        button13.addActionListener(this);

        button14 = new JButton ("C");
        contentPane.add(button14);
        button14.addActionListener(this);

        button15 = new JButton ("=");
        contentPane.add(button15);
        button15.addActionListener(this);

        button16 = new JButton ("+");
        contentPane.add(button16);
        button16.addActionListener(this);



    }

    public static void main(String[] args)
    {
        Calculator guiWindow = new Calculator(); //uses GUI
        guiWindow.setVisible(true); //makes it visible

    }

    public void actionPerformed(ActionEvent e)
    {
        Container contentPane = getContentPane();

        String enteredNumbers = text.getText();

        if (e.getActionCommand().equals("7"))
        text.setText(enteredNumbers + ("7"));

        if (e.getActionCommand().equals("8"))
        text.setText(enteredNumbers + ("8"));

        if (e.getActionCommand().equals("9"))
        text.setText(enteredNumbers + ("9"));

        if (e.getActionCommand().equals("÷"))
        text.setText(enteredNumbers + ("÷"));

        if (e.getActionCommand().equals("4"))
        text.setText(enteredNumbers + ("4"));

        if (e.getActionCommand().equals("5"))
        text.setText(enteredNumbers + ("5"));

        if (e.getActionCommand().equals("6"))
        text.setText(enteredNumbers + ("6"));

        if (e.getActionCommand().equals("x"))
        text.setText(enteredNumbers + ("x"));

        if (e.getActionCommand().equals("1"))
        text.setText(enteredNumbers + ("1"));

        if (e.getActionCommand().equals("2"))
        text.setText(enteredNumbers + ("2"));

        if (e.getActionCommand().equals("3"))
        text.setText(enteredNumbers + ("3"));

        if (e.getActionCommand().equals("-"))
        text.setText(enteredNumbers + ("-"));

        if (e.getActionCommand().equals("0"))
        text.setText(enteredNumbers + ("0"));

        if (e.getActionCommand().equals("C"))
        text.setText("");

        if (e.getActionCommand().equals("="))
        text.setText(enteredNumbers + ("="));

        if (e.getActionCommand().equals("+"))
        text.setText(enteredNumbers + ("+"));




    }

}

Answer 1

对于类似的内容，您只想更改ActionListener。每个其他按钮 - 我强烈建议你提供更有意义的名字 - 顺便说一下 - 只需在计算器字段中添加一个文本标记。 “=”是指您的描述，以解决它。所以，

if (e.getActionCommand().equals("="))
    text.setText(enteredNumbers + ("="));

应该更像这样：

if (e.getActionCommand().equals("=")) {
    String math = text.getText();
    answerField.setText(solve(math));
}

private solve(String math) {
    //This is the tricky part, see description next.
}

对于你的解决方法，我建议两件事之一。第一种方法是使用一系列regular expressions将其分解，您可以创建并尝试（在实施之前）here。从那里，您将知道提供的字段是数字还是运算符。运算符通常最好与枚举和Function字段配对。不幸的是，这仍然存在如何处理运算符优先级打开的问题。

为此，你需要将其分解为Reverse Polish Notation;像Shunting-Yard algorithm这样的东西。之后，您可以按照它出现的顺序单步执行并应用每个操作功能。

如果你赶时间，你可能也可以使用Java Scripting Engine并简单地将数学实现为可解决的JavaScript;可用的教程here。目前，Javascript / ECMAscript可用，但我确信还有很多其他可通过外部库提供。 （当你处理经常变化的动态代码时，这是天赐之物。）

希望其中一个资源能为您提供帮助。我实际上曾经使用第一种方法在Java中实现了一个专门用于求解数学方程的脚本;现在只需使用第二种方法将其插入新应用程序。祝你好运！

Answer 2

这样的事情，通过Java脚本引擎使用JavaScript库math.js。您可以从http://mathjs.org/download.html

下载math.js文件

package org.mathjs;

import java.io.FileNotFoundException;
import java.io.FileReader;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class MathJS {
    protected static String MATHJS_SCRIPT = "./lib/mathjs/math.js";
    protected ScriptEngine engine;

    public MathJS () throws FileNotFoundException, ScriptException {
        ScriptEngineManager manager = new ScriptEngineManager ();
        engine = manager.getEngineByName ("js");

        engine.eval(new FileReader(MATHJS_SCRIPT));
        engine.eval("var parser = math.parser();");
        engine.eval("var precision = 14;");
    }

    public String eval (String expr) throws ScriptException {
        String script = "math.format(parser.eval('" + expr + "'), precision);";
        return (String) engine.eval(script);
    }

    public static void main(String[] args) 
            throws FileNotFoundException, ScriptException {
        MathJS math = new MathJS();
        System.out.println(math.eval("a = 4.5"));
        System.out.println(math.eval("1.2 * (2 + a)"));
        System.out.println(math.eval("5.08 cm in inch"));
        System.out.println(math.eval("sin(45 deg) ^ 2"));   
        System.out.println(math.eval("9 / 3 + 2i") );   
        System.out.println(math.eval("det([-1, 2; 3, 1])"));
    }
}