如何在java代码中使用括号

Question

我正在自学Java，而不是通常的"hello World!"我认为最好是构建一个计算器应用程序，并在构建它的同时学习Java。

"JavaCalculator.png"

我的问题是:我如何编程"("one_answers")"括号键，以便我可以在计算器应用程序中使用它们。

一个典型的数字按钮(2)在我的应用程序是:

private void btnTwoActionPerformed(java.awt.event.ActionEvent evt) {                                       
    if (bFinish == false) {
        String btnTwoText = txtDisplay.getText() + btnTwo.getText();
        dObtainer = Double.parseDouble(btnTwoText);
        txtDisplay.setText(btnTwoText);
    } else {
        txtDisplay.setText("");
        String btnTwoText = txtDisplay.getText() + btnTwo.getText();
        dObtainer = Double.parseDouble(btnTwoText);
        txtDisplay.setText(btnTwoText);
        bFinish = false;
    }
}

一个典型的操作数按钮(-)在我的应用程序是:

private void btnMinusActionPerformed(java.awt.event.ActionEvent evt) {                                         
        String button_text = "-";
        getOperator(button_text);
}

private void getOperator(String btnText) {
        mathOperator = btnText.charAt(0);
        dTotal1 = dTotal1 + dObtainer;
        txtDisplay.setText("");
    }

这是当"Equals"按钮被按下时的结果。

private void btnEqualsActionPerformed(java.awt.event.ActionEvent evt) {                                          
        String sBuffer = "";
        switch (mathOperator) {
            case '+':
                dResult = dTotal1 + dObtainer;
                break;
            case '-':
                dResult = dTotal1 - dObtainer;
                break;
            case '/':
                dResult = dTotal1 / dObtainer;
                break;
            case '*':
                dResult = dTotal1 * dObtainer;
                break;
            case '^':
                dResult = Math.pow(dTotal1, dObtainer);
                break;
            case '(':
                sBuffer =  "(";
                //dResult = dObtainer;
                break;
        }
        mathOperator = '';
        dTotal1 = 0;
        dObtainer = 0;
        DecimalFormat df = new DecimalFormat("#.########");
        if(!"(".equals(sBuffer))
                sBuffer = df.format(dResult);
        if(!")".equals(sBuffer))
                sBuffer = df.format(dResult);
        sBuffer = sBuffer.replace(".0", "");
        txtDisplay.setText(sBuffer);
        bFinish = true;
    }

Answer 1

作为初学者，我建议使用内置的Javascript引擎

ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine engine = mgr.getEngineByName("JavaScript");
String foo = "4+5+(8*4)"; //replace your expression here
System.out.println(engine.eval(foo));

另一个可以用MVEL

Answer 2

您可以使用StringBuilder创建表达式(如2+(1-5))，然后使用:

import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
...
ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine engine = mgr.getEngineByName("JavaScript");
String exp = sb.toString(); //sb is the string builder
int result = 0;
try {
    result = (int) engine.eval(exp);
} catch (ScriptException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}
//from here you can use result in whatever way you want