像RHINO或NASHORN这样的外部库可以用来运行javascript。javascript可以计算简单的公式，而不需要对字符串进行分割。如果代码写得好，也不会对性能造成影响。 下面是一个使用RHINO -的示例

public class RhinoApp {
    private String simpleAdd = "(12+13+2-2)*2+(12+13+2-2)*2";

public void runJavaScript() {
    Context jsCx = Context.enter();
    Context.getCurrentContext().setOptimizationLevel(-1);
    ScriptableObject scope = jsCx.initStandardObjects();
    Object result = jsCx.evaluateString(scope, simpleAdd , "formula", 0, null);
    Context.exit();
    System.out.println(result);
}

package ExpressionCalculator.expressioncalculator;

import java.text.DecimalFormat;
import java.util.Scanner;

public class ExpressionCalculator {

private static String addSpaces(String exp){

    //Add space padding to operands.
    //https://regex101.com/r/sJ9gM7/73
    exp = exp.replaceAll("(?<=[0-9()])[\\/]", " / ");
    exp = exp.replaceAll("(?<=[0-9()])[\\^]", " ^ ");
    exp = exp.replaceAll("(?<=[0-9()])[\\*]", " * ");
    exp = exp.replaceAll("(?<=[0-9()])[+]", " + "); 
    exp = exp.replaceAll("(?<=[0-9()])[-]", " - ");

    //Keep replacing double spaces with single spaces until your string is properly formatted
    /*while(exp.indexOf("  ") != -1){
        exp = exp.replace("  ", " ");
     }*/
    exp = exp.replaceAll(" {2,}", " ");

       return exp;
}

public static Double evaluate(String expr){

    DecimalFormat df = new DecimalFormat("#.####");

    //Format the expression properly before performing operations
    String expression = addSpaces(expr);

    try {
        //We will evaluate using rule BDMAS, i.e. brackets, division, power, multiplication, addition and
        //subtraction will be processed in following order
        int indexClose = expression.indexOf(")");
        int indexOpen = -1;
        if (indexClose != -1) {
            String substring = expression.substring(0, indexClose);
            indexOpen = substring.lastIndexOf("(");
            substring = substring.substring(indexOpen + 1).trim();
            if(indexOpen != -1 && indexClose != -1) {
                Double result = evaluate(substring);
                expression = expression.substring(0, indexOpen).trim() + " " + result + " " + expression.substring(indexClose + 1).trim();
                return evaluate(expression.trim());
            }
        }

        String operation = "";
        if(expression.indexOf(" / ") != -1){
            operation = "/";
        }else if(expression.indexOf(" ^ ") != -1){
            operation = "^";
        } else if(expression.indexOf(" * ") != -1){
            operation = "*";
        } else if(expression.indexOf(" + ") != -1){
            operation = "+";
        } else if(expression.indexOf(" - ") != -1){ //Avoid negative numbers
            operation = "-";
        } else{
            return Double.parseDouble(expression);
        }

        int index = expression.indexOf(operation);
        if(index != -1){
            indexOpen = expression.lastIndexOf(" ", index - 2);
            indexOpen = (indexOpen == -1)?0:indexOpen;
            indexClose = expression.indexOf(" ", index + 2);
            indexClose = (indexClose == -1)?expression.length():indexClose;
            if(indexOpen != -1 && indexClose != -1) {
                Double lhs = Double.parseDouble(expression.substring(indexOpen, index));
                Double rhs = Double.parseDouble(expression.substring(index + 2, indexClose));
                Double result = null;
                switch (operation){
                    case "/":
                        //Prevent divide by 0 exception.
                        if(rhs == 0){
                            return null;
                        }
                        result = lhs / rhs;
                        break;
                    case "^":
                        result = Math.pow(lhs, rhs);
                        break;
                    case "*":
                        result = lhs * rhs;
                        break;
                    case "-":
                        result = lhs - rhs;
                        break;
                    case "+":
                        result = lhs + rhs;
                        break;
                    default:
                        break;
                }
                if(indexClose == expression.length()){
                    expression = expression.substring(0, indexOpen) + " " + result + " " + expression.substring(indexClose);
                }else{
                    expression = expression.substring(0, indexOpen) + " " + result + " " + expression.substring(indexClose + 1);
                }
                return Double.valueOf(df.format(evaluate(expression.trim())));
            }
        }
    }catch(Exception exp){
        exp.printStackTrace();
    }
    return 0.0;
}

public static void main(String args[]){

    Scanner scanner = new Scanner(System.in);
    System.out.print("Enter an Mathematical Expression to Evaluate: ");
    String input = scanner.nextLine();
    System.out.println(evaluate(input));
}

}

本文讨论了各种方法。以下是文中提到的两种关键方法:

Apache的JEXL

允许脚本包含对java对象的引用。

// Create or retrieve a JexlEngine
JexlEngine jexl = new JexlEngine();
// Create an expression object
String jexlExp = "foo.innerFoo.bar()";
Expression e = jexl.createExpression( jexlExp );
 
// Create a context and add data
JexlContext jctx = new MapContext();
jctx.set("foo", new Foo() );
 
// Now evaluate the expression, getting the result
Object o = e.evaluate(jctx);

使用JDK中嵌入的javascript引擎:

private static void jsEvalWithVariable()
{
    List<String> namesList = new ArrayList<String>();
    namesList.add("Jill");
    namesList.add("Bob");
    namesList.add("Laureen");
    namesList.add("Ed");
 
    ScriptEngineManager mgr = new ScriptEngineManager();
    ScriptEngine jsEngine = mgr.getEngineByName("JavaScript");
 
    jsEngine.put("namesListKey", namesList);
    System.out.println("Executing in script environment...");
    try
    {
      jsEngine.eval("var x;" +
                    "var names = namesListKey.toArray();" +
                    "for(x in names) {" +
                    "  println(names[x]);" +
                    "}" +
                    "namesListKey.add(\"Dana\");");
    }
    catch (ScriptException ex)
    {
        ex.printStackTrace();
    }
}

一个可以计算数学表达式的Java类:

package test;

public class Calculator {

    public static Double calculate(String expression){
        if (expression == null || expression.length() == 0) {
            return null;
        }
        return calc(expression.replace(" ", ""));
    }
    public static Double calc(String expression) {
        String[] containerArr = new String[]{expression};
        double leftVal = getNextOperand(containerArr);
        expression = containerArr[0];
        if (expression.length() == 0) {
            return leftVal;
        }
        char operator = expression.charAt(0);
        expression = expression.substring(1);

        while (operator == '*' || operator == '/') {
            containerArr[0] = expression;
            double rightVal = getNextOperand(containerArr);
            expression = containerArr[0];
            if (operator == '*') {
                leftVal = leftVal * rightVal;
            } else {
                leftVal = leftVal / rightVal;
            }
            if (expression.length() > 0) {
                operator = expression.charAt(0);
                expression = expression.substring(1);
            } else {
                return leftVal;
            }
        }
        if (operator == '+') {
            return leftVal + calc(expression);
        } else {
            return leftVal - calc(expression);
        }

    }
    
    private static double getNextOperand(String[] exp){
        double res;
        if (exp[0].startsWith("(")) {
            int open = 1;
            int i = 1;
            while (open != 0) {
                if (exp[0].charAt(i) == '(') {
                    open++;
                } else if (exp[0].charAt(i) == ')') {
                    open--;
                }
                i++;
            }
            res = calc(exp[0].substring(1, i - 1));
            exp[0] = exp[0].substring(i);
        } else {
            int i = 1;
            if (exp[0].charAt(0) == '-') {
                i++;
            }
            while (exp[0].length() > i && isNumber((int) exp[0].charAt(i))) {
                i++;
            }
            res = Double.parseDouble(exp[0].substring(0, i));
            exp[0] = exp[0].substring(i);
        }
        return res;
    }


    private static boolean isNumber(int c) {
        int zero = (int) '0';
        int nine = (int) '9';
        return (c >= zero && c <= nine) || c =='.';
    }

    public static void main(String[] args) {
        System.out.println(calculate("(((( -6 )))) * 9 * -1"));
        System.out.println(calc("(-5.2+-5*-5*((5/4+2)))"));

    }

}

对于JDK1.6，您可以使用内置的Javascript引擎。

import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
import javax.script.ScriptException;

public class Test {
  public static void main(String[] args) throws ScriptException {
    ScriptEngineManager mgr = new ScriptEngineManager();
    ScriptEngine engine = mgr.getEngineByName("JavaScript");
    String foo = "40+2";
    System.out.println(engine.eval(foo));
    } 
}

解决这个问题的正确方法是使用词法分析器和解析器。您可以自己编写这些页面的简单版本，或者这些页面还包含指向Java词法分析器和解析器的链接。

创建递归下降解析器是非常好的学习练习。