如果我们要实现它，那么我们可以使用下面的算法

While there are still tokens to be read in, 1.1 Get the next token. 1.2 If the token is: 1.2.1 A number: push it onto the value stack. 1.2.2 A variable: get its value, and push onto the value stack. 1.2.3 A left parenthesis: push it onto the operator stack. 1.2.4 A right parenthesis: 1 While the thing on top of the operator stack is not a left parenthesis, 1 Pop the operator from the operator stack. 2 Pop the value stack twice, getting two operands. 3 Apply the operator to the operands, in the correct order. 4 Push the result onto the value stack. 2 Pop the left parenthesis from the operator stack, and discard it. 1.2.5 An operator (call it thisOp): 1 While the operator stack is not empty, and the top thing on the operator stack has the same or greater precedence as thisOp, 1 Pop the operator from the operator stack. 2 Pop the value stack twice, getting two operands. 3 Apply the operator to the operands, in the correct order. 4 Push the result onto the value stack. 2 Push thisOp onto the operator stack. While the operator stack is not empty, 1 Pop the operator from the operator stack. 2 Pop the value stack twice, getting two operands. 3 Apply the operator to the operands, in the correct order. 4 Push the result onto the value stack. At this point the operator stack should be empty, and the value stack should have only one value in it, which is the final result.

你也可以试试BeanShell解释器:

Interpreter interpreter = new Interpreter();
interpreter.eval("result = (7+21*6)/(32-27)");
System.out.println(interpreter.get("result"));

我想无论你用什么方法做这个都会涉及到很多条件命题。但是对于单个操作，比如在你的例子中，你可以将它限制为4个if语句

String math = "1+4";

if (math.split("+").length == 2) {
    //do calculation
} else if (math.split("-").length == 2) {
    //do calculation
} ...

当你想要处理像“4+5*6”这样的多个操作时，它会变得更加复杂。

如果你试图构建一个计算器，那么我建议分别传递计算的每个部分(每个数字或运算符)，而不是作为一个单一的字符串。

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)))"));

    }

}

这是另一个有趣的选择 https://github.com/Shy-Ta/expression-evaluator-demo

它的用法非常简单，可以完成工作，例如:

  ExpressionsEvaluator evalExpr = ExpressionsFactory.create("2+3*4-6/2");  
  assertEquals(BigDecimal.valueOf(11), evalExpr.eval()); 

我已经使用迭代解析和分流码算法，我真的很喜欢开发表达式求值器，你可以在这里找到所有的代码

https://github.com/nagaraj200788/JavaExpressionEvaluator

有73个测试用例，甚至工作于大整数，大小数

支持所有关系，算术表达式和两者的组合。 甚至支持三元运算符。

增加了增强，以支持有符号的数字，如-100+89，这是有趣的，详细信息请检查TokenReader.isUnaryOperator()方法，我已经更新了上面链接中的代码