Execute mathematical operation dynamically

Question

I have a string that contains some mathematical formula which I want to execute dynamically and I want to store that value in a string. E.g.

String formula = "10 * 9";

... (execute somehow)

String result = "90"; (after successful execution).

I am using openjdk-18, therefore I cannot use ScriptEngine.

Is there any other way to do it?

Edit: possible operations: +, -, *, /, %, ^, brackets support

e.g. "((12/4) * (3 - 1) ^3)"

Answer 1

Well, you'll have to parse the String. This may be simple for the kind of forumula you're presenting in the question but may get a lot more complicated when it comes to nested expressions. In that case you might want to look at parse trees. Typically people use a tool like ANTLR which generates a parser for you if you give it a grammar. So lots of theoretical computer science stuff you would need to get your head around if you have not done so yet.

For your simple example however you could just split at space, then parse the first and last parts of the String as an int and then perform an operation based on the operator. This could look something like this:

public class Application {
    record ParsedExpression(int op1, int op2, String operator){};

    public static void main(String[] args) {
        final var sum = "10 + 5";
        final var difference = "10 - 5";

        System.out.println(calculate(parseMathExpression(sum)));
        System.out.println(calculate(parseMathExpression(difference)));
    }

    public static ParsedExpression parseMathExpression(String input){
        var expressionParts = input.split(" ");
        // you should catch NumberFormatException here!
        // you might also wanna do some validation here like checking the array length before accessing it
        final var x = Integer.parseInt(expressionParts[0]);
        final var y = Integer.parseInt(expressionParts[2]);
        final var operator = expressionParts[1];
        return new ParsedExpression(x, y, operator);
    }

    public static int calculate(ParsedExpression expression){
        return switch(expression.operator) {
            case "+" -> expression.op1 + expression.op2;
            case "-" -> expression.op1 - expression.op2;
            default -> throw new IllegalArgumentException("Unknown operator");
        };
    }
}

This is just a proof of concept. You will definitely need to add some error handling and validation (NumberFormatException, Overflows, ArrayIndexOutOfBoundsException etc.) here and you might expand on this to use other datatypes like double and add other operators like * or /.