Java. How to evaluate a String expression?

Question

I have a mathematical expression in the form of a String. String exp = "3 + 6 - 10 + 12 + 15"; Now how to calculate the result of this expression as we do with other mathematical expressions. Help me.

you must translate your string in numbers and operators. There is no function for it afaik. Edit: Maybe Script- Engine: http://stackoverflow.com/questions/3422673/evaluating-a-math-expression-given-in-string-form — pL4Gu33, Sep 05 '15 at 14:49
Check out [this question](http://stackoverflow.com/q/4589951/2032064) — Mifeet, Sep 05 '15 at 14:53

score 4 · Answer 1 · answered Sep 05 '15 at 15:07

4

Assuming that there may only be + and - operations, you can remove all the whitespaces and split it to positive and negative integers and sum them.

String expr = "3 + 6 - 10 + 12 + 15";
String[] nums = expr.replaceAll("\\s", "").split("\\+|(?=-)");
int result = Arrays.stream(nums).mapToInt(Integer::parseInt).sum();

If the expressions can be more complex though, including more operators or parentheses, you have to use another solution.

answered Sep 05 '15 at 15:07

Bubletan

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This solution is far better than the script engine. – D. Ben Knoble Sep 05 '15 at 15:18

score 0 · Answer 2 · answered Sep 05 '15 at 19:28

0

You can use BeanShell bsh.Interpreter for the same:

Interpreter interpreter = new Interpreter(); 
interpreter.eval("result = 5+4*(7-15)");
System.out.println(interpreter.get("result"));

It will evaluate complex mathematical expression.

answered Sep 05 '15 at 19:28

seahawk

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Hanzallah Afgan · Answer 3 · 2015-09-05T15:23:55.953

-1

Through JDK 1.6 or Higher.

    ScriptEngineManager mgr = new ScriptEngineManager();
    ScriptEngine engine = mgr.getEngineByName("JavaScript");
    String expression = "3+6-10+12+15";
    try {
        System.out.println(engine.eval(expression));
    } catch (ScriptException ex) {
        System.out.println("Error occured.");
    }

edited Sep 05 '15 at 15:23

answered Sep 05 '15 at 14:52

Hanzallah Afgan

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This is prone to [code injection](https://en.wikipedia.org/wiki/Code_injection)... what if this value is taken from user input and they write `txt = "a";while(1){txt = txt += "a";}`? That will crash you. Also, malicious programs could potentially be run. Using Javascript just to calculate some basic arithmetic seems a bit hardcore, anyway. – bcsb1001 Sep 05 '15 at 14:53

Java. How to evaluate a String expression?

3 Answers3