How to pass arithmetic operator at run time in Java?

Question

I want to decide the arithmetic operator at run time and calculate the result of the operands. Is there a way to achieve it?

public static void main(String[] args) {

String operator = args[0];

int num1 = 10;
int num2 = 20;

int result = num1 operator num2;

System.out.println(result);

}

May be you can. But at this point you need to work with compiled bytecode. — Aliaksei Bulhak, Nov 27 '13 at 06:00

score 5 · Answer 1 · edited May 23 '17 at 11:45

5

If you're using JDK 1.6 or above, you can use the built-in Javascript engine like this.

public static void main(String[] args) throws Exception{
    ScriptEngineManager mgr = new ScriptEngineManager();
    ScriptEngine engine = mgr.getEngineByName("JavaScript");
    String oper = args[0];
    int num1 = 10;
    int num2 = 20;
    String expr = num1 + oper + num2;
    System.out.println(engine.eval(expr));
}

Warning: As seen from this answer, it is probably not the best idea to use it. But this suited the requirement in your question the best and hence the answer.

edited May 23 '17 at 11:45

Community

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1

answered Nov 27 '13 at 05:59

Rahul

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Yes this would work, but it's work mentioning that eval should probably *never* be used. – Sinkingpoint Nov 27 '13 at 06:03
The OP want to use operator at runtime. – Masudul Nov 27 '13 at 06:04
@Quirliom - Could you share a link with me on why it shouldn't be used? I would love to add a proper reference along with the warning. – Rahul Nov 27 '13 at 06:04
@Masud - The OP wants to send the operator as an argument to the main and use it, which is exactly what the answer suggests. – Rahul Nov 27 '13 at 06:05
2

@R.J From a Purely Java stance, it's entirely out of place to include an interpreted statement, but I like [this](http://stackoverflow.com/questions/18269797/what-does-eval-do-and-why-its-evil) question which does a reasonable job. Other than that, I really like this answer. – Sinkingpoint Nov 27 '13 at 06:09
@Quirliom - Thanks for the heads-up. I hadn't read about it myself! :) – Rahul Nov 27 '13 at 06:16
1

+1 for entirely unthinkable approach! – Vinay Lodha Nov 27 '13 at 06:19

score 3 · Answer 2 · answered Nov 27 '13 at 06:05

You can read it in as a String or char and then use if statements to perform the operation, like so:

public static void main(String... args) {
    String operator = args[0];

    int num1 = 10;
    int num2 = 20;

    int result = 0;

    if(operator.trim().equals("+")) {
        result = num1 + num2;
    } else if(operator.trim().equals("-")) {
        result = num1 - num2;
    } else if(operator.trim().equals("*")) {
        result = num1 * num2;
    } else if(operator.trim().equals("/")) {
        result = num1 / num2;
    }
    // More operations here...

    System.out.println(num1 + operator + num2 + "=" + result);
}

I would suggest, however, using the double datatype over int for precision's sake.

How to pass arithmetic operator at run time in Java?

2 Answers2