Question

Java code…

If I have

int num = 10 + 5;
System.out.println(num);

The result is 15

And if I have

String str = "10";
int number = Integer.parseInt(str);
System.out.println(number);

The result is 10

But if I have

String str = "10 + 5"; //here the problem.
int number = Integer.parseInt(str);
System.out.println(number);

The result is error message.

How to do it correctly. What I want to implement is to take expression from a String and calculate it. I am forced to take it from a String because I take it from JTextfield, so I want to make the calculation for the returned String expression.

Answer 1

There is no simple way to do it directly because you need some sort of “expression evaluator”. One way would be to use a javascript engine from the scripting library:

public static void main(String[] args) throws Exception {
    ScriptEngineManager factory = new ScriptEngineManager();
    ScriptEngine engine = factory.getEngineByName("JavaScript");

    String s = "10 + 5";
    int  result = ((Double) engine.eval(s)).intValue();
    System.out.println(result); // 15
}