How to eval a math expression entered as an input string in Java

One way to evaluate simple math expressions in Java just takes a couple lines of code. It uses Javascript’s engine to do that and it is quite useful.

// to use eval
import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
// to use scanner
import java.util.*;

class Main {
  public static void main(String[] args)
    throws Exception {
    ScriptEngineManager mgr;
    mgr = new ScriptEngineManager();
    ScriptEngine engine;
    engine = mgr.getEngineByName("JavaScript");
    Scanner scan = new Scanner(System.in);
    String foo = scan.nextLine();
    System.out.println(engine.eval(foo));
    }
}

If you want to compile and run this example, you need to have Java JDK installed. The commands to compile and run a file called eval.java with the code above are:

$ javac eval.java
$ java Main
# Then you enter a math expression
# and you can see the result below
1 + 3 * 2 - 5 # input
2 # answer

That’s all for this post, thanks for reading!