OCalculator.java

import java.util.Scanner;

public class OCalculator {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        String exp = sc.next();

        int index = 0;
        char op = ' ';

        for (int i = 0; i < exp.length(); i++) {
            char ch = exp.charAt(i);

            if (ch == '+' || ch == '-' || ch == '*' || ch == '/') {
                op = ch;
                index = i;
                break;
            }
        }

        int a = Integer.parseInt(exp.substring(0, index));
        int b = Integer.parseInt(exp.substring(index + 1));

        switch (op) {
            case '+':
                System.out.println(a + b);
                break;
            case '-':
                System.out.println(a - b);
                break;
            case '*':
                System.out.println(a * b);
                break;
            case '/':
                System.out.println(a / b);
                break;
        }

        sc.close();
    }
}