Example of using Polymorphism for Strategy/Command pattern in Java

Main.java

import java.util.Scanner;

interface Operator {
	int calc(int a, int b);
}

class Add implements Operator {
	public int calc(int a, int b) { return a + b; }
}
class Sub implements Operator {
	public int calc(int a, int b) { return a - b; }
}
class Mul implements Operator {
	public int calc(int a, int b) { return a * b; }
}
class Div implements Operator {
	public int calc(int a, int b) { return a / b; }
}

public class Main {
	public static void main(String[] args) {
		
		Scanner s = new Scanner(System.in);

		System.out.print("number #1 : ");
		int n1 = s.nextInt();

		System.out.print("operator  : ");
		String o = s.next();

		System.out.print("number #2 : ");
		int n2 = s.nextInt();

		Operator op = getOperator(o);
		System.out.println(op.calc(n1, n2));

	}
	public static Operator getOperator(String s) {
		if (s.equals("+")) return new Add();
		if (s.equals("-")) return new Sub();
		if (s.equals("*")) return new Mul();
		if (s.equals("/")) return new Div();
		return null;
	}
}