Calculator class for unit test tutorial

Calculator

public class Calculator {

  public int add(final int x, final int y) {
    if (x < 0 || y < 0) {
      throw new IllegalArgumentException("arguments must be positive");
    }
    return x + y;
  }

  public int substract(final int x, final int y) {
    return x - y;
  }

  public int multiply(final int x, final int y) {
    return x * y;
  }
}