public class StackMachine extends IntStack {
  /**
   * Pop the two top integers from the stack, add
   * them, and push their integer sum.
   * @throws EmptyStack if stack runs out
   */
  public void add() throws EmptyStack {
    int i = pop();
    int j = pop();
    push(i + j);
  }

  /**
   * Pop the two top integers from the stack, divide
   * them, and push their integer quotient.
   * @throws EmptyStack if stack runs out
   */
  public void divide() throws EmptyStack {
    int i = pop();
    int j = pop();
    push(i / j);
  }

  /**
   * Pop the two top integers from the stack, multiply 
   * them, and push their integer product.
   * @throws EmptyStack if stack runs out
   */
  public void multiply() throws EmptyStack {
    int i = pop();
    int j = pop();
    push(i * j);
  }

  /**
   * Pop the two top integers from the stack, subtract
   * the second from the first, and push their integer
   * difference.
   * @throws EmptyStack if stack runs out
   */
  public void subtract() throws EmptyStack {
    int i = pop();
    int j = pop();
    push(i - j);
  }
}