/**
 * A stack machine for simple integer arithmetic.
 */
public class StackMachine extends IntStack {
  /**
   * Pop the two top integers from the stack, add 
   * them, and push their integer sum.  There
   * should be at least two integers on the stack
   * when we are called.  If not, we leave the stack 
   * empty and set the error flag.
   */
  public void add() {
    int i = pop();
    int j = pop();
    if (getError()) return;
    push(i + j);
  }

  /**
   * Pop the two top integers from the stack, divide
   * them, and push their integer quotient.  There
   * should be at least two integers on the stack
   * when we are called.  If not, we leave the stack
   * empty and set the error flag.
   */
  public void divide() {
    int i = pop();
    int j = pop();
    if (getError()) return;
    push(i / j);
  }

  /**
   * Pop the two top integers from the stack, multiply
   * them, and push their integer product.  There
   * should be at least two integers on the stack
   * when we are called.  If not, we leave the stack 
   * empty and set the error flag.
   */
  public void multiply() {
    int i = pop();
    int j = pop();
    if (getError()) return;
    push(i * j);
  }

  /**
   * Pop the two top integers from the stack, subtract
   * the second from the first, and push their integer
   * difference.  There should be at least two 
   * integers on the stack when we are called.  If not, 
   * we leave the stack empty and set the error flag.
   */
  public void subtract() {
    int i = pop();
    int j = pop();
    if (getError()) return;
    push(i - j);
  }
}