public synchronized void addBidPoint(int quantity, float unitPrice) {
    if (isSubmitted()) {
      throw new IllegalStateException("Bid already submitted");
    }
    if (unitPrice < 0) {
      throw new IllegalArgumentException("Negative price not allowed");
    }
    // This is a "trick" for checking that this auction allow "sell"
    if (auction < TACAgent.MIN_ENTERTAINMENT && quantity < 0) {
      throw new IllegalArgumentException("Not allowed to sell in auction " +
					 auction);
    }

    realloc();
    this.quantity[len] = quantity;
    this.price[len++] = unitPrice;
    this.bidString = null;
  }