public String highestBid(String paintingID) {
    int x = 1;
    int count = 0;
    try {
        for (Bid b : bid) {
            if (paintingID.equals(b.getPaintingID())) {
                x = b.getPrice();
                count++;
            }
        }
    } catch (Exception ex) {
        return ex.getMessage();
    }
    if (count == 0) {
        return "n" + "Invalid Painting ID. There is no Painting with this ID!";
    } else {
        return paintingID;
    }
}

public void actionPerformed(ActionEvent e) {
if (e.getSource() == tfPid)
    taText.setText("");
    String paintingID = tfPid.getText();
    taText.setText(taText.getText()+a.highestBid(paintingID));
}