Java by API/javax.swing.text/PlainDocument

extends PlainDocument

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;
import javax.swing.text.PlainDocument;
public class MaxLengthDocument extends PlainDocument {
  private int max;
  public MaxLengthDocument(int maxLength) {
    max = maxLength;
  }
  public void insertString(int offset, String str, AttributeSet a) throws BadLocationException {
    if (getLength() + str.length() > max)
      java.awt.Toolkit.getDefaultToolkit().beep();
    else
      super.insertString(offset, str, a);
  }
  public static void main(String[] args) {
    Document doc = new MaxLengthDocument(5);
    JTextField field = new JTextField(doc, "", 8);
    JPanel flowPanel = new JPanel();
    flowPanel.add(field);
    JFrame frame = new JFrame("MaxLengthDocument demo");
    frame.setContentPane(flowPanel);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(160, 80);
    frame.setVisible(true);
  }
}