Force JTextField to convert input to upper case with PlainDocument in Java

import java.awt.FlowLayout;
//  ww  w  . j av a 2s  .co m
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.PlainDocument;

public class Main {
  public static void main(String[] argv) {
    JTextField tf = new JTextField(20);
    UpperCaseDocument ucd = new UpperCaseDocument();

    tf.setDocument(ucd);
    
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLayout(new FlowLayout());
    frame.add(tf);
    frame.setSize(300, 150);
    frame.setVisible(true);
  }
}
class UpperCaseDocument extends PlainDocument {
  private boolean upperCase = true;

  public void setUpperCase(boolean flag) {
    upperCase = flag;
  }

  public void insertString(int offset, String str, AttributeSet attSet)
      throws BadLocationException {
    if (upperCase)
      str = str.toUpperCase();
    super.insertString(offset, str, attSet);
  }

}