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Thinking in Java
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Text fields

This example shows the extra behavior that JTextFields are capable of:

// Text fields and Java events.
// <applet code=TextFields width=375 height=125></applet>
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class TextFields extends JApplet {
  private JButton
    b1 = new JButton("Get Text"),
    b2 = new JButton("Set Text");
  private JTextField
    t1 = new JTextField(30),
    t2 = new JTextField(30),
    t3 = new JTextField(30);
  private String s = new String();
  private UpperCaseDocument ucd = new UpperCaseDocument();
  public void init() {
    ucd.addDocumentListener(new T1());
    b1.addActionListener(new B1());
    b2.addActionListener(new B2());
    DocumentListener dl = new T1();
    t1.addActionListener(new T1A());
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
  class T1 implements DocumentListener {
    public void changedUpdate(DocumentEvent e) {}
    public void insertUpdate(DocumentEvent e) {
      t3.setText("Text: "+ t1.getText());
    public void removeUpdate(DocumentEvent e) {
  class T1A implements ActionListener {
    private int count = 0;
    public void actionPerformed(ActionEvent e) {
      t3.setText("t1 Action Event " + count++);
  class B1 implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      if(t1.getSelectedText() == null)
        s = t1.getText();
        s = t1.getSelectedText();
  class B2 implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      t1.setText("Inserted by Button 2: " + s);
  public static void main(String[] args) { TextFields(), 375, 125);

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);
} ///:~

The JTextField t3 is included as a place to report when the action listener for the JTextField t1 is fired. You’ll see that the action listener for a JTextField is fired only when you press the “enter” key.

The JTextField t1 has several listeners attached to it. The T1 listener is a DocumentListener that responds to any change in the “document” (the contents of the JTextField, in this case). It automatically copies all text from t1 into t2. In addition, t1’s document is set to a derived class of PlainDocument, called UpperCaseDocument, which forces all characters to uppercase. It automatically detects backspaces and performs the deletion, adjusting the caret and handling everything as you would expect.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire