In this small Swing code snippet we demonstrate how to use java.awt.event.KeyAdapter abstract class to handle keyboard event for the JTextField component. The snippet will change the characters typed in the JTextField component to uppercase.

A better approach for this use case is to use the DocumentFilter class. See the following code snippet How do I format JTextField text to uppercase?.

package org.kodejava.swing;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.HeadlessException;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;

public class UppercaseTextFieldDemo extends JFrame {
    public UppercaseTextFieldDemo() throws HeadlessException {
        initComponents();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new UppercaseTextFieldDemo().setVisible(true));
    }

    protected void initComponents() {
        // Set default form size, closing event and layout manager
        setSize(500, 500);
        setTitle("JTextField Key Listener");
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        getContentPane().setLayout(new FlowLayout(FlowLayout.LEFT));

        // Create a label and text field for our demo application and add the
        // component to the frame content pane object.
        JLabel usernameLabel = new JLabel("Username: ");
        JTextField usernameTextField = new JTextField();
        usernameTextField.setPreferredSize(new Dimension(150, 20));
        getContentPane().add(usernameLabel);
        getContentPane().add(usernameTextField);

        // Register a KeyListener for the text field. Using the KeyAdapter class
        // allow us implement the only key listener event that we want to listen,
        // in this example we use the keyReleased event.
        usernameTextField.addKeyListener(new KeyAdapter() {
            public void keyReleased(KeyEvent e) {
                JTextField textField = (JTextField) e.getSource();
                String text = textField.getText();
                textField.setText(text.toUpperCase());
            }
        });
    }
}