KeyListener is an interface in the Java programming language that defines a listener for keyboard-related events, such as key pressed, released, and typed. A KeyListener receives notification when a key is pressed or released on the keyboard.

To use a KeyListener, you typically create an instance of a class that implements the KeyListener interface and then register it with a component that generates keyboard events, such as a JTextField or JTextArea. For example, to create a KeyListener for a JTextField, you might write:

textField.addKeyListener(new KeyListener() {
    public void keyTyped(KeyEvent e) {
        // Perform some action when a key is typed (pressed and released)
    }
    public void keyPressed(KeyEvent e) {
        // Perform some action when a key is pressed
    }
    public void keyReleased(KeyEvent e) {
        // Perform some action when a key is released
    }
});

This code creates an anonymous inner class that implements the KeyListener interface and defines the methods for handling the various keyboard events. Each method is called when the corresponding event occurs, and can be used to perform some action in response to that event.

In Java 8 and later, you can use lambda expressions to define KeyListener instances more concisely. For example:

textField.addKeyListener(new KeyAdapter() {
    public void keyPressed(KeyEvent e) {
        // Perform some action when a key is pressed
    }
});

This code creates a KeyAdapter instance (a class that implements the KeyListener interface with empty method implementations) and overrides only the keyPressed method to perform the desired action. Alternatively, you can use a lambda expression to create the KeyAdapter instance:

textField.addKeyListener(new KeyAdapter() {
    @Override
    public void keyPressed(KeyEvent e) {
        // Perform some action when a key is pressed
    }
});

This code is equivalent to the previous example, but uses a lambda expression to create the KeyAdapter instance.