MouseListener is an interface in the Java programming language that defines a listener for mouse events, such as mouse clicks and mouse movements. A MouseListener receives notification when a user interacts with a GUI component using the mouse.

To use a MouseListener, you typically create an instance of a class that implements the MouseListener interface and then register it with the component that generates the mouse events. For example, to create a MouseListener for a JButton, you might write:

button.addMouseListener(new MouseListener() {
    public void mouseClicked(MouseEvent e) {
        // Perform some action when the button is clicked
    }
    public void mouseEntered(MouseEvent e) {
        // Perform some action when the mouse enters the button
    }
    public void mouseExited(MouseEvent e) {
        // Perform some action when the mouse exits the button
    }
    public void mousePressed(MouseEvent e) {
        // Perform some action when the mouse button is pressed on the button
    }
    public void mouseReleased(MouseEvent e) {
        // Perform some action when the mouse button is released on the button
    }
});

This code creates an anonymous inner class that implements the MouseListener interface and defines the methods for handling the various mouse 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 MouseListener instances more concisely. For example:

button.addMouseListener(new MouseAdapter() {
    public void mouseClicked(MouseEvent e) {
        // Perform some action when the button is clicked
    }
});

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

button.addMouseListener(new MouseAdapter() {
    @Override
    public void mouseClicked(MouseEvent e) {
        // Perform some action when the button is clicked
    }
});

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