In java what are nested classes and what do they do?

Question

In java what are nested classes and what do they do?

Answer 1

They're just classes within other classes. They make it possible to have a hierarchy of classes, and if you make them private they're a convenient way to encapsulate data that isn't exposed outside of the class using them. Sun has a short tutorial about them

Answer 2

One of the most important uses of inner classes in Java are listeners. Instead of writing an entire separate class for an ActionListener or similar, you create it in-place:

button.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        ... your code here ...
    }
}

These inner classes typically call functions in the outer class (they capture a pointer to the object they were created in), and lead to much tidier code than having a single callback with some logic that has to figure out which button etc. was clicked. You also don't have to modify code in several places when you add or remove a button, since the inner class is created where you create the button. It's quite elegant.