The object passed to addActionListener() is instantiated from an anonymous class that implements the java.awt.event.ActionListener interface. Nested interfaces and classes.

Because it’s my bag, I’ve found that many Java developers aren’t aware of how to leverage anonymous classes. Indeed, they are an extremely handy feature, which can yield concise, but ...

You need a whole new class to implement a single method. Furthermore, the logic is separated from the point in which it is needed. This will make troubleshooting and code maintenance more difficult.