Using Event Listeners
Components can generate events, which are occurrences that take place as users interact with an application, such as when they click a button or change a value. There are also events, such as gaining or losing focus, that are not the result of user interactions.
If there is a listener for the event, the event communicates some information about what happened, so that the listener can respond. An event, then, is a component’s way of letting a listener know that something happened.
An event listener is a class that implements one of the listener interfaces and contains code to respond to the event. A component has mechanisms to register and deregister event listeners for the types of events that are generated by the component.