Java Swing

addActionListener() means the class is listenable to some actions.
The listenable classes includes
Swing: JComboBox, Jbutton, JTextField, JMenuItem, JCheckBox, JRadioButton
SwingX: JXDatePicker,

It is common to make a class extends from a listenable Swing class and implements ActionListener for actionPerformed(ActionEvent).

Classes that implements ActionListener becomes a listener that listens to some listenable classes.
When some actions happens to listenable classes, actionPerformed(ActionEvent) will be invoked.
When actionPerformed, ActionListener reacts to the action.
It is like action and reaction pair that Listenable and ActionListener comes together always.

Some common methods of ActionEvent are:
Object EventObject.getSource()
String getActionCommand() like button name
long getWhen()