Question

I’m having a hard time understanding the DEM (Delegation Event Model) in Java. After reading this question with answers I didn’t get the explanation I needed.

What are the motivations for using DEM?

My observations in the examples I’ve seen are:

• There is a model with a list of listeners.
• That model also has a methods like fireThisOrThat() that are called from other methods like addThisOrThat.

I will get this on an upcoming exam, that’s why I need to understand it and how to use it.

Answer 1

Delegation means a source generates an event and sends it to one or more listeners.

Java Swing is an excellent example of delegation. A Swing object, like a JButton, will generate events. In the example of a JButton, an event would be that the button image receives a left click (the button is pressed).

Any number of other objects can listen to the JButton. This allows one or more event actions to take place as a result of the JButton event.

Suppose you have a JTextArea. You can have a listener on the JButton such that when the JButton fires a left mouse click event, the JTextArea listener edits the text and writes the text to a database. In other words, you push (left click) a button on the screen, and the text in a JTextArea is written to a database.

By using delegation, the JButton and the JTextArea aren’t coupled together. You could replace the JTextArea with another Swing component, and the Swing components would still work together.