1. Download this source code and add to your Java project:
http://www.orbital-computer.de/JComboBox/source/AutoCompletion.java

2. Add folowing code to enable autocompletion for your JComboBox:

AutoCompletion.enable(yourComboBox);

For further information about JComboBox and AutoCompletion see:
http://www.orbital-computer.de/JComboBox/
https://today.java.net/pub/a/today/2007/07/19/adding-auto-completion-to-swing-comboboxes.html

You can install the plugin using the Eclipse Update Manager:

Help > Install new Software
Work with: http://hge.javaforge.com/hgeclipse
[ Add... ]
[ OK ]
check HqEclipse
[ Next > ]
[ Next > ]
check "I accept the terms of the license agreement"
[ Finish ]

confirm Security Warnings dialog with [ OK ]

Restart your Eclipse

https://j2m.googlecode.com/hg/
https://hgeclipse-demo.googlecode.com/hg/

http://code.google.com/p/hgeclipse-demo/