The UML Extension gives BlueJ the ability to display more information about Java classes, in the style of a UML class icon. Right-clicking on a BlueJ class and selecting the "Display UML" menu item will pop up a UML class icon of the selected BlueJ class. Clicking anywhere in the new class icon will remove it from the display.
The BlueJ Tools/Preferences/Extensions panel can be used to control the details of what is displayed in the class icon. Individual elements of a class's methods and attributes can be displayed or hidden, as can visibility annotations. Entries in the icon can be displayed using either Java or UML syntax. The default values of all of these properties can be set in BlueJ's properties files.