Kumo's goal is to create a powerful and user friendly Word Cloud API in Java. Kumo directly generates an image file without the need to create an applet (as many other libraries do).
Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window kumo-1.28.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.