For the most up to date overview on the Pusher Java Client API see the Pusher Java Client README.
Packages 
Package Description
com.pusher.client
Classes for connecting to Pusher, configuring the Pusher instance and authenticating subscriptions.
com.pusher.client.channel
Classes related to channels, which are subscriptions to data within Pusher, and channel event listener interfaces.
com.pusher.client.connection
Classes related to the Pusher connection, connection state and monitoring connection state changes.
com.pusher.client.user  
com.pusher.client.user.impl  
com.pusher.client.user.impl.message  
com.pusher.client.util
Contains helper classes.