Java library for Bitcoin Client Application are there Java libraries for Bitcoin available to develop client applications?
BitCoinJ

In addition to @D.H's answer, BitCoinJ provides the following for developers


open source through Subversion
easy to use Java API
easy to integrate via Maven
access to either release or snapshot editions depending on your risk profile


BitCoinJ also uses the Bouncy Castle encryption algorithm implementations. Some of these were missing or poorly implemented in Android which necessitated the inclusion of these libraries into BitCoinJ (currently via the Maven Shade plugin to allow updates to Bouncy Castle to be introduced when they occur). 

These inclusions makes it a very useful library to get started with than simply rolling your own against the JSON API of the standard client. 

Disclaimer: I'm a contributor to the BitCoinJ project so I'm a bit biased regarding it's utility. 

BCCAPI

A transaction and balance monitoring API for Java.

From the website:


  The BCCAPI (BitCoin Client API) allows you to create a light-weight
  secure Bitcoin client in Java. The BCCAPI connects to a server that
  holds the block chain, and which tracks the client’s wallet balance on
  the clients behalf. The server only has knowledge of the clients
  public keys, and is in no position to spend funds owned by the
  client’s wallet.


This could be useful if your application does not require the client to spend coins through your system.

Off topic: A note on Bitcoin clients in Java

Although it's not a library as such, you may benefit from looking at the source code for the MultiBit project to get you started in creating a Java Swing based Bitcoin client based on BitCoinJ with internationalisation and QR code support.