Yesterday Google released version 0.4 of the AdWords API Java Client Library. The new version of Java Client makes it easier to write Java clients to programmatically access AdWords accounts.
The new version incorporates very few code changes and it still supports AdWords API v4 (except a a fix to the AdWordsService interface which was lacking a few methods used for quota management). There are many improvements in the documentation and samples. The javadoc for the library is updated to reflect the new architecture, and includes samples showing how to use all the services, and the most useful methods in the API.
The AdWordsUser class has methods for setting username, password, useragent and token so that you don't have to write the code to set the request headers. The client library uses AspectJ that enables you to work directly with the Axis generated classes without having to worry about implementing quota management manually. The AdWordsUser class provides methods for setting credentials for logging into AdWords accounts as well as for creating instances of the AdWordsService classes.