Bear in mind that the Java-Cocoa bridge has been deprecated since Tiger, so may not be the best choice for new development.
Anyway, there is an 'AlbumData.xml' plist file in the iPhoto library which may or may not be intended to be third-party readable (although various third-parties read it anyway). There's also an ObjC plugin API, although I'm fairly sure that's undocumented, and probably not quite what the OP's needing anyway. The main (official) API for manipulating iPhoto data is iPhoto's Apple event ['AppleScript'] interface. It's less than stellar as APIs go, being somewhat quirky, buggy and/or deficient in various areas, but can probably be cajoled into doing what's needed.
Ideally the OP wants a high-level Java-Apple event bridge that would allow Java programs to control 'AppleScriptable' applications directly. Right now no such beast exists, although I am planning to do a port of objc-appscript to Java next year. That may be too long to wait, however, in which case the OP will need to make their own arrangements.
For simple tasks using
osascript to execute prewritten AppleScripts that take string-based arguments may be adequate, but for anything complex you'd probably find yourself having to do code generation which is pretty evil and prone to error.
From poking around the
MRJAdapter source code it does look as if lower-level Apple Event Manager bindings do already exist, although I've not managed to find any other information on these which makes me think they're probably part of an undocumented API. The MRJAdapter developer may know more about this. That said, working directly with the Apple Event Manager API quickly becomes a real chore as it's rather low-level and the resulting code is very verbose.
Probably the first thing the OP should do is get familiar with AppleScript (Matt Neuburg's 'AppleScript: The Definitive Guide' (2nd edition) is best for this) and scripting iPhoto, and make sure that iPhoto can do everything they want. For more detailed advice and suggestions, we'd need to know more about the planned application and what it needs to do.
HTH