ArcGIS offers a number of SDKs for GIS application development for the Java platform. The links below and in the navigation bar at left direct you to documentation and samples for each SDK.
ArcGIS Server SDK
ArcGIS Server SDK provides APIs to build browser based applications with mapping and spatial capabilities. These APIs provide both server side and client side controls and communicate with ArcGIS Server services. Along with these APIs the SDK is also bundled with other tools and ArcGIS plug-ins for Eclipse and Netbeans IDE for ease of development.
ArcObjects SDK
ArcObjects are the basic components of ArcGIS Engine, ArcGIS Desktop and ArcGIS Server. Using ArcObjects Java API you can build pluggable components and extend the functionality of ArcGIS Server and ArcGIS Desktop. You can also use ArcObjects API to build Swing and Console applications. ArcObjects SDK as a whole consists of help docs and samples as well as ArcGIS Plug-in for Eclipse IDE to get started.