Use your browser
The simplest way to get started with the Activity Streams API (and in fact many REST APIs) is to use some browser based tooling to help analyze and understand the use and effect of the REST API calls. Amongst these are:
- RESTClient - allows you to generate REST requests from within the Firebug browser and view the responses
- JSONView - allows viewing of JSON content in Firefox
- Tools->Developer Tools - comes with Chrome and does mostly the same as firebug
- REST Console - allows you to generate REST requests
- JSONView - allows viewing of JSON content in Chrome
There are many other tools and options to help, but the above are a good selection to help get you started.
From a Java application
There are a lot of good Java libraries available to assist, some examples include:
- Apache HTTP Client - If you want to build your own - now being moved to Apache HTTP Components
- Jersey - Both Client and Server support is offered
- Abdera - an Atom reader but is still useful for HTTP access alone when using JSON as it manages authentication quite well
Regardless of the technology you use, it is useful to have a debugging proxy such as Fiddler to help you look at the requests and responses you are sending/receiving.