I know its silly, but previously this URL used to show me all available google open source API's Pervious working API link but now its showing some other page. I am tired of searching the complete API directory. I want to pick one to learn. I have managed to find a similar page which shows only partial API's directory. Google API directory Can any one provide me the correct URL to see complete directory?
I must admit, it's a mess if you take a fresh start.
Accessing Google Apis can be basically be done in two ways.
The old way : Google Data Java Client Library (a.k.a. "gdata-java-client") : docs here. Implemented services can be found at Google Api Docs (the link you are referring to).
The new way : Google API Client Library for Java (a.k.a. "google-api-java-client") : docs here. The Api explorer shows the generated client libraries using the new infrastructure. Or this list.
Both libraries are available for different languages and for different Google services. GData is the most mature library and supports most services.
The new way however is more flexible : the client libraries and even the docs can be automatically generated for any Google service that is ready to transition to the new infrastructure. On the downside, not all Google services have a client library ready for the new infrastructure (e.g. Contacts). You can access services that are not yet transitioned but you have to write your own XML model.
There is a migration guide to move from GData to new infrastructure.