"The Java Platform APIs are a set of essential interfaces that developers need to build their Java applications and applets.
All Java Platfrom APIs are open and extensible, and are created by JavaSoft and industry-wide specialists in each target technology and market area.
By targeting the Java Platform, developers will be assured that their applications can run anywhere, without any need to port.
The widespread availability of the Java Platfrom and the industry-standard Java Platform APIs have created
the highest-volume software application platform available today". (Sun Microsystems)
Additionally this page shows a lot of class libraries from different sources.