Simple library

From APIDesign

Some libraries, let's name them as simple library, have just one implementation. The designer that defines what the API should do also implements it. Obviously, the proximity between the API and the provider of the implementation is almost zero - it is the same person.

Simple library are usually self contained. They contains the API as well as the implementation. Examples include most of java.util package, or Math class. Of course, there may be some attempts to re-implement these libraries (like GNU Classpath or Harmony), but those are licensing driven efforts, not technical ones. In general, there is little to no reason to re-implement such libraries.