Javanese is the language of the javanese, that is people from
Java (island of
Indonesia where the capital,
Jakarta is located). It is spoken by around 75 million people, which is around 1/3rd of Indonesian
population. All, if not the majority of the speakers would be of javanese descent. Knowing javanese language while not being javanese is like a japanese knowing korean language (Many Indonesians of other decents hate Javanese people, but some have to live with them). This is not uncommon however.
This language is quite broad in terms of it's variation, the western, middle and eastern java has it's own variation of the dialect. Evenmoreso, according to my father who was of javanese descent, There are different dialects used by commoners and regals ("high" javanese), that is the upper-class people of Java. The difference between these two is quite significant, that most regal javanese speaker wouldn't really understand a commoner's dialect. Commoners however, needed to know the regal javanese (they were under control), and they were required to speak "high" javanese to the regals.
The indonesian language (Bahasa indonesia) Contains many javanese words, as it is a rather new language, for example the indonesian word "siapa" was derived from javanese "sopo". The language itself is very similar to bahasa indonesia, due to the fact that most of the people who "rule" Indonesia are of javanese descent, and so is the reason that much of the grammar of the Javanese language is "transferred" to bahasa indonesia.
So, next time you want to express your knowledge of Java the programming language, don't say "I know how to speak Javanese". Or vice versa.