Their native language is Javanese, it is the largest of the Austronesian languages in number of native speakers and also the largest regional language in Southeast Asia. 

There are significant numbers of Javanese diaspora outside of central and eastern Java regions, including the other provinces of Indonesia, and also in another countries such as Suriname, Singapore, Malaysia, Egypt, Saudi Arabia, South Africa, Sri Lanka, Yemen and the Netherlands. The Javanese ethnic group has many sub-groups (based on native Javanese community on the island of Java) that can be distinguished based on their characteristics, customs, traditions, dialects, or even ways of life, such as Banyumasan, Cirebonese, Mataram, Osing, Tenggerese, etc.

The Javanese culture is one of the oldest civilizations and has flourished in Indonesia and Southeast Asia. It has gradually absorbed various elements and influences from other cultures, including native reverence for ancestral and natural spirits, Buddhist and Hindu dharmic values, Islamic civilization, and to a lesser extent, Christianity, Western philosophy and modern ideas.

With a large global population, the Javanese are considered significant as they are the largest Muslim ethnic group in the Far East and the fourth largest in the world after the Arabs, Bengalis and Punjabis.

According to Wikipedia