Java (Indonesian: Jawa) is an island in Indonesia.
Java is Indonesia's fifth-largest island. Its 120 million people make up 65% of Indonesia's entire population, and makes Java the most populated island in the world. Covering an area of 127,569 sq. km (7% of Indonesia's area) and with a population density of 940 people/sq. km, Java is the world's most crowded island, as well.
Jakarta, the capital of Indonesia, is located on northwest region of Java. Surabaya (the second largest city in Indonesia), is located on East Java. Java acts as Indonesia's center of cultural and economic activity.
The main ethnic group in Java is Javanese, except for the West Java region where most people are Sundanese. In the province of East Java, 22% of the population is of Madurese descent. Half of them live on the island of Madura.
Jakarta is the capital of Indonesia. Population: 8 million (within city limits). The total population of Jakarta and its suburbs: 12 million people. The population of Jakarta is a mix of many ethnic groups. The group native to Jakarta is Betawi.
While Indonesian (Bahasa Indonesia) is the lingua franca understood by almost everyone, the majority of Java's population also speak Javanese, a related but mutually incomprehensible language. As the largest single language in Indonesia, Javanese influence on Bahasa Indonesia has been quite significant, and particularly in Central and East Java you'll even hear the local pronunciation change — the most notable change is a turning into o, so eg. Surabaya becomes Suroboyo.
Most visitors arrive at Jakarta, Bandung, Semarang, Surabaya, Solo and Yogyakarta. There are several domestic airlines such as Garuda, Merpati, Lion Air, Mandala, Sriwijaya Air. International airlines: AirAsia, SQ, Qantas, MAS, Cathay Pacific, PAL, Thai International, JAL, KAL, Eva Air, Air India, etc.
PT Angkutan Sungai Danau dan Penyeberangan/ASDP ferry services:
PT Pelayaran Nasional Indonesia/PELNI passenger ship routes include:
You can take a private car to explore java. It will be more satisfied and comfort, because the public transport not so good. You can stop at any place that you want, it cant enjoyed by public transport. 
Java's infrastructure is by far the best in Indonesia, and it's possible to cross the entire island by land in a single (long) day. However, the sheer density of population means that roads, buses and trains can get overcrowded if you're moving at the same time as everyone else.
The main form of long-distance transport, vast armadas of buses cross the island and connect every city and mountain hamlet. Watch out though, as many drivers have near-suicidal driving habits that emphasize speed above all else.
Java has the best railway network in Indonesia, with trains connecting the capital city of Jakarta with most other cities and towns in the island. The eksekutif class is the best class, and consequently the most expensive.
Ticket reservations can be made starting one month in advance. No on-line ticket reservation is available, but availability can be gleaned on PT Kereta Api's ticketing site. Descriptions of various trains are available on Kereta Api website.
Java's toll roads are built nearly to Western standards, with dual grade-separated lanes in both directions and elevated interchanges. Tolls are quite affordable, usually Rp 1,000-5,000 every 10 km or so. However, signage is often lacking and driving habits are atrocious, so it's very wise to pay a little extra and get a car with driver instead of attempting to drive yourself.
Java offers everything a somewhat adventurous traveler is looking for: volcanoes, rainforests, tea and rice plantations, large cities, big waves and even savanna. The scenery in most parts of Java is spectacular and, while the island is overpopulated, there are still plenty of unspoiled places. Even though the beaches are not very appealing in Java and tides can be fatal especially in south part of Java, in nearby archipelagos like Pulau Seribu or Karimunjawa the traveler can find white sand beaches and corals islands.
Boiled water or water from a sealed bottle. Coffee or tea. It's safe. The same applies to Bir Bintang, which combines very well with a hot indonesian dish.
Use common sense, as you would do in your own country. Most common is (fortunately) theft, violent crime is rare in Indonesia. However, it's wise to be extra cautious in Jakarta and other big cities (See also wikitravel: Jakarta section). Avoid crowds.
Traffic can be chaotic. Watch out for trucks, cars and other moving objects. One rule applies: "might is right". Don't expect traffic to respect pedestrian ("Zebra") crossings (even when a police(wo)man is in sight). When crossing a busy street - be confident, make sure your intentions are clear to the oncoming drivers. Cars never stop, but if they know what they can expect from you, they will avoid you. Using hand signal like put one hand in the air or signaling stop might help, too.WikiPedia:Java (island) Dmoz:Asia/Indonesia/Regions/Java World66:asia/southeastasia/indonesia/java