Java lies between Sumatra to the west and Bali to the east. Borneo lies to the north and Christmas Island to the south. It is the world’s 13th largest island. Java is surrounded by Java Sea in the north, Sunda Strait in the west, Indian Ocean in the south and Bali Strait and Madura Strait in the east.
Java is almost entirely of volcanic origin; it contains thirty-eight mountains forming an east-west spine which have at one time or another been active volcanoes. The highest volcano in Java is Mount Semeru (3,676 m). The most active volcano in Java and also in Indonesia is Mount Merapi (2,968 m). See Volcanoes of Java.
More mountains and highlands help to split the interior into a series of relatively isolated regions suitable for wet-rice cultivation; the rice lands of Java are among the richest in the world.
Java was the first place where Indonesian coffee was grown, starting in 1699. Today, Coffea arabica is grown on the Ijen Plateau by small-holders and larger plantations.
The area of Java is approximately 139,000 km2.] It is about 650 miles (1,050 km) long and up to 130 miles (210 km) wide. The island’s longest river is the 600 km long Solo River. The river rises from its source in central Java at the Lawu volcano, then flows north and eastward to its mouth in the Java Sea near the city of Surabaya. Temperatures throughout the year average 22°C to 29°C and humidity average 75%. The northern coastal plains are normally hotter averaging 34°C during the day in the dry season. The south coast is generally cooler than the north, and highland areas inland are cooler again. The wet season begins in October ending in April during which rain falls mostly in the afternoons and intermittently during other parts of the year. The wettest months are January and February.
West Java is wetter than East Java and mountainous regions receive much higher rainfall. The Parahyangan highlands of West Java receive over 4,000 mm annually, while the north coast of East Java receives 900 mm annually. (Wikipedia)