His publications have focused on the history of Mataram, Kartasura, Yogyakarta, Surakarta, locations in Central Java. He has also regularly updated his history of Indonesia - A History of Modern Indonesia, ca. 1300 to the present. Professor Ricklefs has dedicated most of his academic career to understanding how Indonesian society reacted to both the European presence  and the spread of Islam , with an emphasis on cultural as well as political history. Few other living English speaking writers can claim the scope of his knowledge of the history of Java from the 17th to the 21st century. In 2010 he edited and co-authored the New History of Southeast Asia, which continues the work of his friend and mentor D.G.E. Hall, who first published his own History of South East Asia in 1955. From 2004 to 2015, Professor Ricklefs was sectional editor for Southeast Asia for the new 3rd edition of Encyclopaedia of Islam . He is currently a member of the editorial boards of History Today, Studia Islamika, Journal of Indonesian Islam and Journal of Southeast Asian Studies. He co-edits the monograph series Handbook of Oriental Studies/Handbuch der Orientalistik and Brill's Southeast Asia Library .
Question: How many areas in central Java has Ricklefs focused on?
Answer: 4