Political expansions and trade activities enabled migration among ethnic groups in the Java Sea region. In this connection, not only the Javanese migrated to regions around the Java Sea but also other ethnic groups. In the pre-colonial Makassar for example, Javanese, Banjarese, Madurese, Malays, and Balinese could be found easily. Besides, ethnic groups from the Outer Islands could also be easily found in most harbor cities along the north coast of Java such as those of Bantam, Cirebon, Demak, Tuban, Gresik, and Surabaya. In those cities, Buginese, Madurese, Malays, and Makassarese settlements were not a strange thing. Internal migration was actually one of the most important basics for getting cultural understanding among social groups in the region around the Java Sea.
Islamic education and missionary also became the cultural foundation of Javenese influences in the Outer Islands. Beside Aceh and Minangkabau, the influence of Islam from Java in the region around the Java Sea was also significant. Although the existence of Islam in Java itself was younger compared to Islam in Aceh (Samudra Pasai) for example, the institutionalization of Islamic education in Java, i.e. Islamic boarding schools (pesantren), was more develop than elsewhere in the archipelago. Since the 15th century one of the cities in the north coast of Java, i.e. Gresik (Pesantren Giri) became an important center of Islamic education in the archipelago.