The high density of population in the Nile Valley and the Island of Java is primarily due to intensive agriculture.

1. Intensive agriculture: The Nile Valley, particularly in Egypt, has historically been a highly fertile and productive agricultural region due to the annual flooding of the Nile River. The availability of water and fertile soil has facilitated intensive agricultural practices, including the cultivation of crops such as wheat, barley, and various fruits and vegetables. The agricultural productivity of the Nile Valley has supported a dense population over centuries.

Similarly, the Island of Java in Indonesia is known for its rich volcanic soil and favorable climate, which have made it a highly productive agricultural region. The cultivation of crops such as rice, tea, coffee, and spices has supported a high population density on the island.

2. Industrialization and urbanization: While industrialization and urbanization can contribute to population density, they are not the primary factors in the case of the Nile Valley and the Island of Java. These regions have seen some level of industrial and urban development, but the historical and ongoing significance of intensive agriculture has played a more significant role in supporting high population densities.

3. Topographic constraints: Topographic constraints, such as rugged terrain or limited habitable land, can sometimes limit population density. However, in the case of the Nile Valley and the Island of Java, the availability of fertile land and favorable agricultural conditions has outweighed any potential topographic constraints, allowing for the development of high-density human settlements.

In summary, the primary reason for the high population density in the Nile Valley and the Island of Java is intensive agriculture, supported by the availability of fertile land and favorable agricultural conditions.