I agree with the other answers; Casuarina equisetifolia.
This species is a common coastal species in the region and readily colonizes disturbed ground and is recorded for Krakatau.
See:
Partomihardjo, T., Mirmanto, E. and Whittaker, R.J., 1992. Anak Krakatau's Vegetation and Flora circa 1991, with Observations on a Decade of Development and Change. GeoJournal, 28(2), pp.233-248.
Whittaker, R.J., Bush, M.B. and Richards, K., 1989. Plant recolonization and vegetation succession on the Krakatau Islands, Indonesia. Ecological Monographs, 59(2), pp.59-123.