Small initial heaps reduce fragmentation by encouraging the collector to perform compactions early, when the heap is small and the cost of compaction is lower.

G1 compacts sufficiently to completely avoid the use of fine-grain free lists for allocation, which considerably simplifies parts of the collector and mostly eliminates potential fragmentation issues.

The gencon policy avoids fragmentation by using a copying collector that efficiently compacts the nursery area as a side-effect of every collection.