Magic Factors

Growth Factor

If array list or hash container has growth factor gf, it's expected memory footprint is gf * log(gf) / (gf - 1) (slots per element) and expected number of reinsertions during grows is gf * log(gf) / (gf - 1)^2 (reinsertions per element). In particular:

Growth Factor	Memory Footprint	Reinsertions
1.5	1.21	2.43
2.0	1.38	1.38
3.0	1.64	0.82

For example, average effective load factor of hash containers with formal load factor 0.5 and default growth factor 2.0 is 0.5 / 1.38 = 0.36.

Reinsertions into array list are very cheap (Arrays.copyOf), reinsertions into hash container are only aboout 30% cheaper than ordinary insertions.