【JAVA进阶】HashMAP中加载因子0.75的问题

An instance of HashMap has two parameters that affect its performance: initial capacity and load 

factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply
 the capacity at the time the hash table is created. The load factor is a measure of how full the 

hash table is allowed to get before its capacity is automatically increased. When the number of 

entries in the hash table exceeds the product of the load factor and the current capacity, the hash 

table is rehashed (that is, internal data structures are rebuilt) so that the hash table has 

approximately twice the number of buckets.

As a general rule, the default load factor (.75) offers a good tradeoff between time and space 

costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of

 the operations of the HashMap class, including get and put). The expected number of entries in the

 map and its load factor should be taken into account when setting its initial capacity, so as to 

minimize the number of rehash operations. If the initial capacity is greater than the 
maximum number of entries divided by the load factor, no rehash operations 
will ever occur.



As with all performance optimizations, it is a good idea to avoid optimizing 
things prematurely (i.e. without hard data on where the bottlenecks are).