>A common mistake I see Java developers make is, when they know they'll be inserting n items into a HashSet or HashMap is to initialize the structure with a capacity of n.
The common mistake is using the c-tor allocating N size. The c-tor does way more harm than good. Most HashMaps in Java have few elements with tons of empty array space, many of them are pre-allocated to quite large sizes too. (At the very least nowadays the empty c-tor doesn't immediately allocate the Node[], so empty HashMaps are ok).
One of the strengths of HashMap is that resize/rehash is extremely fast/efficient and it doesn't dereference the keys (i.e. no hashCode/equals called). It doesn't reallocate Nodes, etc.
The common mistake is using the c-tor allocating N size. The c-tor does way more harm than good. Most HashMaps in Java have few elements with tons of empty array space, many of them are pre-allocated to quite large sizes too. (At the very least nowadays the empty c-tor doesn't immediately allocate the Node[], so empty HashMaps are ok).
One of the strengths of HashMap is that resize/rehash is extremely fast/efficient and it doesn't dereference the keys (i.e. no hashCode/equals called). It doesn't reallocate Nodes, etc.