Best HashMap initial capacity while indexing a List

Question

I have a list (List list) and I want to index its objects by their ids using a map (HashMap map). I always use list.si

Answer 1

If you wish to avoid rehashing the HashMap, and you know that no other elements will be placed into the HashMap, then you must take into account the load factor as well as the initial capacity. The load factor for a HashMap defaults to 0.75.

The calculation to determine whether rehashing is necessary occurs whenever an new entry is added, e.g. put places a new key/value. So if you specify an initial capacity of list.size(), and a load factor of 1, then it will rehash after the last put. So to prevent rehashing, use a load factor of 1 and a capacity of list.size() + 1.

EDIT

Looking at the HashMap source code, it will rehash if the old size meets or exceeds the threshold, so it won't rehash on the last put. So it looks like a capacity of list.size() should be fine.

HashMap map = new HashMap(list.size(), 1.0);

Here's the relevant piece of HashMap source code:

void addEntry(int hash, K key, V value, int bucketIndex) {
    Entry e = table[bucketIndex];
    table[bucketIndex] = new Entry<>(hash, key, value, e);
    if (size++ >= threshold)
        resize(2 * table.length);
}