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So I read about HashMap. At one point it was noted:
"Immutability also allows caching the hashcode of different keys which makes the overall retrieval process very fast and suggest that String and various wrapper classes (e.g.,
Integer) provided by Java Collection API are very goodHashMapkeys."
I don't quite understand... why?
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Immutable objects are also useful because they are inherently thread-safe. Other benefits are that they are simpler to understand and reason about and offer higher security than mutable objects.
Immutabiility is required, in order to prevent changes on fields used to calculate hashCode() because if key object return different hashCode during insertion and retrieval than it won't be possible to get object from HashMap.
Strings immutability makes it possible to share them, this is more efficient memory-wise.
Dicts and sets must use a hash for efficient lookup in a hash table; the hash values must be immutable, because changing the hash will mess up the data structures and cause the dict or set to fail.
String#hashCode:
private int hash; ... public int hashCode() { int h = hash; if (h == 0 && count > 0) { int off = offset; char val[] = value; int len = count; for (int i = 0; i < len; i++) { h = 31*h + val[off++]; } hash = h; } return h; } Since the contents of a String never change, the makers of the class chose to cache the hash after it had been calculated once. This way, time is not wasted recalculating the same value.
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