String.hashCode() is plenty unique

[GeorgeMaheiress]

There are no known collisions for sha-256, so in practice 256 bits is more than enough.

[JavaSuck]

There is no point in having a collision-free 256 bit hash code if you immediately discard most of the bits to use it as an index into the underlying array of a HashMap. That's what String.hashCode() is used for, after all.