Question

I have two kinds of objects identified by a long and these objects should be mapped to column or row indices in a matrix of type int. There are thousands of objects to be mapped and only the combination of the IDs is unique (e.g. (1,1) -> 0; (1,2) -> 1; … (2,2)->…)).

Is a Map<String, Integer> where the key is "long1<a separator>long2" the way to go or are there better solutions?

Thanks

Answer 1

how to create the hash from the two longs

You can create a hash from a long with

long l = ...
int hash = (int) ((l >> 32) ^ l);

for two longs you can combine there

long a = ..., b = ...
long l = a * 31 + b;
int hash = (int) ((l >> 32) ^ l);

Its not guaranteed to be unique, but it doesn’t use any memory.