Logan Logan - 1 year ago 69
Java Question

Is it faster to check if a map contains a key or just put that value?

If I know that the value being entered into an entry of the map is the same, would I be better off checking if it's not in the map or just doing the put?
In other words, is

if (!map.containsKey(key)){
map.put(key, value);

better than just doing
map.put(key, value)
if I know that the value is the same for the key.

I am trying to calculate all of the overlaps for a map that maps id to a set of ints. For example: given
{A:1,2,3; B:2,4,6; C:2,3,4; D:1,3,5}
I need to get this map:
{AnB:2; AnC:2,3; AnD:1,3; BnC:2,4; AnBnC:2; AnCnD:3;}
. Doing this recursively is not an option as for larger sets we run of heap space. So I am doing it iteratively by adding the next "layer" onto the existing layer. So to get AnBnC I take AnB and calculate the intersection with C. But this also means I take AnC and calculate the intersection with B which will both give the same result.


Answer Source

I'm not sure about the answer to your direct question about which is faster, but if you're trying to optimize, I suggest avoid doing unnecessary set intersections. Something like;

intersectionKey = setA.key + setB.key;
if (!map.containsKey(intersectionKey)) {
    intersection = A | B;
    map.put(intersectionKey, intersection);
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