List的Java映射值到Hashmap中

Question

I have a HashMap:

K1, false
K2, false
K3, false
...
K1000, false

Initially all the values of keys are false.

I have an ArrayList:

K3, K10, K33,..., K417, K834, K998

I need to get the values of the list and mark just those values as "true" in the Hashmap for the corresponding keys. Now I know we can do this:

Iterate HashMap:
    Iterate List:
        if HashMap.contains(List[i])
            HashMap.put(List[i], true)

But we are using 2 iterations in this logic and maybe for 1000 keys it is not much of a problem, but if I want to scale it to a million keys, is there any efficient algorithm I can use to achieve above?

Thanks.

Answer 1

You don't need to iterate HashMap explicitly. hashMap.containsKey(key) checks if a key present in the map or not in an optimized way. You need to study a bit on how HashMap works.

Iterate List:
    if HashMap.containsKey(List[i])
        HashMap.put(List[i], true)

This will work.

Answer 2

Use Map.replace() :

for (E element : list) {
    map.replace(element, true);
}

This will only update existing keys with matching elements in list .

As a side note, a Map<K, Boolean> can often be replaced by a lighter Set<K> .