Java map values of List into a Hashmap

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.

Why iterate the map? Just iterate the list and use it's value as key to set the value to true. for (Object k : list) map.put(k, true) — tsolakp
– tsolakp, Commented Aug 6, 2017 at 23:25
This sounds more like a set than a map. You could just build a Set from the ArrayList's contents; then anything that's not in the set is false. Two reasons to keep the Map instead: 1. Some other interface requires a Map; 2. You need to throw an exception for unexpected keys like "KMFDM" instead of just returning false. — Kevin J. Chase
– Kevin J. Chase, Commented Aug 7, 2017 at 0:44

RP- · Accepted Answer · 2017-08-06 23:24:56Z

3

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.

edited Aug 6, 2017 at 23:24

answered Aug 6, 2017 at 23:22

RP-

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1 Comment

RP- Over a year ago

No, there are no stupid questions :) just a matter of time to learn.

shmosel · Accepted Answer · 2017-08-06 23:49:16Z

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>.

edited Aug 6, 2017 at 23:49

answered Aug 6, 2017 at 23:38

shmosel

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