Merge two map with different types

Question

I hope all is good for you.

I have two maps Map<Type1, Type2> and a Map<Type3, Type1>. I want to merge them and have at the end a Map<Type3, Type2>.

How can I do that? I make this code but the result is Map<Type1, Type2>

Map<Type3, Type2> result =
    Stream.of(map1.keySet(), map2.values())
        .flatMap(Collection::stream)
        .toList()
        .stream()
        .filter(
            id ->
                map1.containsKey(id)
                    && map2.containsValue(id))
        .collect(
            Collectors.toMap(
                map2::get,
                map1::get,
                (v1, v2) -> StringUtils.isNotEmpty(v1) ? v1 : v2));

Thanks for your help.

One way I can think of is iterating over the entries of the second map (Map<Type3, Type1>) and using the values (Type1) to look up the corresponding values (Type2) in the first map (Map<Type1, Type2>) — esQmo_
– esQmo_, Commented Sep 6, 2023 at 23:44

rzwitserloot · Accepted Answer · 2023-09-07 13:51:16Z

containsValue is very expensive; it's a full loop. So definitely do not do that.

Collecting into a map requires that you design a function that turns a stream element into a key, and a second function that turns a stream element into a value. That's not very difficult here. T3 remains, it's just that the value of your T3->T1 map needs to be adjusted.

import java.util.AbstractMap.SimpleImmutableEntry;

Map<Type3, Type1> map1 = ...;
Map<Type1, Type2> map2 = ...;

Map<Type3, Type2> result = map1.entrySet().stream()
  .map(entry -> new SimpleImmutableEntry<>(entry.getKey(), map2.get(entry.getValue())))
  .filter(entry -> entry.getValue() != null)
  .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

This:

goes through every K/V pair in your T3->T1 map.
Turns each such pair into a T3->T2 pair instead, by looking up the T1 value in the T1->T2 map.
Assuming you want any k/v pairs in the T3->T1 map where the value is not in the T1->T2 map to just be removed, we then filter it out, (ab)using null as a go-between. We could have filtered with .filter(entry -> map2.containsKey(entry.getValue()) too, but this way we save 1 lookup.
These map entries are already exactly what we want, so we just reconstitute that back into a map.

esQmo_ · Accepted Answer · 2023-09-06 23:49:25Z

0

One way I can think of is iterating over the entries of the second map (Map<Type3, Type1>) and using the values (Type1) to look up the corresponding values (Type2) in the first map (Map<Type1, Type2>)

Map<Type3, Type2> result = new HashMap<>();
for (Map.Entry<Type3, Type1> entry : map2.entrySet()) {
    Type3 key = entry.getKey();
    Type1 value = entry.getValue();
    Type2 otherValue = map1.get(value);
    result.put(key, otherValue);
}

answered Sep 6, 2023 at 23:49

esQmo_

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