Java8 Lambda compare two List and trasform to Map - list

Question

Suppose I have two class:
class Key {
private Integer id;
private String key;
}
class Value {
private Integer id;
private Integer key_id;
private String value;
}
Now I fill the first list as follows:
List<Key> keys = new ArrayLisy<>();
keys.add(new Key(1, "Name"));
keys.add(new Key(2, "Surname"));
keys.add(new Key(3, "Address"));
And the second one:
List<Value> values = new ArrayLisy<>();
values.add(new Value(1, 1, "Mark"));
values.add(new Value(2, 3, "Fifth Avenue"));
values.add(new Value(3, 2, "Fischer"));
Can you please tell me how can I rewrite the follow code:
for (Key k : keys) {
for (Value v : values) {
if (k.getId().equals(v.getKey_Id())) {
map.put(k.getKey(), v.getValue());
break;
}
}
}
Using Lambdas?
Thank you!
‐------UPDATE-------
Yes sure it works, I forget "using Lambdas" on the first post (now I added). I would like to rewrite the two nested for cicle with Lamdas.

Answer 1

Here is how you would do it using streams.
stream the keylist
stream an index for indexing the value list
filter matching ids
package the key instance key and the value instance value into a SimpleEntry.
then add that to a map.
Map<String, String> results = keys.stream()
.flatMap(k -> IntStream.range(0, values.size())
.filter(i -> k.getId() == values.get(i).getKey_id())
.mapToObj(i -> new AbstractMap.SimpleEntry<>(
k.getKey(), values.get(i).getValue())))
.collect(Collectors.toMap(Entry::getKey, Entry::getValue));
results.entrySet().forEach(System.out::println);
prints
Address=Fifth Avenue
Surname=Fischer
Name=Mark
Imo, your way is much clearer and easier to understand. Streams/w lambdas or method references are not always the best approach.
A hybrid approach might also be considered.
allocate a map.
iterate over the keys.
stream the values trying to find a match on key_id's and return first one found.
The value was found (isPresent) add to map.
Map<String,String> map = new HashMap<>();
for (Key k : keys) {
Optional<Value> opt = values.stream()
.filter(v -> k.getId() == v.getKey_id())
.findFirst();
if (opt.isPresent()) {
map.put(k.getKey(), opt.get().getValue());
}
}