Map<?, Optional<T>> to Map<?, T>

Using Optional as a value in a Map will lead you to unnecessary complexity and confusion — as you can see.

The primary intention of Optional is to serve as a return type, indicating that a value may or may not be present.

To achieve what you are looking for, you need to unwrap the values:

final var result = input.entrySet()
    .stream()
    .filter(it -> it.getValue().isPresent())
    .collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue().get()));

posted 3 months ago

CC0

ɯıpɐʌ‭

154 reputation 4 3 18 2

Based on the code snippet in the question (which uses collect etc), I'm assuming you want to use streams. I'm also infering that "empty values" means those values for which Optional.isEmpty() returns false (and consequently Optional.isPresent() returns true).

In that case, just create a stream for the map entries, filter the non-empty ones and collect to a new map, by getting the Optional's values:

Map<K, T> result = input
    // get stream of map entries
    .entrySet().stream()
    // check if value is present (which means it's not empty)
    .filter(e -> e.getValue().isPresent()) // alternative: .filter(e -> ! e.getValue().isEmpty())
    // collect to a new map, getting the values from the Optional
    .collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue().get()));

But of course you can also do it without streams, using the traditional loop approach:

Map<K, T> result = new HashMap<>();
for (Map.Entry<K, Optional<T>> entry : input.entrySet()) {
    // value is present (AKA: not empty)
    if (entry.getValue().isPresent()) { // alternative: if (! entry.getValue().isEmpty())
        // add the Optional's value to result
        result.put(entry.getKey(), entry.getValue().get());
    }
}

IMO, the second approach makes it easier to change the Map implementation - if you want a TreeMap instead of a HashMap, for example, just need to change the line Map<K, T> result = new HashMap<>(); to use whatever implementation type you want.

With streams, it's also possible, but not so straighforward. You'd have to change the collector to:

Map<K, T> result = input
  ....
  .collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue().get(), (a, b) -> a, TreeMap::new));

The fourth parameter tells what implementation type we'd like to return. But to use this, we must provide the third parameter, which is used to resolve collisions between values associated with the same key. In this case I'm assuming there will be no collisions (as the question has no indication that such situation can happen), so I'm just returning the first occurrence.

posted 3 months ago

CC BY-SA 4.0

3mo ago

hkotsubo‭

5110 reputation 20 69 575 228