Java Tip: ComputeIfAbsent

Jan 01, 0001
Java

Often we have to loop through a list of items and convert it into a map, with the key being some sort of category and the value being a set of the associated items ie Map<Key, Set<Object>>. The old way of doing this looped through the list and before adding checked if the key was present. Using computeIfAbsent we can clean up this syntax considerably.

Instead of


// Map of item types to items
Map<String, Set<Items>> map = new HashMap<>();

for (Item item: items) {
    if (map.containsKey(item.getType())) {
        map.get(item.getType().add(item));
    } else {
        Set newSet = new HashSet<>();
        newSet.add(item);
        map.put(item.getType(), newSet));
    }
}

Do

// Map of item types to items
Map<String, Set<Items>> map = new HashMap<>();

for (Item item: items) {
    map.computeIfAbsent(item.getType(), v -> new HashSet<>()).
    add(item);
}