Java 8: Get default value from HashMap and update underlying map [duplicate]

Question

I'm looking for the best way (readability and efficiency) of providing a default value for a HashMap get operation but to also have the underlying map updated with that default value if a value is not already present.

I understand there are 3rd party libraries out there, but I'd prefer to stick with standard Java. Here are some different ways I've thought of:

Old school

Map<String, List<Integer>> someIntegerListLookup = new HashMap<>();
String key = "key";
...
List<Integer> integerList = someIntegerListLookup.get(key);

if (integerList == null) {
    integerList = new ArrayList<>();
    someIntegerListLookup.put(key, integerList);
}

Java 8 Map methods

// getOrDefault
List<Integer> integerList = someIntegerListLookup.getOrDefault(key, new ArrayList<>());
someIntegerListLookup.put(key, integerList);

// putIfAbsent
someIntegerListLookup.putIfAbsent(key, new ArrayList<>());
List<Integer> integerList = someIntegerListLookup.get(key);

Using Java 8 Optionals

List<Integer> integerList = 
    Optional.ofNullable(someIntegerListLookup.putIfAbsent(key, new ArrayList<>()))
    .orElse(someIntegerListLookup.get(key));

Not a fan of this approach from a readability point of view.

Using an 'or else' supplier:

List<Integer> integerList = Optional.ofNullable(someIntegerListLookup.get(key))
    .orElseGet(() -> {
        // Only runs if the map contains no value for the key
        List<Integer> defaultIntegersList = new ArrayList<>();
        someIntegerListLookup.put(key, defaultIntegersList);
        return defaultIntegersList;
    });

Right now I'm leaning towards the above solution being the best option. Are there other (better) solutions out there for this that I haven't thought of or come across?

Answer 1

The computeIfAbsent method is designed for exactly this purpose:

List<Integer> value = 
    someIntegerListLookup.computeIfAbsent("a", x -> new ArrayList<>());

Answer 2

You don't want to create a new ArrayList unnecessarily if the key is present.

Therefore use :

List<Integer> integerList = someIntegerListLookup.computeIfAbsent(key, s -> new ArrayList<>());