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

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?

Thanks @shmosel. Nice and simple, that's exactly what I needed. I'm still trying to get familiar with all the Java 8 apis. — Stephen Asherson, Dec 29 '16 at 08:48

Eran · Answer 1 · 2016-12-29T08:50:14.420

5

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<>());

edited Dec 29 '16 at 08:50

answered Dec 29 '16 at 08:40

Eran

387,369
54
702
768

1

The second example doesn't compile. See the discussion I'm having with Andreas on [my answer](http://stackoverflow.com/a/41375974/2422776). – Mureinik Dec 29 '16 at 08:46
1

There's a fundamental difference between `putIfAbsent()` and `computeIfAbsent()`: the former returns null in the absent case, which isn't what OP wants. Also, `ArrayList::new` wont compile unless `key` is `Integer`, in which case you'd be better off if it didn't compile. – shmosel Dec 29 '16 at 08:47
1

@Mureinik My bad, I didn't read the signature of that method carefully. Will fix. – Eran Dec 29 '16 at 08:49
1

@shmosel I missed that. Thanks for the comment. – Eran Dec 29 '16 at 08:52

Mureinik · Accepted Answer · 2016-12-29T08:45:30.283

5

The computeIfAbsent method is designed for exactly this purpose:

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

edited Dec 29 '16 at 08:45

answered Dec 29 '16 at 08:41

Mureinik

297,002
52
306
350

As you can see in [answer by Eran](http://stackoverflow.com/a/41375969/5221149), you can use a method reference instead of the lambda expression. – Andreas Dec 29 '16 at 08:42
3

@Andreas This is wrong. The mapping function parameter takes the key as an argument. Since `ArrayList` doesn't have a constructor from `String`, using `ArrayList::new` isn't appropriate here. You have to use a lambda, or something equivilant. – Mureinik Dec 29 '16 at 08:45

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

2 Answers2