Should I use put() or putIfAbsent() after using getOrDefault()?

Question

Java8 introduced those nice methods getOrDefault() and putIfAbsent(), allowing to write code like:

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Answer 1

getOrDefault is suitable if you want to use a stand-in for an absent value without modifying the map. If you want to add a new value for absent keys, you can do it right in one operation.

List bars = itemsByFoo.computeIfAbsent(key, x -> new ArrayList<>());
bars.add(someNewBar);

or even

itemsByFoo.computeIfAbsent(key, x -> new ArrayList<>()).add(someNewBar);

In the best case, when being overridden by the Map implementation, like with HashMap, this will bear a single hash lookup only.

Not that putIfAbsent only bears two lookups when using the default implementation, but, of course, most Map implementations will provide a single lookup implementation for it. Still, the combination of getOrDefault and putIfAbsent would still bear two lookups in the best case, whereas an optimized computeIfAbsent does only one.