When Map's compute, computeIfAbsent and computeIfPresent methods are provided with a lambda that always returns null, they will fail to create new entries for the provided key.

This may go against the expected behavior. Consider changing this code to directly insert a key with a null value instead.

The compute methods are a convenient and powerful way to perform certain actions that are predicated on whether an element exists in the map or not.

If a null value is returned from the lambda provided to these methods, no new element will be created for the corresponding key, which may not be the assumed behavior.

Bad Practice

Map<String, String> someMap = new HashMap<>();

someMap.computeIfAbsent("key", key -> null);
someMap.computeIfPresent("key", (key, value) -> { return null; });
someMap.compute("key", (key, oldValue) -> null);

assertEquals(0, someMap.size()); // Passes.

All three of the calls above make no changes to the map because no new entry is created by the methods.

Recommended

If you intend to create a new null valued entry in the map, use Map.put() to insert the entry:

if (!someMap.containsKey("key")) {
    someMap.put("key", null);
}

assertEquals(1, someMap.size());

References

Oracle Java SE 8 JavaDocs - java.util.Map.compute()

ben-manes / caffeine

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