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Map.merge

As explained in 100 Java Mistakes and How to Avoid Them

Map.merge is a Java Map interface method that inserts a non-null value for an absent key or combines it with an existing value using a remapping function.

Map.merge inserts a non-null value when a key is absent or mapped to null; otherwise, it combines the existing and supplied values with a remapping function. section 8.4

Treat these cases as distinct:

  • The remapping function is skipped when the key has no mapping or an existing null mapping. section 8.4
  • A non-null existing value invokes the function with the old and new values. section 8.4
  • A null result from the function removes the mapping. section 8.4
  • A null value argument is not accepted and causes NullPointerException; merge does not write a null mapping. section 8.4

Null mappings change the apparent control flow

With map.put("x", null), map.merge("x", "1", (x, y) -> null) stores "1" without calling the function. A second merge calls the function, receives null, and deletes the entry. Passing null as the value argument instead throws NullPointerException. section 8.4

Atomic occurrence counting

For a concurrently updated map, replace a get-and-put sequence with map.merge(key, 1, Integer::sum). This performs the increment atomically when the concurrent map implementation provides the required operation. section 9.12

Table 8.2 Behavior of java.util.Map interface methods with respect to null
java.util.Map interface method
Distinguishes between existing null mapping and no mapping at all?
Puts null mapping if the value parameter is null?
Removes existing mapping if lambda returns null?
containsKey
yes
—
—
getOrDefault
yes
—
—
get
no
—
—
put
no
yes
—
putIfAbsent
no
yes
—
compute
no
—
yes
computeIfPresent
no
—
yes
computeIfAbsent
no
—
no
merge
no
no
yes

Compare Map methods by whether they distinguish null mappings, accept null values, and remove mappings after a null function result. section 8.4

How 100 Java Mistakes and How to Avoid Them covers it

How deep it goes
Explains null mappings, remapping results, exceptions, and concurrent atomicity through focused examples, without detailing implementation internals.
Examples
Uses null-valued maps, deletion through a null remapping result, and concurrent occurrence counting.
What you can do after
Apply Map.merge for null-aware value combination and replace non-atomic concurrent get-and-put sequences.
How it connects
Connects Map.merge with null mappings, compute, putIfAbsent, computeIfAbsent, and ConcurrentMap updates.
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