Map Compute Function Always Returns Null

ID

java.map_compute_returns_null

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Java

Tags

collections, reliability

Description

Reports a Map.computeIfAbsent(key, function) call whose function argument is a lambda that unconditionally evaluates to null.

Map.computeIfPresent(key, function) is deliberately out of scope: returning null from it is the documented way to remove the entry, so the same shape there is correct code rather than a defect.

Rationale

computeIfAbsent and computeIfPresent look like a convenient way to conditionally add an entry with a null value, but neither method behaves that way. When the function returns null, computeIfAbsent does not add the key → null entry at all, and computeIfPresent goes further: if the key was present, its entry is removed. A lambda that always returns null therefore does nothing useful, or actively deletes data, while reading as if it stores a null.

Map<String, String> cache = new HashMap<>();
cache.computeIfAbsent(key, k -> null);           // entry is never added
cache.computeIfPresent(key, (k, old) -> null);   // entry is removed if present

A lambda that returns null on some paths only is not reported — that is ordinary conditional logic, not the "always null" shape this rule targets — and neither is a method reference argument, whose body is not visible to the rule.

Remediation

Use the traditional containsKey check followed by an explicit put, which does add the null value.

Map<String, String> cache = new HashMap<>();
if (!cache.containsKey(key)) {
  cache.put(key, null);
}

If the intent was never to store null, compute a real value instead.

References

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