MixedMutabilityReturnType
This method returns both mutable and immutable collections or maps from different paths. This may be confusing for users of the method.

The problem

It is dangerous for a method to return a mutable instance in some circumstances, but immutable in others. Doing so may lead users of your API to make incorrect assumptions about the mutability of the return type. For example, consider this method:

List<Integer> primeFactors(int n) {
  if (isPrime(n)) {
    return Collections.singletonList(n);
  }
  List<Integer> factors = new ArrayList<>();
  for (...) {
    factors.add(i);
  }
  return factors;
}

If someone were to add another method to include the trivial factor 1, a bug will be introduced.

List<Integer> primeFactorsAndOne(int n) {
  List<Integer> primeFactors = primeFactors(n);
  primeFactors.add(1);
  return primeFactors;
}

primeFactorsAndOne will behave as intended for composite numbers, but throw an exception for primes.

Suppression

Suppress false positives by adding the suppression annotation @SuppressWarnings("MixedMutabilityReturnType") to the enclosing element.