Collection.contains(Object) tests whether a collection contains an element equal to the supplied value. Its Object parameter permits calls that compile even when the argument cannot match the collection’s element type. section 8.1
Primitive arrays become one list element
Passing a primitive array to Arrays.asList creates a list containing the entire array, not its primitive elements. Therefore Arrays.asList(arr).contains(0) searches for an Integer inside a List<int[]> and returns false. Declare an intermediate variable with its intended type so this mismatch becomes visible. section 2.9.3
Avoid contains failures that compile cleanly:
- Check that the searched object has the same semantic type as the stored elements; mixing File, Path, and String path representations produces a not-found result rather than an immediate error. section 8.1
- Do not assume a Collection declaration reveals null behavior. Some collections return false for contains(null), while others throw NullPointerException; use obj != null && collection.contains(obj) when null is possible. section 8.3
- Review warnings for incompatible collection calls; IntelliJ IDEA reports them through Suspicious Collections Method Calls, and SonarLint provides rule S2175. section 2.9.3
The same expression can change behavior after replacing an older unmodifiable collection with List.of or Set.of: a nullable search argument may change from false to NullPointerException. section 8.3
static boolean containsZero(int[] arr) {
List<int[]> list = Arrays.asList(arr);
return list.contains(0);
}
Inspect the inferred List<int[]> declaration that makes the failed primitive-array search apparent. section 2.9.3
