get the last non null object from List of objects

Question

I have a List of objects and want to get the last element from List for which property "personStatus" value is not null.

List<Person> personList = getAllPersons();
List<Person> personActive = personList.stream().filter(person -> person.getPersonStatus()!=null);

I tried the above code which gives List of Person whose status is not null, but i want to get the last Person object with status not null.

Answer 1

You solved 80%: first collect and filter. This will give a new collection using Java 8 streams.

Literally: last

Then pick the last from the filtered collection, e.g. on list using list.get(lastIndex) where int lastIndex = list.size()-1 .

The last is a bit ambiguous, so I have 2 approaches ️

List<Person> personList = getAllPersons();
// typo: name of List should be plural e.g. `personActiveList` or `activePersons`
List<Person> personActive = personList.stream()
    .filter(person -> person.getPersonStatus()!=null)
    .toList();

// the last in order of appearance
Person lastInActiveList = personActice.get(personActice.size()-1); // may throw NPE if list empty


// the last who was active by some Instant timestamp
List<Person> sortByLastActive = personList.stream()
    .filter(person -> person.getPersonStatus()!=null)
    .sorted(Person::getTimestamp())  # sort by Instant property
    .toList();

Person lastRecentlyActice = sortByLastActive.get(sortByLastActive.size()-1); // may throw NPE if list empty

Note: For brevity I used toList() as terminator (since Java 16).

You could also use collect(Collectors.toList()) from older Java versions. Beware: Differences of Java 16's Stream.toList() and Stream.collect(Collectors.toList())?

⚠️ Caution: all random get operations on list can throw a NullPointerException (NPE) if the list is empty. This should be tested using if (list.isEmpty()) before to prevent illegal access.

Trick: reverse to first

The trick here as commented by Boris is to reverse the list before opening the stream, thus before it becomes filtered/collected as result. Afterwards you can directly pick the first.

List<Person> personActiveReversed = Collections.reverse(personList).stream()  // reverse to get last elements first
    .filter(person -> person.getPersonStatus()!=null)
    .toList();

// may throw NPE if list empty
Person last = personActiveReversed.get(0);

// safer: directly on stream
Optional<Person> last = Collections.reverse(personList).stream()  // reverse before streaming
    .filter(person -> person.getPersonStatus()!=null)
    .findFirst();  // originally the last or empty 

// evaluate optional
if (last.isPresent()) {
   System.out.println("found the last: " + last.get());
} else {
   System.out.println("nobody found at last :(");
}

Explained:

• see comments by Boris below and on question: use Collections.reverse before .stream to reverse an ordered list (from last to first). Then reversedList.get(0) will get the originally last element.

• findFirst() is the new terminator, returning Java 8 type-wrapper Optional. This is null-safe but requires a further test or convenience-methods like orElse(defaultObject), e.g. last.orElse(null).

See also

• collections - comparing Instants in Java8 - Stack Overflow
• How to use Comparator in Java to sort, comparing a peopleList
• Java 8 stream reverse order, contains answer 'How NOT to do it': e.g. don't use ⚠️ lambda (a, b) -> -1 to reverse the elements in stream (swapping). It violates Comparator contract and might break depending on underlying implementation.
• How to get ordered stream from a list in reverse order in Java 8, interesting discussion on generic list sorting
• How to Get the Last Element of a Stream in Java? | Baeldung, alternatives using skip or reduce with warning note on stream types

Answer 2

int lastNotNull = -1;
for(int i = 0; i < personList.size();i++){
    if(person.getPersonStatus()!=null)
        lastNotNull = i;
}

lastNotNull is the index of the last not null Person