What changes occur if the flatMap method is used to process a list of Longs inside an object?

The flatMap method in Java Stream API is used to transform each element of a stream into another stream, and then combine all these streams into one.

If you have an object that contains a list of Long, and you apply flatMap to this list, you will get a flat stream of Long elements instead of a stream of lists. This simplifies further processing, such as filtering or aggregation.

Example:

List<MyObject> objects = ...; // each object contains List<Long> numbers

List<Long> allNumbers = objects.stream()
    .flatMap(obj -> obj.getNumbers().stream())
    .collect(Collectors.toList());

As a result, allNumbers will contain all the numbers from all lists, combined into a single list without nesting.