1
JavaBeginner#streams
JavaIntermediate#java8#streams
What is the difference between map() and flatMap() in streams?
map() transforms each element to another value one-to-one, potentially producing nested structures. flatMap() flattens nested structures (like Stream<List<T>>) into a single flat stream by merging inner streams.
Example
List<List<Integer>> nested = List.of(List.of(1,2), List.of(3,4)); List<Integer> flat = nested.stream() .flatMap(List::stream) .collect(Collectors.toList()); // [1, 2, 3, 4]