Java: What is the difference between Stream.map() and Stream.flatMap()?

map() — one-to-one transformation. flatMap() — one-to-many, flattens nested structures. Think of map() as a machine that replaces each item with one new item, while flatMap() replaces each item with a bag of items, then pours all bags into one pile.

map():

• Each element → one result.
• Returns Stream<R>.
• No flattening.

1Stream<Integer> lens = words.stream()
2    .map(String::length);
3// ["hello", "world"] → [5, 5]

flatMap():

• Each element → Stream of results.
• Flattens into single stream.

1Stream<String> chars = words.stream()
2    .flatMap(w -> Arrays.stream(w.split("")));
3// ["ab", "cd"] → ["a", "b", "c", "d"]

Key difference:

• map() — transform, one-to-one.
• flatMap() — transform + flatten, one-to-many.