What is the difference between map and flatMap operations?

map transforms each element of the stream into a new element, returning a stream with the same number of elements.

flatMap transforms each element of the stream into a stream of elements, and then flattens these nested streams into a single flat stream. It is used when the transformation of one element results in multiple elements.

Example:

Suppose we have a list of lists of strings: List<List<String>> listOfLists.

Using map:

// Transforms each list of strings into its size
listOfLists.stream()
           .map(list -> list.size())
           .forEach(System.out::println);
// Will print the size of each inner list: 2, 3, 1, etc.

Using flatMap:

// Transforms each list of strings into a stream of strings of that list
// and flattens (merges) them into a single stream of all strings
listOfLists.stream()
           .flatMap(list -> list.stream())
           .forEach(System.out::println);
// Will print all strings from all inner lists: "a", "b", "c", "d", "e"...

Comparison table:

Operation	Input	Transformation	Output	Purpose
map	Stream<T>	T -> R	Stream<R>	Transforming each element
flatMap	Stream<T>	T -> Stream<R>	Stream<R>	Transforming each element into a stream and flattening