Java Stream map() vs flatMap()

In Java, Stream interface has >map() and flatMap() methods and both are intermediate stream operations and return another stream as method output. The primary difference between map() vs flatMap() is the return type of both methods.

map() is used for transformation only, but flatMap() is used for both transformation and flattening.

flatMap() = map() + Flattening

1. Difference between map() and flatMap()

The map() method produces one output value for each input value in the stream. So if there are n elements in the stream, map() operation will produce a stream of n output elements.

List<String> listOfStrings = Arrays.asList("1", "2", "3", "4", "5");

List<Integer> listOfIntegers = listOfStrings.stream()
                                .map(Integer::valueOf)
                                .collect(Collectors.toList());

System.out.println(listOfIntegers); 	//[1, 2, 3, 4, 5]

flatMap() is two step process i.e. map() + Flattening. It helps in converting Collection<Collection<T>> to Collection<T>.

List<Integer> list1 = Arrays.asList(1,2,3);
List<Integer> list2 = Arrays.asList(4,5,6);
List<Integer> list3 = Arrays.asList(7,8,9);

List<List<Integer>> listOfLists = Arrays.asList(list1, list2, list3);

List<Integer> listOfAllIntegers = listOfLists.stream()
                            .flatMap(x -> x.stream())
                            .collect(Collectors.toList());

System.out.println(listOfAllIntegers);		//[1, 2, 3, 4, 5, 6, 7, 8, 9]

2. Usage of map() vs flatMap()

We can use map() operation when we have a stream of objects, and we need to get some unique value for each element in the stream. There is one-to-one mapping between input and output element. For example, we can write a program to find the date of birth of all employees in a stream of employees.

In case of flatMap(), a one-to-many mapping is created where for each input element/stream, we first get a multiple values and then we flatten the values from all such input streams into a single output stream. For example, we may write program to find all district words from all lines in a text file.

Reference :

Java Stream Interface