Skip to content
The Code Storm
All videos
Stream StormPart 1 of 6

Java Streams Explained Visually: filter, map, flatMap, sorted | Stream Storm Part 1

A stream sends each item through a few steps. filter keeps items that pass a test, map changes every item, flatMap unpacks lists inside a list, and sorted puts them in order. Nothing runs until you ask for a result.

Watch on YouTube

The code from the video

java
List<Integer> numbers = List.of(3, 8, 1, 6, 5, 2);
 
List<Integer> result = numbers.stream()
    .filter(n -> n % 2 == 0)   // keep the even numbers
    .map(n -> n * 10)          // multiply each by ten
    .toList();
 
System.out.println(result);    // [80, 60, 20]

flatMap unpacks lists inside a list, and sorted puts the result in order:

java
List<List<Integer>> nested = List.of(List.of(3, 8), List.of(1), List.of(6, 5, 2));
 
nested.stream()
    .flatMap(List::stream)
    .sorted()
    .toList();                  // [1, 2, 3, 5, 6, 8]

Next in the series

Java reduce() and groupingBy() Explained Visually | Stream Storm Part 2

Read the write-up

Related videos