Data Source

private static List> teams = new ArrayList<>();
    private static List aTeam = new ArrayList<>();
    private static List bTeam = new ArrayList<>();
    static {
        aTeam.add("Java");
        aTeam.add("C++");
        aTeam.add("PHP");
        aTeam.add("Scala");

        bTeam.add("Go");
        bTeam.add("R");

        teams.add(aTeam);
        teams.add(bTeam);
    }

MapDemo

@Test
    public void mapDemo() {
        System.out.println("=== Demo Lambda Map ===");
        List> result1 = teams.stream().map(team -> {
            System.out.println("Team: " + team);
            return team;
        }).collect(Collectors.toList());

        System.out.println("Result: " + result1);
    }

=== Demo Lambda Map ===
Team: [Java, C++, PHP, Scala]
Team: [Go, R]
Result: [[Java, C++, PHP, Scala], [Go, R]]

FlatMap Demo

@Test
    public void flatMapDemo() {
        System.out.println("=== Demo Lambda FlatMap ===");
        List result2 = teams.stream().flatMap(team -> {
            System.out.println("Team: " + team);
            return team.stream();
        }).collect(Collectors.toList());

        System.out.println("Result: " + result2);
    }

=== Demo Lambda FlatMap ===
Team: [Java, C++, PHP, Scala]
Team: [Go, R]
Result: [Java, C++, PHP, Scala, Go, R]