Java Community | Help. Code. Learn.•12mo ago

what is the actual difference between map() and flatMap()?

hi guys. can smb help me out with stream().map() and stream().flatmap? In the google i saw that flatmap flattens smth. What does it even mean? Can smb explain? Thanks alot

44 Replies

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Kyo-chan•12mo ago

To map() and flatMap(), you pass converters. Converting each item of the stream to something else, making the original stream a new stream where all is converted. But with flatMap(), the converter has to convert to a stream of something. If you gave the same converter to just map(), that would result in a stream of streams of stuff, which isn't useful. flatMap() applies the converter given, but flattens the result. Instead of giving out a stream of streams of stuff, it gives out just a stream of stuff.

0x150•12mo ago

-> with flatMap, one element becomes many eg if you have a stream of “ab”, “cd” flatMapping it with a function that turns the string into their individual characters and returns a stream of them returns in the stream “a”, “b”, “c”, “d”

tjoener•12mo ago

Yes, but to be more precise, 0 and 1 are also perfectly fine

bambyzasOP•12mo ago

can u guys give me an example? if i have List<Vegetables> and i do flatmap on it. what would i get?

tjoener•12mo ago

Well does a vegetable itself contain a list of some kind?

bambyzasOP•12mo ago

no. Vegetable is just a pojo

tjoener•12mo ago

yeah, flatmap isn't really useful then Lets say you have a list of users, and each user has a list of emails, then you can use flatmap eg: users.stream().flatMap(user -> user.getEmails().stream()).collect(Collectors.toList()) This will give you a list of all users' emails

bambyzasOP•12mo ago

ok i did some improvements. lets say i ahve such case:

Vegetable apple1 = new Vegetable(1, "apple1", 1);
        Vegetable pear1 = new Vegetable(11, "pear1", 11);
        Vegetable egg1 = new Vegetable(111, "egg1", 111);

        Farmer farmer1 = new Farmer();
        farmer1.setFirstName("Name1");
        farmer1.setId(1);
        farmer1.setLicense("license1");
        farmer1.setLastName("LastName1");
        farmer1.setVegetables(List.of(apple1,pear1,egg1);

        Vegetable apple2 = new Vegetable(2, "apple2", 2);
        Vegetable pear2 = new Vegetable(22, "pear2", 22);
        Vegetable egg2 = new Vegetable(222, "egg2", 222);

        Farmer farmer2 = new Farmer();
        farmer2.setFirstName("Name2");
        farmer2.setId(2);
        farmer2.setLicense("license2");
        farmer2.setLastName("LastName2");
        farmer2.setVegetables(List.of(apple2, pear2, egg2));

        List<Farmer> farmerList = new ArrayList<>();
        farmerList.add(farmer1);
        farmerList.add(farmer2);

        //List<String> farmerNames = farmerList.stream().map(farmer -> farmer.getFirstName()).collect(Collectors.toList());

        farmerList.stream().flatMap(farmer -> farmer.getVegetables(

Vegetable apple1 = new Vegetable(1, "apple1", 1);
        Vegetable pear1 = new Vegetable(11, "pear1", 11);
        Vegetable egg1 = new Vegetable(111, "egg1", 111);

        Farmer farmer1 = new Farmer();
        farmer1.setFirstName("Name1");
        farmer1.setId(1);
        farmer1.setLicense("license1");
        farmer1.setLastName("LastName1");
        farmer1.setVegetables(List.of(apple1,pear1,egg1);

        Vegetable apple2 = new Vegetable(2, "apple2", 2);
        Vegetable pear2 = new Vegetable(22, "pear2", 22);
        Vegetable egg2 = new Vegetable(222, "egg2", 222);

        Farmer farmer2 = new Farmer();
        farmer2.setFirstName("Name2");
        farmer2.setId(2);
        farmer2.setLicense("license2");
        farmer2.setLastName("LastName2");
        farmer2.setVegetables(List.of(apple2, pear2, egg2));

        List<Farmer> farmerList = new ArrayList<>();
        farmerList.add(farmer1);
        farmerList.add(farmer2);

        //List<String> farmerNames = farmerList.stream().map(farmer -> farmer.getFirstName()).collect(Collectors.toList());

        farmerList.stream().flatMap(farmer -> farmer.getVegetables(

so now i can use flatmap, right? bc i have a list of lists?

tjoener•12mo ago

yes

bambyzasOP•12mo ago

ok. thanks. but i dont understand what im doing wrong here farmerList.stream().flatMap(farmer -> farmer.getVegetables());

JavaBot•12mo ago

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tjoener•12mo ago

check what flatmap wants It wants a function that returns a stream a collection (or a list) is not a stream

bambyzasOP•12mo ago

how do u know? <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);

tjoener•12mo ago

? extends Stream

bambyzasOP•12mo ago

doesnt it mean that Function takes two things? wildcard extending a type, and wildcard extending a stream?(idk what that even means)

tjoener•12mo ago

No Function doesn't change, one return value with one parameter Don't worry about the ? extends or ? super too much Just ignore them for now

bambyzasOP•12mo ago

ok. so i have to provide a mapper for flatmap function. And what is the data type of the mapper? that whole Function<... things is very confusing to read and understand

tjoener•12mo ago

If you ignore the ? super and ? extends, it's basically Function<T, Stream<R>> That should be understandable

bambyzasOP•12mo ago

not very clearly tbh. i see that for this interface, if i want to write a lambda (for example) i need to comply to apply method

bambyzasOP•12mo ago

but i dont understand how to comply to Function<T, Stream<R>>?

tjoener•12mo ago

So basically, the T here, becomes the T from the flatmap parameter, the R becomes the Stream<R> from the flatmap parameter I think you need some good generics/lambda/stream tutorials, becuase you asked the same questiosn yesterday about map

bambyzasOP•12mo ago

but i know how lambdas and generics work. and i can write needed lambda for map. but flatmap is a little bit more difficult

tjoener•12mo ago

Well, I'm doubtful about the generics part, because that's what you just asked 🙂

bambyzasOP•12mo ago

generics are used when u want ur method to be able to accept/return any type

public class Generic<T> {
    T object;

    public void createList(){
        List<T> genericList = new ArrayList<T>();
        genericList.add(object);
        System.out.println("my list is: "+genericList.get(0));
    }
   }

public static void main(String[] args) {
        Generic generic = new Generic("AAA");
        generic.createList();
    }

public class Generic<T> {
    T object;

    public void createList(){
        List<T> genericList = new ArrayList<T>();
        genericList.add(object);
        System.out.println("my list is: "+genericList.get(0));
    }
   }

public static void main(String[] args) {
        Generic generic = new Generic("AAA");
        generic.createList();
    }

tjoener•12mo ago

Yeah but the whole ? super ? extends is a big part of generics Basically what those mean: ? super means that whatever you're presenting needs to be that type, or any parent of it ? extends means that whatever you're presenting needs to be that type, or any subtype of it

bambyzasOP•12mo ago

upper and lower bounds correct

tjoener•12mo ago

See, you get it 🙂

bambyzasOP•12mo ago

not really. i cant put everything together into one place

0x150•12mo ago

basically: ? super something means that the generic type accepts any parent class of something, eg with something being String, the generic bound ? super String would accept either String itself, or CharSequence for example. Read: ? super X means "i can assign a value with type X to a variable having that generic type" ? extends something means that the generic type accepts any child class or something, so in the exact other direction. ? extends X means "i can assign a value of that type to a variable of type X"

bambyzasOP•12mo ago

i have theoretical knowledge, but cant even write a lambda that complies to the needed functional interface

tjoener•12mo ago

In flatMap they do that so that if you have a method that takes a charSequence as a parameter, I can apply that on a stream of strings, because a String is a CharSequence That's the ? super And the return value needs to be at least a Stream, and the types of that stream need to be at least what I want the flatMap operation to return If I want it to return a Number, I can pass in a method that returns Integer, because Integer extends Number That's the ? extends

bambyzasOP•12mo ago

if i use map, i ctrl-click on it, and see this: <R> Stream<R> map(Function<? super T, ? extends R> mapper); and it basically means, that i need to provide a mapper. its type is Function that operates on two types. then i ctrl-click on function, and see R apply(T t);. so it means that my lambda for .stream().map() has to be smth like .map(farmer->farmer.getName()). but if i do .stream.flatMap() now im stuck and cant write lambda in a similar manner. i ctrl-click on flatMap and i get <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);. so it means that mapper types is a Function that operates on datatype and a stream. then again i ctr-click on Function and see the same R apply (T t). and thats it. idk what to do next, because for mapper i need a Function with 2 args, and here i see that it can actually take only one.

tjoener•12mo ago

Where's the second arg?

0x150•12mo ago

the function args are the exact same now your function accepts anything above T, and returns a stream of anything below R same thing

tjoener•12mo ago

https://discord.com/channels/648956210850299986/1200415367571439667/1200426565289394196

Phloof_boll•12mo ago

So basically, the T here, becomes the T from the flatmap parameter, the R becomes the Stream<R> from the flatmap parameter

tjoener•12mo ago

What I said here basically

bambyzasOP•12mo ago

this would be the logic?

0x150•12mo ago

yes

bambyzasOP•12mo ago

ok, but whose stream is it? where do i get it?

0x150•12mo ago

you make it you make a new stream consisting of the elements you want to put into the stream you convert one element to a stream of other elements, which are then "appended" to the current stream or rather, the element you get as an argument is replaced with the elements in the stream you returned for that element

tjoener•12mo ago

Collection and Optional have .stream() Or you could roll your own

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what is the actual difference between map() and flatMap()?