Java Community | Help. Code. Learn.•15mo ago

how to read and understand the map() method signature?

hey guys. can smb help me out? Im messing around with streams and map() and im trying to understand how it works. in the ide i see this

<R> Stream<R> map(Function<? super T, ? extends R> mapper);

<R> Stream<R> map(Function<? super T, ? extends R> mapper);

can smb explain what the f is this? as always, google is worthless and gooling raises more questins than it gives u answers. 1. as i understand, map function returns Stream, but what do those R mean? 2. map function has a parameter called mapper right? and its of type Function<? super T, ? extends R>? 3. how to undertand this stuff? Function<? super T, ? extends R> thanks alot guys.

68 Replies

JavaBot•15mo ago

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dan1st•15mo ago

Do you know how generics work?

bambizas19OP•15mo ago

yeah. you can apply generics to a method, and then it wont return/receive one specific data type. and also you can extend that generic type up and down aka upper and lower bounds

dan1st•15mo ago

so the class is Stream<T> hence T refers to the type of the stream

bambizas19OP•15mo ago

wait theres no Stream<T>

dan1st•15mo ago

and map changes a Stream<T> to a Stream<U>

bambizas19OP•15mo ago

dan1st•15mo ago

the class surrounding map or interface technically

bambizas19OP•15mo ago

doesnt ring any bells, sorry

dan1st•15mo ago

look at the code and scroll up

bambizas19OP•15mo ago

public interface Stream<T> extends BaseStream<T, Stream<T>> {

public interface Stream<T> extends BaseStream<T, Stream<T>> {

dan1st•15mo ago

public interface Stream<T>{
    <R> Stream<R> map(...);
}

public interface Stream<T>{
    <R> Stream<R> map(...);
}

yes so T refers to the type of the current stream

bambizas19OP•15mo ago

ok. so far so good

dan1st•15mo ago

and the goal of map() is to transform a Stream of some type to another Stream<T> to Stream<R>

bambizas19OP•15mo ago

wait

bambizas19OP•15mo ago

map function transforms mapper (not a stream) to a Stream

dan1st•15mo ago

the method is in Stream<T> so it already requires a Stream<T> it needs both a Stream<T> and the mapper and then it gives you a Stream<R>

bambizas19OP•15mo ago

but why in the parameters only mapper is mentioned? wtf. doesnt make any sense

dan1st•15mo ago

Do you know what lambdas are?

bambizas19OP•15mo ago

kinda. anonymous class (or function?) that has to comply to a functinoal interface

dan1st•15mo ago

yes so mapper is a function from T to R it takes a T and returns an R

bambizas19OP•15mo ago

um, theres no such signature

dan1st•15mo ago

the only important thing about Function is the apply method

bambizas19OP•15mo ago

why apply method? why not any other method? apply method isnt mentioned here

dan1st•15mo ago

it takes one thing and changes it to another it takes a T and gives back an R

bambizas19OP•15mo ago

based on what logic?

dan1st•15mo ago

that's specified in the lambda you pass the caller can decide how that happens

bambizas19OP•15mo ago

wait. im completely lost now

dan1st•15mo ago

Stream<Integer> someIntStream;
Stream<String> resultStream = someIntStream.map(someInt -> ""+someInt);

Stream<Integer> someIntStream;
Stream<String> resultStream = someIntStream.map(someInt -> ""+someInt);

0x150•15mo ago

the mapper is to be implemented by the caller

bambizas19OP•15mo ago

you guys are inconsistent and all over the place. i cant keep up with u

0x150•15mo ago

it accepts anything T can assign to (? super T), returns anything that can be assigned to R. result is a stream with elements of type R eg with a stream of type String, T would be String. If you were to write a method that maps that string to an Integer, R would be Integer and the mapper would return a new Integer for every String in the stream

bambizas19OP•15mo ago

so for example what does this part mean? map returns Stream of something? what is that something? and why is there <R> in front of a Stream?

0x150•15mo ago

<R> at the start declares one generic variable to be used for this method specifically, R. Stream<R> is the return type, it's a stream with the R as component type -> a stream containing elements of type R what R is depends on what the mapper function returns

bambizas19OP•15mo ago

so just like we have <T> in this generic method, which means this is a generic method, we have the same stuff in this case with Stream?

0x150•15mo ago

exact same concept in the image you sent the type is called T, in the case of map it's R the name is free to choose but your image actually demonstrates this perfectly, fromArrayToList has a generic type T, which depends on what type the array you pass in has. the same type is then returned in a List doing that helps the java compiler understand what types are in play, to ensure type safety

bambizas19OP•15mo ago

ffs. finally. but why these are different letters? whats wrong with using <T> Stream <T> map ...?

0x150•15mo ago

T's already used convention is single letter uppercase names, and since the type is mainly involved with deciding the return type, R for Return fit best (or so i would assume) as mentioned, the name of generic types is entirely up to you things like <GenericType1> GenericType1 returnTheSame(GenericType1 val) { return val; } are valid, but not encouraged

bambizas19OP•15mo ago

why is it so difficult but ok, this part is clear.

0x150•15mo ago

its pretty easy to understand once you know what type means what and how they come to be java's generic type system is not that complex

bambizas19OP•15mo ago

yeah, but if you dont use generics, lambdas, streams and other advanced stuff and have only theoretical knowledge, then u can say u have no knowledge at all haha

tjoener•15mo ago

Well... Invariance makes it more complex than it needs to be

bambizas19OP•15mo ago

and then moving on to this part. it means that map function takes a parameter mapper of type Function<...?

tjoener•15mo ago

Theoretical knowledge in programming doesn't mean anything 😁

bambizas19OP•15mo ago

and when u dont have places/cases when u can use it, you cant learn stuff at all

0x150•15mo ago

yep, Function is an interface providing one method to convert a value from the first type to the second, so in this case a value of type ? super T to ? extends R

tjoener•15mo ago

Yes, you can pass either a lambda or any other class that implements that interface Find a project, and just make it Start small though

bambizas19OP•15mo ago

come on. where can you use lambdas or generics, if you are building a basic api? i cant think of use cases for generics, lambdas, etc

0x150•15mo ago

oh they come up all the time

tjoener•15mo ago

All the time lol A list of strings, there's your generic usage And modifying data with streams, there's your lambdas (and more generics)

bambizas19OP•15mo ago

ive been working in the bank for 2 years, and i used them maybe 5 times but u dont have to overcomplicate things if u dont need to. and thats usually not necessary

0x150•15mo ago

i somehow doubt that, especially modern java uses lambdas a lot but for all i know, you could be stuck with java 5 over there

tjoener•15mo ago

Generics and lambdas make your code clearer. If they don't you need to use them more I mean it is a bank

bambizas19OP•15mo ago

i know all that theoretical stuff, but when u r using 17th java, and u get a task, start coding, and intellij suggestions do all the work for you, u are not forced to actually learn this stuff. becuase functional interfaces are already developed by previous colleagues.

bambizas19OP•15mo ago

in this interface, default and static methods dont count as abstract, right? so my lambda (if i will write it) and mapper parameter from map() has to comply to apply method, right?

tjoener•15mo ago

Yep just the apply

bambizas19OP•15mo ago

im trying to translate everything into easier words. so this Function<? super T, ? extends R> mapper would mean mapper that takes one data type and returns another (i.e. complies to apply method?;

tjoener•15mo ago

Yup

bambizas19OP•15mo ago

and how do i know if smth complies to apply method? bc this kinda complies too:

public double intToDouble(int number){
  return number*1.0;
}

public double intToDouble(int number){
  return number*1.0;
}

tjoener•15mo ago

Well, java is pretty clever about it and does some adaptation for you so it fits Little bit

0x150•15mo ago

it technucally does, but primitives dont work with the generics system. you can imitate primitives with the wrapper types tho, those work

bambizas19OP•15mo ago

u mean autoboxing? okay. so without using intellisense and using my brain, how can i complete this filtering?

public static void main(String[] args) {
        Farmer farmer1 = new Farmer();
        farmer1.setFirstName("Name1");
        farmer1.setId(1);
        farmer1.setLicense("license1");
        farmer1.setLastName("LastName1");

        Farmer farmer2 = new Farmer();
        farmer2.setFirstName("Name2");
        farmer2.setId(2);
        farmer2.setLicense("license2");
        farmer2.setLastName("LastName2");

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

        farmerList.stream().map()
    }

public static void main(String[] args) {
        Farmer farmer1 = new Farmer();
        farmer1.setFirstName("Name1");
        farmer1.setId(1);
        farmer1.setLicense("license1");
        farmer1.setLastName("LastName1");

        Farmer farmer2 = new Farmer();
        farmer2.setFirstName("Name2");
        farmer2.setId(2);
        farmer2.setLicense("license2");
        farmer2.setLastName("LastName2");

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

        farmerList.stream().map()
    }

i know that map returns stream, and inside of map i need to write stuff that complies to apply method from Function interface, right? is this sequence of thoughts correct?

tjoener•15mo ago

Yeah So for example 'f -> f.getFirstName()"

bambizas19OP•15mo ago

so since Function is a functional interface, i need/can write lambda. so it means that inside of farmerList.stream().map() map part, ill need to have a lambda ?

tjoener•15mo ago

Yeah Like the little one I showed

bambizas19OP•15mo ago

finally everything is falling into the places and it starts making sense

tjoener•15mo ago

Or, any object as long as it matches that Function thing

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how to read and understand the map() method signature?

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