One of the greatest strong points of Kotlin can also be its weakness. When we talk about data classes today, and in the same way java records, we tend to focus, at least many people I know do, on the complete elimination of the boilerplate code. Boilerplate code has been something that has been annoying developers for ages. It exists not only in Java, but other languages have that as well. If we think about creating data access objects any way, it doesn’t really matter if we do it in Kotlin or Java or any other language. Frequently we repeat the same process: repository creation, service and controller. A lot of what we do is boilerplate any way still. However boilerplate code may have been a part of the decision to create data classes or java records, a more important paradigm of software engineering was the actual focal point of creating these types of data structures. It all boils down to immutability or in any case the possibility thereof.Let’s rollback the time and go back to 1995 where we started at some point to create data structures where we would specify different things. We would define the fields, the getters, the setters, the properties, the accessors, the attributes and the parameters in a way to be able to pass our arguments to either the methods, the functions or the contructors.Fot this part we will have some fun programming using the characters of the mid-80’s series: The Golden Girls. Having said this, let’s see how a class would look like for about 15 years: public class GoldenGirlsJava {

    public String goldenGirl1;
    private final String goldenGirl2;
    private final String goldenGirl3;
    private final String goldenGirl4;


    public GoldenGirlsJava() {
        this.goldenGirl1 = "Dorothy Zbornak";
        this.goldenGirl2 = "Rose Nylund";
        this.goldenGirl3 = "Blanche Devereaux";
        this.goldenGirl4 = "Sophia Petrillo";

    }

    public GoldenGirlsJava(
            String goldenGirl1,
            String goldenGirl2,
            String goldenGirl3,
            String goldenGirl4
    ) {
        this.goldenGirl1 = goldenGirl1;
        this.goldenGirl2 = goldenGirl2;
        this.goldenGirl3 = goldenGirl3;
        this.goldenGirl4 = goldenGirl4;
    }

    public String getGoldenGirl1() {
        return goldenGirl1;
    }

    public void setGoldenGirl1(String goldenGirl1) {
        this.goldenGirl1 = goldenGirl1;
    }

    public String getGoldenGirl2() {
        return goldenGirl2;
    }

    public String getGoldenGirl3() {
        return goldenGirl3;
    }

    public String getGoldenGirl4() {
        return goldenGirl4;
    }

    @Override
    public String toString() {
        return "GoldenGirlsJava{" +
                "goldenGirl1='" + goldenGirl1 + '\'' +
                ", goldenGirl2='" + goldenGirl2 + '\'' +
                ", goldenGirl3='" + goldenGirl3 + '\'' +
                ", goldenGirl4='" + goldenGirl4 + '\'' +
                '}';
    }
} This was a tremendous amount of code to type in only to create two contructors. Although the hashcode and equals weren’t necessarily a must in all occasions, we almost always had to implememt a no arguments contructor next to a more generalistic constructor. The good thing about creating all of this boilerplate code is that we knew very clearly how everything would look like after compiling to bytecode. However, and in any case, boilerplate code was always a contentious issue for many developers and so in 2009, lombok came along and revolutionized the way we create data structures. It introduced the concept of using an annotation processor and interpreting specific annotations that would give the qualities we needed for our classes and so a lombok annotated class would look like this: @Getter
@Setter
@AllArgsConstructor
@ToString
public class GoldenGirlsLombok {
    public String goldenGirl1;
    private final String goldenGirl2;
    private final String goldenGirl3;
    private final String goldenGirl4;

    public GoldenGirlsLombok() {
        this.goldenGirl1 = "Dorothy Zbornak";
        this.goldenGirl2 = "Rose Nylund";
        this.goldenGirl3 = "Blanche Devereaux";
        this.goldenGirl4 = "Sophia Petrillo";
    }
} And for a while, people were very happy about Lombok! Finally the weight of having to create all of that boilerplate code was gone. But that felling endured for about 7 years, when a new player arrived and this time it was something the software development industry didn’t expected. It was called Kotlin and in 2016 it debuted with the immediate introduction of data classes. Our golden girls implementation in Kotlin would now look like this: data class GoldenGirls(
    var goldenGirl1: String = "Dorothy Zbornak",
    private val goldenGirl2: String = "Rose Nylund",
    private val goldenGirl3: String = "Blanche Devereaux",
    private val goldenGirl4: String = "Sophia Petrillo"
) Although Kotlin slowly started gathering fans, Java on the other hand realized that something else was in the market and some developments on that side started gaining some steam like project Loom which had been in the making but also left in the back burner for a while. That is why, with the release of Java 14 in 20202, Java introduced java records, and data structures in Java would now look like this: public record GoldenGirlsRecord(
        String goldenGirl1,
        String goldenGirl2,
        String goldenGirl3,
        String goldenGirl4
) {
    public GoldenGirlsRecord() {
        this(
                "Dorothy Zbornak",
                "Rose Nylund",
                "Blanche Devereaux",
                "Sophia Petrillo"
        );
    }
} And to this day code simplification and code reduction seems only to continue. However with the reduction of the boilerplate code, the concepts of fields, getters, setters, properties, accessors, attributes and parameters became far less visual and less easy to map in our minds. Whether we like it or not those concepts are still how the JVM works and organizes code with the bytecode.But code oversimplification is really about making code easier to read and in the case of data classes and java records the idea is also to create data structures that are immutable or partially immutable. Java records are truly immutable in the sense that all values it contains or references it contains cannot be modified. Kotlin data classes can be also truly immutable for the same reasons but they don’t have to which in a way gives permission to developers to create complicated and worst of all, mutable code anyway.In any case this is all good until we then need to work with frameworks like the Spring Framework, which rely heavily on annotations. Here is an example: @Entity
@Table(name = "shelf_case")
data class Case(
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    val id: Long?,
    var designation: String?,
    var weight: Long?
) {
    constructor() : this(0L, null, null)

    override fun toString(): String {
        return super.toString()
    }
} This example works fine. To the unsuspecting eye, there isn’t a lot special going on here. This works in Kotlin, just in the same way as it would work in Java. But now let’s have a look at a similar example, but this time with Jackson annotations: data class AccountNumbersPassiveDto(
    @NotNull
    val accountNumberLong: Long?,
    val accountNumberNullable: Long?,
    @DecimalMax(value = "10")
    @DecimalMin(value = "5")
    val accountNumber: BigDecimal,
    val accountNumberEven: Int,
    val accountNumberOdd: Int,
    @Positive
    val accountNumberPositive: Int,
    @Negative
    val accountNumberNegative: Int,
    @DecimalMax(value = "5", groups =  [LowProfile::class])
    @DecimalMax(value = "10", groups = [MiddleProfile::class])
    @DecimalMax(value = "15", groups = [HighProfile::class])
    @DecimalMax(value = "20")
    val accountNumberMaxList:Int
) This example will fail and the reason for that is that in Kotlin, there is no way to tell the compiler where exactly do we want our annotation to be applied to. In the previous example with annotation , which is a jakarta persistence annotation, the annotations are applied correctly to the fields. If we decompile` that code we’ll find this: @Entity public final class Case {
   @Id
   @GeneratedValue(
      strategy = GenerationType.SEQUENCE
   )
   @Nullable
   private final Long id;
   @Nullable
   private String designation;
   @Nullable
   private Long weight; But for the latter, which is a jakarta validation example, we find this: public final class AccountNumbersPassiveDto {
   @Nullable
   private final Long accountNumberLong;
   @Nullable
   private final Long accountNumberNullable;
   @NotNull
   private final BigDecimal accountNumber;
   private final int accountNumberEven;
   private final int accountNumberOdd; This means that has not been affected by those annotations on its fields. We were just lucky in the first example. It might have actually also failed.In order to specify where our annotation needs to go to, Kotlin gives us the possibility to use use-site targets as prefix to any annotation. The requirement is of course that conditions are met. In this case, one way to fix this is to prefix all of the annotations with like this: AccountNumbersDto @field data class AccountNumbersPassiveDto(
    @field:NotNull
    val accountNumberLong: Long?,
    val accountNumberNullable: Long?,
    @field:DecimalMax(value = "10")
    @field:DecimalMin(value = "5")
    val accountNumber: BigDecimal,
    val accountNumberEven: Int,
    val accountNumberOdd: Int,
    @field:Positive
    val accountNumberPositive: Int,
    @field:Negative
    val accountNumberNegative: Int,
    @field:DecimalMax(value = "5", groups =  [LowProfile::class])
    @field:DecimalMax(value = "10", groups = [MiddleProfile::class])
    @field:DecimalMax(value = "15", groups = [HighProfile::class])
    @field:DecimalMax(value = "20")
    val accountNumberMaxList:Int
) If we now try to decompile the resulting bytecode using IntelliJ, we’ll find that it results in this code in Java: public final class AccountNumbersPassiveDto {
   @NotNull
   @Nullable
   private final Long accountNumberLong;
   @Nullable
   private final Long accountNumberNullable;
   @DecimalMax("10")
   @DecimalMin("5")
   @org.jetbrains.annotations.NotNull This means that the annotations have been applied to field and that our code works as it is supposed to. You can probably imagine at this point that people who have been working on Java for a long time, will have no problem in adapting to Kotlin because we all know what fields, parameters, properties, etc, are. What we observe today and I witness that too, is that it seems like the code simplification has a potential to backfire. There have been quite a few occasions where I have become aware of the time spent in projects trying to figure how why in some projects or modules, the annotations just don’t seem to work. And it all seems to boil down to not making use of the use-site targets. My theory on what is  most likely, is that new generations of developers will probably look at things that my generation didn’t think was complicated and learned out of instinct, as really complicated material. Such is the case with use-site targets. These used to be things that we could very easily visualize. However in current times, the generated confusion with this seems to be delaying projects from being developed on time in some occasions. I wouldn’t be able to generalize of course but there is potential for good and there is potential for bad with java records and data classes. How data classes and records will shape our future is hard to tell, but one thing that is clear is that because of this problematic, other frameworks are betting on going completely annotation free as is the case with Ktor. I have created documentation about this on Scribed: also on slide-share: . Fields-in-Java-and-Kotlin-and-What-to-Expectand fields-in-java-and-kotlin-and-what-to-expect You can also find the examples I’ve given on GitHub. The golden girls example can be found here: and the jakarta persistence and validation annotations examples can be found here: . jeorg-kotlin-test-drives https://github.com/jesperancinha/jeorg-spring-master-test-drives Finally I also created a video about this on YouTube that you can have a look at right over here: https://youtu.be/rTCjlyGVDGE?embedable=true Also published . here

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How to Fix Kotln Problems When Dealing With Use-site Targets

since the early Beginnings in Java we would create 
our data structures using Getters and Setters we  

would declare our Fields like this and we would 
create our pams like this then we would create  

our instances passing out the arguments to the 
Constructor this way in 2009 a project made its  

debut to the jvm world that would revolutionize 
the way we create data structures this was called  

long walk and Long Walk would allow us to 
reduce the bottle plate code that we have  

been used to creating up until this time that way 
we would create Getters in a different way using  

annotations like this another language was being 
developed at the same time called cotlin and its  

debut was in 2016 where cotlin introduced for 
the first time data classes data classes didn't  

have to use any annotation and with them you could 
create something like this which looks much much  

simpler but cotlin was still not as popular as it 
is today so in 2020 with the release of java 14  

records were introduced and with records we could 
do exactly the same this way just as much as in  

data classes so let's go back to the traditional 
way of creating data structors in Java here we  

can see where we have created a getter and here 
we see where we have created a setter this is a  

property because we are using accessors to reach 
it this is a field because we access it publicly  

and these are the parameters of our Constructor 
fast forward and we can get to records and data  

classes and let's look at data classes when we 
look at data classes we just don't see any of  

this if we do compile this class we can see all 
of these Concepts implemented in this gigantic  

Java class which is something we don't really see 
in the data class as we don't see it in this data  

class in my career I hardly ever had bumped into 
so many people questioning what is a getter what  

is a Setter what is a field and and what it's Pam 
and when we see this happening is when we try to  

apply annotations Frameworks like the spring 
framework usually needs annotations applied to  

either a field or either a getter either a Setter 
they have very specific places where they need  

to be applied in a data class that is not visible 
and because it's not visible lots of people have a  

lot of questions and a lot of trouble figuring out 
what is going on why are they not behaving the way  

The Decorator is telling them to behave usually 
the reason is just because we do need to specify  

in cotlin for example where does this annotation 
needs to be applied to so we do need to specify  

either a field either a Pam either a getter either 
Center and so forth it is one of these negative  

signs of a long-term effect of using cutland that 
we forget what is a field what is a parameter what  

is a getter what is a Setter maybe Getters and 
Setters is a completely different discussion  

but I would expect property in field not to 
be such a huge discussion and in it actually

is so can cotlin be an answer to all 
our problems it can be an answer to  

some problems definitely but for 
other things it might actually be a

curse so that was a dramatic ending to the 
first part of our video for this part I want  

to show you an example about how annotations 
actually work for those examp examples that  

we have seen so in this case for my project 
spring Master test drives I have this class  

account lists asserts dto this is part of a 
project and this is called size size is part  

of the Jakarta annotations package and with 
Jakarta we can decorate our data objects when  

we want to send data Through the Wire via rest 
services in this case we want to essentially  

validate our field according to groups now 
groups is something different but what's  

important here is to make these annotations 
work as you can see right over here on the  

left bottom corner of the screen we see that 
all tests are in the clear if for example I do

this and remove field from it and run the tests 
again we're going to see that these tests are  

going to fail just as I expected they fail and 
if we go here down to the bottom of it this test  

what it does it expects a bad request because 
we are sending data that it's not valid to be  

sent through the wire but in this case we get a 
200 and the reason for that is that of course as  

we have seen before the size annotation doesn't 
have a specific prefix letting it know that we  

need to apply this to field but here's the 
catch when we look at size The annotation  

itself we see that the target annotation 
that it contains thus comprises of method  

field anotation type Constructor parameter 
and type use important to note is that it  

is already stereotyped with a target of field 
but unfortunately in cotland it doesn't really  

work that way when we apply certain annotations 
what usually happens is something different so  

now let's have a look at the decompiled code so 
when we decompile this code so code decompile we  

can see an interesting thing so the field 
that we want to annotate is postcode post  

code is located here and we already see that 
the field is not annotated because this is a  

final variable with Val we also are not going 
to generate Setters we only have Getters in  

this case and none of the Getters is annotated 
with the size annotation but we can see that the

Constructor is the Constructor has all the 
annotations that we need for post code and  

curiously enough the parameter of the copy 
function is also annotated with size but  

the crucial thing that needs to be annotated is 
field and field isn't there I can can see this  

very clearly with the Java code but with cotlin 
I don't see anything so when I put field here and  

let's put field on all of the three annotations 
why do I have it this way it doesn't really matter  

I have a whole video about it that explains what 
this group is I put in a link in the description  

um so that you can see a tutorial I made about 
it but for now it's important that we get our  

tests running again and that I can show you 
why are these tests now working why do I get  

them why do we get them all green and what was the 
change so now let's decompile our class again by  

code decompile exactly so here we have the post 
code it's annotated with the three options with  

the size annotation and if we look further down 
we can see that in postcode now we have nothing  

and obviously in copy we don't have it as well 
so the only thing that's annotated now is field  

and another important aspect of this is that for 
this case these Jackson annotations work really  

well because for our specific case they only need 
field so that's why this works and of course each  

manufacturer or its brand has its way of using 
annotations in this case for our case we need  

field it doesn't necessarily have to be the 
case all the time so now let's have a look at  

what happens with other annotations that don't 
need this if we look at for example a many to  

many many to many is an annotation that only has 
a starget method in FEI field and this works this  

works without the need to put in field on it or 
anything like that and the reason for that is that  

if we then decompile this we can directly see that 
many to many along with the other annotations is  

already applied to a field in this case receipts 
and also shop yeah shops and in this case there  

is no confusion where the confusion exists is for 
annotations like in the case of Jakarta where the  

cotton compiler doesn't really know what to do 
with it and so we don't see size applied to field  

and these are the cases that we need to specify 
that we want our annotation applied to a field  

and I can tell you that using field is something 
that is not necessarily the first thing that pops  

into mind when someone is used to programming in 
Java and of course the literature is filled with  

examples of using simple simply just size and that 
doesn't necessarily have have to be the case all  

the time I'm also not saying that field in this 
case is the only possible fix there is maybe with  

configuration of a certain Plug-In or something 
else external but the bottom line is when these  

errors happen it's very important for us to 
understand these Concepts so we can find the  

source of the of the book understand the problem 
and be able to fix it which is the subject of this  

whole video and this concludes my presentation on 
possible problems using cotlin data structures for  

some people of course I've been using cotlin 
already for a long time and I cannot say that  

I understood this problem immediately but it 
became clear after the compiling what didn't  

became clear of course is what I said already 
in the video that so many questions came up  

when trying trying to explain the source of the 
problem and the fix all right so this wraps up our  

video I hope this video made a difference to you 
I hope that you were able to learn something with  

it and that you can use this in your future career 
whether it is in cotlin Java or any other language  

I think it's important never to forget what a 
field and what a parameter is what a property  

and especially what an argument and a parameter 
is these concepts are important to know and if  

you enjoyed this video don't forget to leave a 
like the usual don't forget to subscribe if you  

dislike this video please give me your dislike 
and if you are interested and want to give an  

input I will be very happy to hear your comment 
in the comment section find out the links to  

these projects especially the spring Master Test 
R project with these examples that I just showed  

How to Fix Kotln Problems When Dealing With Use-site Targets

Too Long; Didn't Read

João Esperancinha

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How to Fix Kotln Problems When Dealing With Use-site Targets

Too Long; Didn't Read

João Esperancinha

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Guide

Review

DYOR

Code License

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Languages

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