Difference between accessing via property access and destructuring in kotlin data class - kotlin

I have a data class like this
data class Task(
var id: Int,
var description: String,
var priority: Int
)
I implement it the following
val foo = Task(1, "whatever", 10)
I read about accessing whatever like this
foo.description
or
foo.component2()
What is the difference?

There is no difference in behaviour, but use foo.description.
It's extremely rare to use a componentN() function directly. If you know which component you're accessing, it's just way more readable to use the property directly.
The componentN() functions are mostly a tool to implement actual destructuring declarations like:
val (id, desc, prio) = task
Which is a shortcut that is equivalent to:
val id = task.component1()
val desc = task.component2()
val prio = task.component3()
..which you should probably never write in source code.

Related

Kotlin equivalent for "with" keyword in C# (data class)

In C# there is a kind of class that is called "record" which is more or less the same as a "data" class in Kotlin.
When using a record in C# you can use the keyword "with" to create a new instance of the your record with some properties set to specific values (see https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/with-expression )
I was wondering if there is a similar way to do it in kotlin ? I cannot find anything regarding that, and the way I do it for now is by defining function that do the job, but it can be kind of boilerplate sometimes, and using data class is supposed to avoid me that boilerplate work.
Also I'd prefer to avoid using "var" properties (to have immutable instances), hence my question.
With a data class, you can use the copy method:
val someData = SomeClass(a = 1, b = 2)
val modifiedData = someData.copy(b = 0) // modifiedData = SomeClass(a = 1, b = 0)
See the official data class documentation.
Kotlin has Data classes, that are close to C# records.
As you can see from the Kotlin documentation:
It is not unusual to create classes whose main purpose is to hold
data. In such classes, some standard functionality and some utility
functions are often mechanically derivable from the data. In Kotlin,
these are called data classes and are marked with data:
data class User(val name: String, val age: Int)
The compiler automatically derives the following members from all
properties declared in the primary constructor:
equals()/ hashCode() pair
toString() of the form "User(name=John, age=42)"
componentN() functions corresponding to the properties in their order of declaration.
copy() function (see below).
The last line shows that the compiler automatically generates a copy function for a Data class, and this is what you are looking for
Again from the documentation, the usage would be:
Use the copy() function to copy an object, allowing you to alter some
of its properties while keeping the rest unchanged.
[...]
You can then write the following:
val jack = User(name = "Jack", age = 1)
val olderJack = jack.copy(age = 2)
Do you mean something like copy() function that is auto-generated for data classes?
fun main() {
val foo1 = Foo("foo", 5)
val foo2 = foo1.copy(value1 = "bar")
val foo3 = foo2.copy(value2 = 10)
}
data class Foo(
val value1: String,
val value2: Int,
)

How to pass the current instance into the constructor of another class withon class declaration?

I have a service that returns me a list of entities, for example:
data class TypeDto(
val type: Type,
val stages: List<StageDto>)
After applying several filter and flatMap operations, I get the desired data that has the following structure:
data class CustomerDto(
val id: String,
val name: String)
In order to extract the logic of filtering and mapping, I made a CustomerDtoWrapper class that takes List<TypeDto> as a constructor argument and does all the collection manipulation. So, in the end it looks as follows:
val types = service.getTypes()
val customers = CustomerDtoWrapper(types).filteredCustomers()
But I would like to make it more fluent and easy to read. Is it possible to call a certain function after getTypes(), so that types will be of the CustomerDtoWrapper type and look as follows:
val types = service.getTypes().someFun { ... }
val customers = types.filteredCustomers()
You can write for example extension function for this:
fun List<TypeDto>.toFilteredCustomers() = CustomerDtoWrapper(this).filteredCustomers()
and use it like this:
val customers = service.getTypes().toFilteredCustomers()

Kotlin multiple class for data storage

I am developing a simple Android app, that will display an icon of a vehicle and the user can click on the icon to display the vehicle information. I want to load the data dynamically when I build the app i.e. the data will come from an external source including the picture for the icon.
I am new to Kotlin and not sure what to search for to understand a suitable solution. What is the correct way to define the data, is it best to create an class as below then create an array of the class (not sure if this is possible)
public class VehicleSpec()
{
var OEM: String? = null
var ModelName: String? = null
var EngineSize: String? = null
}
Or would be better to create a multiple dimension array and then link the data to the cells?
var VehicleSpec = arrayOf(20,20)
VehicleSpec[0][0] = Null //OEM
VehicleSpec[0][1] = Null //ModelName
VehicleSpec[0][2] = Null //EngineSize
What is the best way to set up the data storage, is there any good references to understand how this should be setup?
What is the correct way to define the data, is it best to create an class as below then create an array of the class
Using an array for the properties of an object is not making the full use of the type safety you have in Kotlin (and even Java for that matter).
If what you want to express is multiple properties of an object, then you should use a class to define those properties. This is especially true if the properties have different types.
There is no performance difference between an array and a class, because you'll get a reference to the heap in both cases. You could save on performance only if you convert your multi-dimensional array approach to a single-dimension array with smart indexing. Most of the time, you should not consider this option unless you are handling a lot of data and if you know that performance is an issue at this specific level.
(not sure if this is possible)
Defining lists/arrays of classes is definitely possible.
Usually, for classes that are only used as data containers, you should prefer data classes, because they give you useful methods for free, and these methods totally make sense for simple "data bags" like in your case (equals, hashcode, component access, etc.).
data class Vehicle(
val OEM: String,
val ModelName: String,
val EngineSize: String
)
Also, I suggest using val instead of var as much as possible. Immutability is more idiomatic in Kotlin.
Last but not least, prefer non-null values to null values if you know a value must always be present. If there are valid cases where the value is absent, you should use null instead of a placeholder value like empty string or -1.
First at all, using the "class aprocah" makes it easy for you to understand and give you the full benefits of the language itself... so dont dry to save data in an array .. let the compiler handle those stuff.
Secondly i suggest you have maybe two types (and use data classes ;-) )
data class VehicleListEntry(
val id: Long,
val name: String
)
and
data class VehicleSpec(
val id: Long,
val oem: String = "",
val modelName: String = "",
val engineSize: String = ""
)
from my perspective try to avoid null values whenever possible.
So if you have strings - which you are display only - use empty strings instead of null.
and now have a Model to store your data
class VehicleModel() {
private val specs: MutableMap<Long, VehicleSpec> = mutableMapOf()
private var entries: List<VehicleListEntry> = listOf()
fun getSpec(id: Long) = specs[id]
fun addSpec(spec: VehicleSpec) = specs[spec.id] = spec
fun getEntries(): List<VehicleListEntry> = entries
fun setEntries(data: List<VehicleListEntry>) {
entries = data.toMutableList()
}
}
You could also use a data class for your model which looks like
data class VehicleModel(
val specs: MutableMap<Long, VehicleSpec> = mutableMapOf(),
var entries: List<VehicleListEntry> = listOf()
)
And last but not least a controller for getting stuff together
class VehicleController() {
private val model = VehicleModel()
init{
// TODO get the entries list together
}
fun getEntries() = model.entries
fun getSpec(id: Long) : VehicleSpec? {
// TODO load the data from external source (or check the model first)
// TODO store the data into the model
// TODO return result
}
}

Filter out null in immutable objects list of fields

I have an immutable object:
class Foo(
val name: String,
val things: List<Thing>
)
A third party lib creates the Foo object with some 'null' Thing objects.
I am creating a new object:
val foo = thirdPartyGetFoo()
val filteredFoo = Foo(foo.name, foo.things.filterNotNull())
That works, however AndroidStudio greys out the filterNotNull function call and presents a warning:
Useless call on collection type: The inspection reports filter-like
calls on already filtered collections.
Is this the right way to filter that list? Should I ignore the warning or is there a better way?
You do not specify what library creates the object with nulls. Some deserialization libraries can use static factory methods which you could configure, and then have the factory method strip the null. For example, if this were Jackson you would simply:
class Foo(val name: String, val things: List<Thing>) {
companion object {
#JsonCreator
#JvmName("createFromNullable")
fun create(name: String, things: List<Thing?>) = Foo(name, things.filterNotNull())
fun create(name: String, things: List<Thing>) = Foo(name, things)
}
}
Then...
val goodFoo = jacksonObjectMapper().readValue<Foo>(someJsonWithNulls)
Maybe your library has options that are similar?
If not, and you don't have 100 of these things with this problem, I would probably create a temporary class to hold the results and convert that to the final class:
open class FooNullable(val name: String, open val things: List<Thing?>) {
open fun withoutNulls(): Foo = Foo(name, things.filterNotNull())
}
class Foo(name: String, override val things: List<Thing>) : FooNullable(name, things) {
override fun withoutNulls(): Foo = this
}
Then you can deserialize into FooNullable and just call withoutNulls() to get the other flavor that is clean. And if you accidentally call it on one without nulls already, it just does nothing.
val goodFoo = Foo("", emptyList<Thing>())
val alsoGoodFoo = goodFoo.withoutNulls() // NOOP does nothing
val badFoo = thirdPartyGetFoo()
val betterFoo = badFoo.withoutNulls() // clean up the instance
val safeFoo = thirdPartyGetFoo().withoutNulls() // all at once!
Not the cleanest, but does work. The downsides is this second step, although it looks like you were already planning on doing that anyway. But this model is safer than what you proposed since you KNOW which type of object you have and therefore you continue to be typesafe and have the compiler helping you avoid a mistake.
You don't have to use inheritance as in the above example, I was just trying to unify the API in case there was a reason to have either version in hand and know which is which, and also act upon them in a similar way.

How to make a builder for a Kotlin data class with many immutable properties

I have a Kotlin data class that I am constructing with many immutable properties, which are being fetched from separate SQL queries. If I want to construct the data class using the builder pattern, how do I do this without making those properties mutable?
For example, instead of constructing via
var data = MyData(val1, val2, val3)
I want to use
builder.someVal(val1)
// compute val2
builder.someOtherVal(val2)
// ...
var data = builder.build()
while still using Kotlin's data class feature and immutable properties.
I agree with the data copy block in Grzegorz answer, but it's essentially the same syntax as creating data classes with constructors. If you want to use that method and keep everything legible, you'll likely be computing everything beforehand and passing the values all together in the end.
To have something more like a builder, you may consider the following:
Let's say your data class is
data class Data(val text: String, val number: Int, val time: Long)
You can create a mutable builder version like so, with a build method to create the data class:
class Builder {
var text = "hello"
var number = 2
var time = System.currentTimeMillis()
internal fun build()
= Data(text, number, time)
}
Along with a builder method like so:
fun createData(action: Builder.() -> Unit): Data {
val builder = Builder()
builder.action()
return builder.build()
}
Action is a function from which you can modify the values directly, and createData will build it into a data class for you directly afterwards.
This way, you can create a data class with:
val data: Data = createData {
//execute stuff here
text = "new text"
//calculate number
number = -1
//calculate time
time = 222L
}
There are no setter methods per say, but you can directly assign the mutable variables with your new values and call other methods within the builder.
You can also make use of kotlin's get and set by specifying your own functions for each variable so it can do more than set the field.
There's also no need for returning the current builder class, as you always have access to its variables.
Addition note: If you care, createData can be shortened to this:
fun createData(action: Builder.() -> Unit): Data = with(Builder()) { action(); build() }.
"With a new builder, apply our action and build"
I don't think Kotlin has native builders. You can always compute all values and create the object at the end.
If you still want to use a builder you will have to implement it by yourself. Check this question
There is no need for creating custom builders in Kotlin - in order to achieve builder-like semantics, you can leverage copy method - it's perfect for situations where you want to get object's copy with a small alteration.
data class MyData(val val1: String? = null, val val2: String? = null, val val3: String? = null)
val temp = MyData()
.copy(val1 = "1")
.copy(val2 = "2")
.copy(val3 = "3")
Or:
val empty = MyData()
val with1 = empty.copy(val1 = "1")
val with2 = with1.copy(val2 = "2")
val with3 = with2.copy(val3 = "3")
Since you want everything to be immutable, copying must happen at every stage.
Also, it's fine to have mutable properties in the builder as long as the result produced by it is immutable.
It's possible to mechanize the creation of the builder classes with annotation processors.
I just created ephemient/builder-generator to demonstrate this.
Note that currently, kapt works fine for generated Java code, but there are some issues with generated Kotlin code (see KT-14070). For these purposes this isn't an issue, as long as the nullability annotations are copied through from the original Kotlin classes to the generated Java builders (so that Kotlin code using the generated Java code sees nullable/non-nullable types instead of just platform types).