I am using a POJO data class with GSON to parse the data which is being called from the Firestore database.
For example, I have a POJO class with few non-nullable, and nullable values like userID
data class Users(id:String="", userID:String="" ...)
I am then using GSON to parse the JSON data to object for that class
val gson = Gson()
val jsonObjects = gson.toJson(querySnapshot.data)
val parseData = gson.fromJson(jsonObjects,Users::class.java)
My question is if someone uploads data in the database and forgets to add the userID (i.e. it is null), is there a way I can check if the User data class is valid when being parsed?
I am using a check like if(userID == ""){return false} . But as the number of non-nullable fields grows it gets tedious and there must be a better way to check this.
My question is if someone uploads data in the database and forgets to add the userID (i.e. it is null), is there a way I can check if the User data class is valid when being parsed?
If you don't want to have null values at all, why would you then let the user the possibility to provide incomplete data? The simplest solution would be to restrict the data that is added to the database. How? Simply by creating some constraints. For example, your users cannot get access to a feature in your app if they do not fill in all the required fields. That's an operation that is widely used.
If you already have null values, then checking against nullity is a solution you can go ahead with. In Kotlin, null safety can be handled in many ways, either by checking for null in conditions, using safe calls, elvis operator or even using the !! operator.
Maybe the extension method of Kotlin is an accepted approach.
Let me show a demo, I assume the class User looks like this:
data class User(
val id: String,
val userId: String? // nullable
)
Create a extension method User.isValid() to verify the fields you want.
fun User.isValid() = when {
userId.isNullOrEmpty() -> false
else -> true
}
The method isNullOrEmpty() is in stdlib of Kotlin. I test the code, it works fine.
val user1 = User("id 001", null)
val user2 = User("id 002", "userId 001")
println(user1.isValid()) //==> print false
println(user2.isValid()) //==> print true
Now, back to your worry point:
...But as the number of non nullable fields grow it gets tedious
I changed the class User
data class User(
val id: String,
val userId: String?,
val email: String?,
val name: String?
)
it means that when the fields of userId, email, name, any of them is null, the User is invalid.
Just add conditions in extention method, like this:
fun User.isValid() = when {
userId.isNullOrEmpty()
|| email.isNullOrEmpty()
|| name.isNullOrEmpty() -> false
else -> true
}
We just need to maintain the method of isValid.
conclusion
Kotlin Extension Method can be used in your case.
It's better don't use id="", also can create an extension method for this empty string if need.
fun String.Companion.empty() = ""
data class User(
val id: String,
val userId: String? = String.empty()
...
)
All the extension methods can be placed in a class, like UserExt.kt for easy maintaining.
Related
Let's say I have an object
data class Person(
val name: String,
val surname: String,
val street: String,
val postalCode: String,
val telephoneNumber: String,
)
And then I have a list of persons :
val personsList = listOf(
Person(name="John", surname="Hams", street="Gariolg", postalCode="929429", telephoneNumer="+2142422422",),
Person(name="Karl", surname="Hamsteel", street="Gariolg", postalCode="124215", telephoneNumer="+3526522",),
Person(name="Stepf", surname="Hiol", street="Bubmp", postalCode="5342", telephoneNumer="+7574535",),
Person(name="Germa", surname="Foo", street="Hutioa", postalCode="235236", telephoneNumer="+112355",)
)
So now if the user types for instance Hams it should return John and Karl, because both have the word "Hams" inside the object. What I mean is doesn't matter if the user types postalCode, name, or whatever I'd like to loop throughout the object to check if there's any coincidence.
How i would do it, is create a function inside the data class, say, for example, like this. This will check if any field inside your data class matches with the given string.
In my example i check if whole string matches, but you can change this however you want. You probably want it.contains(searchString) inside the any block.
fun checkIfStringMatches(searchString: String) : Boolean =
setOf(this.name, this.surname, this.strees, this.postalCode, this.telephone).any { it == string }
Then, you can use this function on your list of persons to filter if any object matches your string search.
personList.filter{it.checkIfStringMatches(mySearchString)} // this will return a list with all the objects that match your search criteria
The problem is that if you add more fields, you will have to change this function and add it to the listOf() block. But i don't know any way to do this automatically without reflection, which is not really recommended to use. If you still want to use it, here is a question on this topic. Kotlin: Iterate over components of object
Try this, it will work.
personsList.filter { it.surname.startsWith("Hams") }.map {
Log.d("filter_name", it.name)
}
Hey You can apply filter method on list and grab the expected output like below :
val filtered = personsList.filter { it.toString().contains("Hams", true) }
I would like to convert $document.data details to useful format so that I could use it for further applications. This is data from firestore documents.
private val mFireStore = FirebaseFirestore.getInstance()
mFireStore.collection("Users").whereEqualTo("lastName","H").whereEqualTo("firstName", "Uc").get()
.addOnSuccessListener{ documents ->
for(document in documents){
Log.d("TAG","${document.id}=>${document.data}")
Toast.makeText(applicationContext, "${document.id} => ${document.data}",
Toast.LENGTH_LONG).show()
}
}
.addOnFailureListener{exception ->
Log.w("TAG","Error getting documents:",exception)
Toast.makeText(applicationContext, "Failed",
Toast.LENGTH_LONG).show()
}
This is my code. Now when I run the code get this in the logcat
OL0rD4UfgHSh2K8UoTMnX6Xea6P2=>{lastName=H, image=, firstName=Uc, B=L, gender=, organization=, profileCompleted=0, mobile=0, blood group=o+, id=OL0rD4UfgHSh2K8UoTMnX6Xea6P2, email=jojoy09#gmail.com}
Now I want to convert this result to a useful format so that I could use it later. I wpuld like to convert the data so that I could load it in listview.
In the following for-loop:
for(document in documents) { ... }
The "document" object is of type DocumentSnapshot. When you call getData() on such an object, the type of object that is returned is a Map<String, Object>.
In Kotlin, this object is represented by Map<String, Any>. In order to get the data, you can simply iterate over the Map and get the data accordingly, using the following lines of code:
val map = document.data
for ((key, value) in map) {
Log.d("TAG", "$key = $value")
}
Or even simpler, using:
map.forEach { (key, value) -> Log.d("TAG", "$key = $value") }
However, if you only need, the value of a particular property, for example, the value of the email address, you can simply get it by using DocumentSnapshot#getString(String field) method:
val email = document.getString("email")
Log.d("TAG", email)
The result in the logcat will be:
jojoy09#gmail.com
.................
As I see in your screenshot, almost all of the properties are of type String. However, you can find different flavors for each type of field, like getLong(String field), getDouble(String field), getDate(String field), getTimestamp(String field), and so on.
Furthermore, if you need to get the entire document, and you want to convert it into an object of a specific class, as also #mehulbisht mentioned in his answer, you should use DocumentSnapshot#toObject(Class valueType). So assuming that you have a data class that looks like this:
data class User(
var email: String? = null,
var firstName: String? = null,
var lastName: String? = null,
//Other necessary fields
)
To convert the "document" object into an object of the "User" class, please use the following line of code:
val user = document.toObject(User::class.java)
Log.d("TAG", user.email)
The result in the logcat will be the same as above.
If you want to display a list of "User" objects in a ListView, then please check my answer from the following post:
What miss, with connect Firestore and ListView for random results in sample?
It's really simple to convert the Java code into Kotlin.
The Model that you used to set this data will be used here. You can convert the documents to your Model class using the .toObjects() method on it. Just use:
val myObjs = documents.toObjects(Model::class.java)
EDIT
For displaying this as a Log in Logcat use:
Log.d("myObjs ","""
$myObjs
""".trimIndent())
Do tell if this doesn't work for you :)
I'm using a simple data class Track, then saving it as an object so that fields and values are automatically saved. This is working as intended. However later when I want to query on 1 of those property/field names, I need to provide the String value.
How do I refer to the name of that property in the data class, so that I maintain the "single source of truth" for that value, without hard coding it a second time in the query? Example uses "spotifyId":
data class Track(
val spotifyId: String,
val name: String,
val artist: List<String>,
val duration: String
)
Save a track:
set(trackDocRef, track)
Query for a track:
db.collection("tracks").whereEqualTo("spotifyId", "sdfgsdfswer4543w5yer345").get()
Thank you!
I figured out one way of doing it, not sure how "correct" it is. I used reflection in the TracksContract object to refer to the Track model:
data class Track(
val spotifyId: String,
val name: String,
val artist: List<String>,
val duration: String
)
Single reference to Track model via TracksContract:
object TracksContract {
internal const val COLLECTION_NAME = "Tracks"
object Fields {
val SPOTIFY_ID = Track::spotifyId.name
val NAME = Track::name.name
val ARTIST = Track::artist.name
val DURATION = Track::duration.name
}
}
now if I need to run the query based on that spotifyId, I refer to through the TracksContract:
db.collection(TracksContract.COLLECTION_NAME)
.whereEqualTo(TracksContract.Fields.SPOTIFY_ID, "ID_VALUE")
.get()
Finally if I decide to change the property names in the Track model then the TracksContract will show a compiler error, and I can change the name and refactor references from there if I want to.
I'm new to coding in kotlin and want to implement an immutable class that represents a project with various fields inside.
The easiest way to do this is by using a data class and using the copy() method so that anytime one of the app user modifies a field it results in the backend in a call to the copy method with the modified field producing the new project.
My problem is that this way does not allow for prior checking of parameters (eg : limit string size of the owner, making sure the number of people added to the project is reasonable etc).
If this was java, I'd use a builder pattern but this seems to defeat the purpose of kotlin, and i've read articles that are positive to using builders in kotlin (https://www.baeldung.com/kotlin/builder-pattern)
and others that are completely against (https://code-held.com/2021/01/23/dont-use-builder-in-kotlin/).
I haven't found any way to "modify" the copy method and to add the parameter sanitization checks that are needed for each parameter. I would appreciate any "smooth" idea to implement this, if anybody has found it. The goal would also be to throw exeptions/sealed classes variables so that the app UI can tell the user what went wrong instead of a generic error message just mentioning that the project was not modified.
I agree with the second link. If you look at the comments on the Baeldung article, you'll see even they were convinced and pledged to revise the article.
You can throw exceptions in an init block but if these are exceptions that are not caused by programmer error, it would be more Kotlin-idiomatic to expose a single constructor-like function that returns a wrapper or just null for invalid input.
Examples:
data class Person(val name: String, val age: Int = 0) {
init {
if (age < 0) {
throw IllegalArgumentException("Age $age is less than 0.")
}
}
}
If you want to return a wrapper or nullable, a data class isn't suitable for preventing invalid input because the generated copy() function will always return a fully constructed object. Sadly, Kotlin does not support overriding the generated copy() function.
sealed class Result<T>
data class Success<T>(val value: T): Result<T>()
data class Failure<T>(val reason: String): Result<T>()
class Person private constructor(val name: String, val age: Int = 0) {
companion object {
fun build(name: String, age: Int = 0): Result<Person> {
return when {
age < 0 -> Failure("Age $age is less than 0.")
else -> Success(Person(name, age))
}
}
}
fun buildCopy(name: String = this.name, age: Int = this.age) = build(name, age)
}
I have text values I retrieve from text inputs. I want to allow the user to not fill in these inputs. But if the user has not filled one or more values I want to display default values for these inputs.
I have a data class that looks something like this:
#Parcelize
data class Profile(
val firstName: String = "",
val lastName: String = "",
val description: String = "",
val imageUri: String = ""
) : Parcelable
On click I call a method from my ViewModel class and pass it the current input values which is then persisted using a Repository class:
viewModel.createProfile(
etFirstName.text.toString(),
etLastName.text.toString(),
etProfileDescription.text.toString(),
profileImageUri.toString()
)
// The createProfile function itself
fun createProfile(
firstName: String = "John",
lastName: String = "Doe",
description: String = "Default Description",
imageUri: String = ""
) {
val profile = Profile(firstName, lastName, description, imageUri)
// Persist data
}
In a another fragment I set some UI data using this persisted data like this:
private fun observeProfile() {
viewModel.getProfile()
viewModel.profile.observe(viewLifecycleOwner, Observer {
val profile = it
tvName.text = getString(R.string.profile_name, profile.firstName, profile.lastName)
tvDescription.text = profile.description
ivProfileImage.setImageURI(Uri.parse(profile.imageUri))
})
}
So currently createProfile expects 4 arguments. I'm able to pass less because I have optional parameters, but how can I conditionally pass arguments to createProfile based on if the value is non null or empty. I can of course create checks for each value, but what is the best way to approach this?
Update
I don't think I was clear enough in my original question, because I don't only pass values from text inputs to createProfile. profileImageUri is a class variable of type Uri? and is initially set to null. The user can select an image and this variable is updated with the image data. The reason I'm passing and storing the image data as a String is because all the profile data also gets persisted to Firestore so Strings are easier to work with.
Compared to your own answer, I'd create a helper function
fun CharSequence?.ifNullOrEmpty(default: String) = if (this.isNullOrEmpty()) default else this.toString()
And use it as
viewModel.createProfile(
etFirstName.text.ifNullOrEmpty("John"),
etLastName.text.ifNullOrEmpty("Doe"),
etProfileDescription.text.ifNullOrEmpty("Default Description"),
profileImageUri.ifNullOrEmpty("Default Uri")
)
EDIT: given the update, I'd consider
fun Any?.ifNullOrEmpty(default: String) =
if (this == null || (this is CharSequence && this.isEmpty()))
default
else
this.toString()
I have found a workaround.
Turns out it's possible to pass if-else statements as parameters, because if statements are expressions in Kotlin:
viewModel.createProfile(
if (!etFirstName.text.isNullOrEmpty()) etFirstName.text.toString() else "John",
if (!etLastName.text.isNullOrEmpty()) etLastName.text.toString() else "Doe",
if (!etProfileDescription.text.isNullOrEmpty()) etProfileDescription.text.toString() else "Default Description",
if (profileImageUri != null) profileImageUri.toString() else ""
)
Using this approach I also don't have to set default values for my data class variables and for my createProfile function parameters.
I additionally added a check in my observeProfile function so if profileImageUri is null it won't try to set the image:
// ...
if (profile.imageUri.isNotEmpty()) {
ivProfileImage.setImageURI(Uri.parse(profile.imageUri))
}
// ...
My initial idea doesn't seem to be possible using a data class. It does seem to be possible using a regular class and varargsbut it has problems:
#Parcelize
class Profile(
vararg val params: String
) : Parcelable
...
val params = arrayOfValues.filter{ !it.isNullOrBlank() } // filter out all unwanted data
val profile = Profile(*params) // pass every param separately using spread operator
Main problem here is that the parameters themselves are obfuscated. I can still get the reference to individual parameters using an index and do stuff with them, but it doesn't work as nicely.
I think what you want to use is the Elvis Operator in Kotlin: ?:.
val test = exampleExpression ?: "alternative value"
If the expression to the left of ?: is not null, the elvis operator returns it, otherwise it returns the expression to the right. Note that the right-hand side expression is evaluated only if the left-hand side is null.
viewModel.createProfile(
etFirstName.text.toString() ?: "John",
etLastName.text.toString() ?: "Doe",
etProfileDescription.text.toString() ?: "Default Description",
profileImageUri.toString() ?: "Default Uri"
)