I am trying to assign data to my class using the variable that I created. I want to use the variable as an instance of the membership class. Every time I pass any values I get an error.
error: no value passed for parameter '_number'
var member1 = membership()
error: unresolved reference: _number
member1._number = 1
I am getting this for all properties I try to pass anything to.
I tried to create the class without any constructors but I would still get errors. My goal is to create a list of the variables.
class membership(_number:Int, _name:String, _address:String, _zip:String, _phone:String, _memberSince:String, _memberType:Char)
{
var number: Int = _number;
var name: String = _name;
var address: String = _address;
var zip: String = _zip;
var phone: String = _phone;
var memberSince: String = _memberSince;
var memberType: Char = _memberType;
}
fun main(args: Array<String>) {
var member1 = membership()
member1._number = 1
member1._name = "George Jetson";
member1._address ="123 Main St.";
member1._zip = "99207";
member1._memberSince = "12/01/1997";
member1._memberType = 'L';
}
You have declared the class membership with the primary constructor that expects 7 parameters. Hence, you need to provide values for these parameters when you're instantiating this class:
var member1 = membership(
_number = 1,
_name = "George Jetson",
_address ="123 Main St.",
_zip = "99207",
_memberSince = "12/01/1997",
_memberType = 'L'
)
If you want to create an instance of the class first and then initialize its properties one-by-one, you need it to have a parameterless constructor:
class membership() {
However then you'll have to make all its properties nullable or lateinit because now you cannot provide their initial values upon construction:
class membership() {
var number: Int? = null
var name: String? = null
var address: String? = null
// etc
}
This way you'll be able to initialize them as you want in your question.
Related
class ZabbixAPIResult<T> {
var jsonrpc = ""
var result = T()
var id = ""
}
yields
type parameter T cannot be called as a function
Is it possible to declare something like this in Kotlin? I.e., a class that embeds an instance of a class supplied as a parameter?
Of course you can have a field with type T. But you need to pass either the instance itself or a way to construct it. E.g.
class ZabbixAPIResult<T>(var result: T) {
var jsonrpc = ""
var id = ""
}
or
class ZabbixAPIResult<T>(resultFun: () => T) {
var jsonrpc = ""
var result = resultFun()
var id = ""
}
See https://kotlinlang.org/docs/classes.html#constructors for the details of this syntax.
But Kotlin can't assume T is a class with a default constructor, so your version doesn't work.
(On a side note, it's generally better to avoid using var by default.)
How can I create an instance of InfoA that contains also title. Do I need to modify the classes?
Can't specify the title.
Also, do I need to create setters for it? To not access with the _
val info = InfoA(_subtitle = "SUBTITLE", title = ...)
open class Info(
open val action: Action = Action(),
open val title: String? = ""
) {
fun hasAction(): Boolean = action.hasAction()
}
class InfoA(
private val _subtitle: String? = "",
private val _image: String? = "",
private val _backgroundImage: String? = "",
private val _backgroundColor: String? = null,
private val _foregroundColor: String? = null,
private val _borderColor: String? = null
) : Info() {
val subtitle: String
get() = _subtitle.orEmpty()
val image: String
get() = _image.orEmpty()
val backgroundImage: String
get() = _backgroundImage.orEmpty()
val backgroundColor: Int?
get() = if (_backgroundColor != null) convertRgbStringToColorInt(_backgroundColor) else null
val foregroundColor: Int?
get() = if (_foregroundColor != null) convertRgbStringToColorInt(_foregroundColor) else null
val borderColor: Int?
get() = if (_borderColor != null) convertRgbStringToColorInt(_borderColor) else null
}
As the code is written, title is a val, so it can't be changed from its initial value — which is empty string if (as in the case of InfoA) something calls its constructor without specifying another value.
If it were changed to be a var, then it could be changed later, e.g.:
val info = InfoA(_subtitle = "SUBTITLE").apply{ title = "..." }
Alternatively, if you want to keep it a val, then InfoA would need to be changed: the most obvious way would be to add a title parameter in its constructor, and pass that up to Info:
class InfoA(
title: String? = "",
// …other fields…
) : Info(title = title) {
Note that this way, InfoA can never use Info's default value for title, so you may need to duplicate that default in InfoA's constructor.
The need to duplicate superclass properties in a subclass constructor is awkward, but there's currently no good way around it. (See e.g. this question.) If there are many parameters, you might consider bundling them together into a single data class, which could then be passed easily up to the superclass constructor — but of course users of the class would need to specify that. (Some people think that having more than a few parameters is a code smell, and that bundling them together can often improve the design.)
I got this (contrived) sample from Packt's "Programming Kotlin" on using secondary constructor with inheritance.
Edit: from the answer it is clear that the issue is about backing field. But the book did not introduced that idea, just with the wrong example.
open class Payment(val amount: Int)
class ChequePayment : Payment {
constructor(amount: Int, name: String, bankId: String) : super(amount) {
this.name = name
this.bankId = bankId
}
var name: String
get() = this.name
var bankId: String
get() = this.bankId
}
val c = ChequePayment(3, "me", "ABC")
println("${c} ${c.amount} ${c.name}")
When I run it this error is shown.
$ kotlinc -script class.kts 2>&1 | more
java.lang.StackOverflowError
at Class$ChequePayment.getName(class.kts:10)
at Class$ChequePayment.getName(class.kts:10)
at Class$ChequePayment.getName(class.kts:10)
Line 10 does seems to be a infinite recursion, how to solve it?
You have a recursion in your code:
class ChequePayment : Payment {
constructor(amount: Int, name: String, bankId: String) : super(amount) {
this.name = name
this.bankId = bankId
}
var name: String
get() = this.name // recursion: will invoke getter of name (itself)
var bankId: String
get() = this.bankId // recursion: will invoke getter of bankId (itself)
}
If you don't need custom logic for your getter, just leave your properties like this:
var name: String
var bankId: String
They will have a default getter, which does nothing more than returning the value of the backing field.
Note: The code as it is can/should be refactored to this:
class ChequePayment(amount: Int, var name: String, var bankId: String) : Payment(amount) {
// ...
}
This uses the primary constructor and is much less redundant.
To access the backing field you have to use the keyword field instead of this.name see https://kotlinlang.org/docs/reference/properties.html#backing-fields
this.name references the getter, which references this.name which is an infinite recursion, as you already noted. In code:
var name: String
get() = field
var bankId: String
get() = field
Side note: Android Studio and Idea will complain rightfully that you don't need a getter in this case. So you can simplify even more:
var name: String
var bankId: String
I have an object User defined as below
class User(){
var id: Int? = null
var name: String? = null}
For certain reasons, I have to create new object User of same parameters and I have to copy data from old to new type.
class UserNew(){
var id: Int? = null
var name: String? = null}
I was looking for easiest way to convert from old type to a new one. I want to do simply
var user = User()
var userNew = user as UserNew
But obviously, I am getting This cast can never succeed. Creating a new UserNew object and set every parameter is not feasible if I have a User object with lots of parameters. Any suggestions?
as is kotlin's cast operator. But User is not a UserNew. Therefore the cast fails.
Use an extension function to convert between the types:
fun User.toUserNew(): UserNew {
val userNew = UserNew()
userNew.id = id
userNew.name = name
return userNew
}
And use it like so
fun usingScenario(user: User) {
val userNew = user.toUserNew()
If you don't want to write a boilerplate code, you can use some libraries that will copy values via reflection (for example http://mapstruct.org/), but it's not the best idea.
To achieve you can Simply use Gson and avoid boilerplate code:
var user = User(....)
val json = Gson().toJson(user)
val userNew:UserNew =Gson().fromJson(json, UserNew::class.java)
you should follow this logic for this case.
note: #Frank Neblung answer i implemented
fun main(args: Array<String>) {
val user = User()
user.id = 10
user.name = "test"
var userNew = user.toUserNew()
println(userNew.id) // output is 10
println(userNew.name)// output is test
}
class User()
{
var id: Int? = null
var name: String? = null
fun toUserNew(): UserNew {
val userNew = UserNew()
userNew.id = id
userNew.name = name
return userNew
}
}
class UserNew() {
var id: Int? = null
var name: String? = null
}
You have two options. Either create interface and implement it in both classes. then you can use this interface in both places (User,UserNew) If this is not what you want, i would use copy constructor in UserNew taking User as parameter, You can create new
NewUser nu = new UserNew(userOld)
if you have lots of properties answer from ppressives is way to go
To achieve that you can use the concept of inheritance:
https://www.programiz.com/kotlin-programming/inheritance
Example:
open class Person(age: Int) {
// code for eating, talking, walking
}
class MathTeacher(age: Int): Person(age) {
// other features of math teacher
}
I have an object
class Person {
#JsonProperty("name")
var name: String? = null
#JsonProperty("id")
lateinit var id: String}
There is an only empty constructor and I want to create a person so I wrote:
val person = Person()
person.name = "someName"
person.id = "SomeId"
I'm pretty sure that there is a prettier syntax, something like
val person = Person {name = "someName" , id = "someId"}
but I can't find an example.
am I missing something? should I create a secondary constructor to use this syntax or is there another way?
Please check apply method.
Your code will be like this:
val person = Person().apply {name = "someName", id = "someId"}
Another way - you can change declaration of Person to (e.g. just change brackets, replace var to val and remove lateinit):
class Person (#JsonProperty("name") val name: String? = null,
#JsonProperty("id") val id: String )
Then you will able to do this:
val person = Person(name = "someName", id = "someId")
You can achieve it with the constructor parameter.
class Person(
#JsonProperty("name")
var name: String? = null,
#JsonProperty("id")
var id: String
)
val person = Person(name = "someName", id = "someId")
Another way is make your class and desired variables open to be overridden.
open class Person {
#JsonProperty("name")
open var name: String? = null
#JsonProperty("id")
open var id: String = ""
}
val person = object : Person() {
override var name: String? = "SomeName"
override var id = "SomeId"
}