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
}
Related
I am trying to save data into a kotlin object but I feel that it's not working.
the object is
object User {
var userInfo: UserInfo? = null
var listOfVehicles: MutableList<Vehicle> = mutableListOf()
init {}
fun clear(){
userInfo = null
listOfVehicles = mutableListOf()
}
}
and I would like to be able to access it from everywhere and be able to modify it from any activities, fragments....
to add a value inside, I just do : User.userInfo.name = "blabla" but it keep being null
any idea how to fix it ? I tried as well to use val user: User? = null but it's the same.
I want to have User a single data structure used by any part of the code to retrieve or add data.
What did I miss ?
Thanks
You need to instantiate the userInfo before assigning values to it.
object User {
var userInfo: UserInfo? = UserInfo()
var listOfVehicles: MutableList<Vehicle> = mutableListOf()
init {
}
fun clear() {
userInfo = null
listOfVehicles = mutableListOf()
}
}
Now to use this, User.userInfo?.name = "blabla"
Updated: added some clarifications from the comments
I would like to use the same 'mapping' code for the primary constructor and copy() method of an immutable data class. How can I do this without creating an empty object first, and then using copy() on it?
The issue with how it is now is that if I add a new attribute with default value to Employee and EmployeeForm it would be easy to only add it in one of the two mapping functions and forget about the other (toEmployeeNotReusable / copyEmployee).
These are the data classes I'd like to map between:
#Entity
data class Employee(
val firstName: String,
val lastName: String,
val jobType: Int,
#OneToMany(mappedBy = "employee", cascade = [CascadeType.ALL], fetch = FetchType.EAGER)
private val _absences: MutableSet<Absence> = mutableSetOf(),
#Id #GeneratedValue(strategy = GenerationType.IDENTITY)
var id: Long = 0 // prevents #Joffrey's answer from working
) {
init {
_absences.forEach { it.employee = this }
}
val absences get() = _absences.toSet()
fun addAbsence(newAbsence: Absence) {
newAbsence.employee = this
_absences += newAbsence
}
#Entity
#Table(name = "absence")
data class Absence(
// ... omitted fields
) {
#ManyToOne(fetch = FetchType.EAGER)
#JoinColumn(name = "employee_id")
lateinit var employee: Employee
}
}
data class EmployeeForm(
var firstName: String = "",
var lastName: String = "",
var jobType: Int = 0
) {
// not reusable
fun toEmployeeNotReusable(): Employee {
return Employee(firstName, lastName, jobType)
}
// works but hacky
fun toEmployee(): Employee {
return copyEmployee(Employee("", "", 0))
}
fun copyEmployee(employee: Employee): Employee {
return employee.copy(
firstName = firstName,
lastName = lastName,
jobType = jobType
)
}
}
While mutability would be fine, in my case, I'd be interested to know how this would be possible.
One way to avoid listing the attributes 4 times would be to declare Employee as an interface instead, and use the "mutable" version, the form, as the only data class implementing it. You would have the "read-only" view using the interface, but you would technically only use the mutable instance behind the scenes.
This would follow what Kotlin designers have done for List vs MutableList.
interface Employee {
val firstName: String
val lastName: String
val jobType: Int
}
data class EmployeeForm(
override var firstName: String = "",
override var lastName: String = "",
override var jobType: Int = 0
): Employee {
fun toEmployee(): Employee = this.copy()
fun copyEmployee(employee: Employee): Employee = this.copy(
firstName = firstName,
lastName = lastName,
jobType = jobType
)
}
However, this implies that the form has all fields of an employee, which you probably don't want.
Also, I would personally prefer what you had done in the beginning, listing twice the field would not be a problem, just write tests for your functions, and when you want to add functionality, you'll add tests for that functionality anyway.
You should be able to do this using reflection: check list of properties in Employee and EmployeeForm, call the constructor by the matching names (using callBy to handle default parameters). The drawback, of course, is that you won't get compile-time errors if any properties are missing (but for this case, any test would probably fail and tell you about the problem).
Approximate and untested (don't forget to add the kotlin-reflect dependency):
inline fun <reified T> copy(x: Any): T {
val construct = T::class.primaryConstructor
val props = x::class.memberProperties.associate {
// assumes all properties on x are valid params for the constructor
Pair(construct.findParameterByName(it.name)!!,
it.call(x))
}
return construct.callBy(props)
}
// in EmployeeForm
fun toEmployee() = copy<Employee>(this)
You can make an equivalent which is compile-time checked with Scala macros, but I don't think it's possible in Kotlin.
I am trying to find a solution for a nice kotlin data class solution. I have already this:
data class Object(
var classMember: Boolean,
var otherClassMember: Boolean,
var example: Int = 0) {
fun set(block: Object.() -> kotlin.Unit): Object {
val copiedObject = this.copy()
copiedObject.apply {
block()
}
return copiedObject
}
fun touch(block: Object.() -> kotlin.Unit): Object {
return this.set {
classMember = true
otherClassMember = false
block() }
}
}
val test = Object(true,true,1)
val changedTest = test.touch { example = 2 }
the result of this method is that the changedTest object has classMember = true, otherClassMember = false and example = 2
The problem with this solution is, the class properties are not immutable with var declaration. Does somebody have an idea how to optimize my methods to change var to val?
val says that a variable can't change it's value after initialization at the definition point. Kotlin's generated copy method does not modify an existing copy after construction: this method actually uses retrieved values from an object, replaces these values with ones that provided in copy method (if any), and after that just constructs a new object using these values.
So, it is not possible to perform such an optimization if you are going to change object's state after construction.
If I understood what you want correctly, you can do
data class Object(
val classMember: Boolean,
val otherClassMember: Boolean,
val example: Int = 0) {
fun touch(example: Int = this.example): Object {
return copy(
classMember = true,
otherClassMember = false,
example = example)
}
}
val test = Object(true,true,1)
val changedTest = test.touch(example = 2)
Though you need to repeat parameters other than classMember and otherClassMember but without reflection you can't do better.
I'm start the learn jooq. I have mssql server. I create some class the represent table on my server. But I don't understand what is the benefit when I was using getPrimaryKey and getReferences methods in my table class?
class User : TableImpl<Record>("users") {
companion object {
val USER = User()
}
val id: TableField<Record, Int> = createField("id", SQLDataType.INTEGER)
val name: TableField<Record, String> = createField("name", SQLDataType.NVARCHAR(50))
val countryId: TableField<Record, Short> = createField("country_id", SQLDataType.SMALLINT)
override fun getPrimaryKey(): UniqueKey<Record> = Internal.createUniqueKey(this, id)
override fun getReferences(): MutableList<ForeignKey<Record, *>> =
mutableListOf(Internal.createForeignKey(primaryKey, COUNTRY, COUNTRY.id))
}
class Country : TableImpl<Record>("country") {
companion object {
val COUNTRY = Country()
}
val id: TableField<Record, Short> = createField("id", SQLDataType.SMALLINT)
val name: TableField<Record, String> = createField("name", SQLDataType.NVARCHAR(100))
override fun getPrimaryKey(): UniqueKey<Record> =
Internal.createUniqueKey(this, id)
}
The generated meta data is a mix of stuff that's useful...
to you, the API user
to jOOQ, which can reflect on that meta data for a few internal features
For instance, in the case of getPrimaryKey(), that method helps with all sorts of CRUD related operations as you can see in the manual:
https://www.jooq.org/doc/latest/manual/sql-execution/crud-with-updatablerecords/simple-crud
If you're not using the code generator (which would generate all of these methods for you), then there is no need to add them to your classes. You could shorten them to this:
class User : TableImpl<Record>("users") {
companion object {
val USER = User()
}
val id: Field<Int> = createField("id", SQLDataType.INTEGER)
val name: Field<String> = createField("name", SQLDataType.NVARCHAR(50))
val countryId: Field<Short> = createField("country_id", SQLDataType.SMALLINT)
}
However, using the code generator is strongly recommended for a variety of advanced jOOQ features which you might not get, otherwise.
Let's take the class of a data class:
data class User(
val userNumber: Int = -1,
val name: String,
val userGroups; List<String> = emptyList(),
val screenName: String = "new-user"
)
When calling this function from Kotlin, it is pretty straightforward. I can simply use the named-argument syntax to do so. Calling from Java, I have to specify all values, or use the #JvmOverloads annotation, which generates the following constructors (in addition to the constructor that kotlin generates with the bit-mask for default values):
User(int userNumber, #NotNull String name, #NotNull List userGroups,
#NotNull String screenName)
User(int userNumber, #NotNull String name, #NotNull List userGroups)
User(int userNumber, #NotNull String name)
User(#NotNull String name)
Now, if I want to create a User object in Java equivalent to User(name="John Doe", userGroups=listOf("admin", "super") I can't do it with the above constructors. I CAN however do it if I put val userNumber: Int = -1 at the end in the data class declaration (the generation of constructors seems to depend on the order the optional arguments are defined in). Which is fine, because expecting kotlin to generate all permutations is going to heavily bloat some classes.
The biggest problem that tools like Jackson simply don't work as they have no idea which constructor to use (and not like I can annotate one of the generated ones specially).
So, is there a way to generate a (single) constructor like:
User(Integer userNumber, String name, List<String> userGroups, String screenName) {
this.userNumber = (userNumber == null) ? -1 : userNumber;
this.userGroups = (userGroups == null) ? Collections.emptyList() : userGroups;
//...
}
Currently I am using the above approach, but manually defining the constructors where I need them.
EDIT
I should clarify, creating a similar constructor doesn't work, obviously because both the signatures would clash on the JVM. This is what it would like in my case:
data class User(
val userNumber: Int = -1,
val name: String,
val userGroups; List<String> = emptyList(),
val screenName: String = "new-user"
) {
companion object {
#JvmStatic
#JsonCreator
fun constructionSupport(
#JsonProperty("userNumber") userNumber : Int?,
#JsonProperty("name") name : String,
#JsonProperty("userGroups") userGroups : List<String>?,
#JsonProperty("screenName") screenName : String?
) = User(
userNumber = userNumber ?: -1,
name = name,
userGroups = userGroups ?: emptyList(),
screenName = screenName ?: "new-user"
)
}
}
Also note the redundancy where I have to write the default values for the properties twice. I Now that I look at it, I doubt there exists a solution for this. Maybe this is a good use-case for a kapt based side-project of mine :)
Better solution is to add possibility to library understand Kotlin functional. For example, for Jackson exists jackson-module-kotlin. With this library we can use default arguments in data classes.
Example:
data class User(
val userNumber: Int = -1,
val name: String,
val userGroups: List<String> = emptyList(),
val screenName: String = "new-user"
)
fun main(args: Array<String>) {
val objectMapper = ObjectMapper()
.registerModule(KotlinModule())
val testUser = User(userNumber = 5, name = "someName")
val stringUser = objectMapper.writeValueAsString(testUser)
println(stringUser)
val parsedUser = objectMapper.readValue<User>(stringUser)
println(parsedUser)
assert(testUser == parsedUser) {
println("something goes wrong")
}
}
After kicking this around for a minute, I think I found a solution that may work well here. Simply define a top level function in the same source file, that will build the object. Perhaps like so:
fun build_user(userNumber: Int?, name: String, userGroups: List<String>?, screenName: String?) : User {
return User(if(userNumber !== null) userNumber else -1, name, if(userGroups !== null) userGroups else emptyList(),
if(screenName !== null) screenName else "new-user")
}
Then when you need it, you simply call it from Java:
User user = UserKt.build_user(null, "Hello", null, "Porterhouse Steak");
System.out.println(user);
Output from the example:
User(userNumber=-1, name=Hello, userGroups=[], screenName=Porterhouse Steak)
The method is somewhere between a constructor and a builder. It beats hammering out a full-blown Builder object, and avoids cluttering your data class with unnecessary Java-interop glue code messiness.
See Package Level Functions for more information.