Companion Objects (2)

Companion Objects: used for factory objects and static members

Kotlin classes do not have static members

'static' methods:

class Person {
    companion object {
        fun main(){...}
    }
}

'factory' methods:

class Student {
    companion object {
        fun createUndergrad(name: String) : Undergraduate {...}
        fun createPostgrad(name: String) : Postgraduate {...}
    }
}

Using Companion Objects:

fun main() {
    var charles = Student3("charles", "Sun", 1)
    var charles2 = Student3("charles", "Sun", 1, "some tutor")
    println(charles.id)

    Student3.createPostgrad("Charles")
    Student3.createUndergrad("Eagle")
}

open class Student3(firstName: String, lastName: String, _id: Int, var tutor: String = "") : Person3(firstName, lastName) {
    val id: Int
    init{
        id = _id
    }

    fun enrole(courseName: String) {
        val course = Courses.allCourses
            .filter{it.title == courseName}
            .firstOrNull()
    }

    companion object : XmlSerializer<Student3> {
        override fun toXml(item: Student3) {

        }


        fun createUndergrad(name: String) : Undergraduate {
            return Undergraduate(name)
        }

        fun createPostgrad(name: String) : Postgraduate {
            return Postgraduate(name)
        }
    }
}

class Undergraduate(firstName: String) : Student3(firstName, "", 1) {

}

class Postgraduate(firstName: String) : Student3(firstName, "", 1) {

}

interface XmlSerializer<T> {
    fun toXml(item: T)
}

Run externally: Run -> Edit Configurations -> + (add a Kotlin class) -> Main class: rsk.Program

it can be run in java

class Program {
    companion object {
        //args: Array<String> must be included
        @JvmStatic
        fun main(args: Array<String>) {
            var charles = Student3("charles", "Sun", 1)
            var charles2 = Student3("charles", "Sun", 1, "some tutor")
            println(charles.id)

            Student3.createPostgrad("Charles")
            Student3.createUndergrad("Eagle")
        }
    }
}

Companion Objects

Static methods:

• Kotlin does not have static methods
• however can have 'singletons'
• use 'object' keyword
• use 'companion object' to get 'statics'

Object keyword:

• create a singleton
• actually defines a class and creates an instance

Objects:

• can have properties, methods, initializers
• cannot have constructors

class Course(val id: Int, val title: String) {

}
//create a singleton
object Courses {
    var allCourses = arrayListOf<Course>()

    fun initialize() {
        allCourses.add(Course(1, "Kotlin Fundasmentals"))
    }
}

class Student3(firstName: String, lastName: String, _id: Int, var tutor: String = "") : Person3(firstName, lastName) {
    val id: Int
    init{
        id = _id
    }

    fun enrole(courseName: String) {
        val course = Courses.allCourses
            .filter{it.title == courseName}
            .firstOrNull()
    }
}

Objects:

• can derive from other classes/interfaces
• can be used where any 'instance' is used
• can be declared inside another class
•  

import java.io.File

object CaseInsensitiveComparator: Comparator<File> {
    override fun compare(...) : Int {}
}

class Person {
    object CaseInsensitiveComparator: Comparator<File> {
        override fun compare() : Int {}
    }
}

Programming with types (4)

Data Classes:

• provide a convenient way to override euqals, hashCode and toString
• typically immutable classes
• kotlin also generates 'copy' method

data class Meeting(val name: String, val location: String)
val aMeeting = Meeting("A Meeting", "Denver")
val anotherMeeting = aMeeting.copy(location = "Atlanta")

fun main() {
    var charles = Student4(1, "Charles")
    var sun = Student4(1, "Charles")

    //class shows reference address
    //data class shows "Student4(id=1, name=Charles)"
    println(charles)

    //because references are different for class
    //will be equal for data class. Kotlin just compares each value in the class
    if (charles == sun) {
        println("Equal")
    }
    else {
        println("Not Equal")
    }

    var newCharles = charles.copy(name = "Eagle")
    println(newCharles)
}

data class Student4(val id: Int, val name: String) {}