Kotlin Programming 2020 Fall - Lecture 7
===
###### tags: `Kotlin`

This lecture intends to cover the materials in Chapter 12.

## Define a Class

A class may contain
1. Constructors and initialization blocks
2. Properties
3. Functions
4. Nested & inner classes (not in this lecture)
5. Object declaration (not in this lecture)

import java.util.Scanner

class Score {
    var point = 0
    fun gradePoint() = when (point) {
        in 90..100 -> 4.3
        in 85..89 -> 4.0
        in 80..84 -> 3.7
        in 77..79 -> 3.3
        in 73..76 -> 3.0
        in 70..72 -> 2.7
        in 67..69 -> 2.3
        in 63..66 -> 2.0
        in 60..62 -> 1.7
        in 50..59 -> 1.0
        else -> 0.0
    }
    fun grade() = when (point) {
        in 90..100 -> "A+"
        in 85..89 -> "A"
        in 80..84 -> "A-"
        in 77..79 -> "B+"
        in 73..76 -> "B"
        in 70..72 -> "B-"
        in 67..69 -> "C+"
        in 63..66 -> "C"
        in 60..62 -> "C-"
        in 50..59 -> "D"
        in 1..49 -> "E"
        else -> "X"
    }
    fun isPass(isGraduate: Boolean = false): Boolean {
        return (!isGraduate && point >= 60) || point >= 70
    }
    fun isFail(isGraduate: Boolean = false) = !isPass(isGraduate)
}

fun main() {
    val scanner = Scanner(System.`in`)
    print("What is your score? ")
    val yourScore = Score()
    yourScore.point = scanner.nextInt()
    println("Your grade is ${yourScore.grade()}.")
    println("Your grade point is ${yourScore.gradePoint()}.")
    println("Pass as a graduate student: ${yourScore.isPass(true)}")
    println("Fail as an undergraduate student: ${yourScore.isFail(false)}")
}

## Class Functions

+ `this`
+ Can also be defined outside of `class` block

import java.util.Scanner

class Score {
    var point = 0
    fun gradePoint() = when (point) {
        in 90..100 -> 4.3
        in 85..89 -> 4.0
        in 80..84 -> 3.7
        in 77..79 -> 3.3
        in 73..76 -> 3.0
        in 70..72 -> 2.7
        in 67..69 -> 2.3
        in 63..66 -> 2.0
        in 60..62 -> 1.7
        in 50..59 -> 1.0
        else -> 0.0
    }
    fun grade() = when (point) {
        in 90..100 -> "A+"
        in 85..89 -> "A"
        in 80..84 -> "A-"
        in 77..79 -> "B+"
        in 73..76 -> "B"
        in 70..72 -> "B-"
        in 67..69 -> "C+"
        in 63..66 -> "C"
        in 60..62 -> "C-"
        in 50..59 -> "D"
        in 1..49 -> "E"
        else -> "X"
    }
}

fun Score.isPass(isGraduate: Boolean): Boolean {
    return (!isGraduate && this.point >= 60) || this.point >= 70
}
fun Score.isFail(isGraduate: Boolean) = !this.isPass(isGraduate)

fun main() {
    val scanner = Scanner(System.`in`)
    print("What is your score? ")
    val yourScore = Score()
    yourScore.point = scanner.nextInt()
    println("Your grade is ${yourScore.grade()}.")
    println("Your grade point is ${yourScore.gradePoint()}.")
    println("Pass as a graduate student: ${yourScore.isPass(true)}")
    println("Fail as an undergraduate student: ${yourScore.isFail(false)}")
}

## Class Properties

+ Can be declared as `val` or `var`
+ Getter: the function to report the value of the property.
+ Setter: the function to update the value of the property (for `var` only).
+ `field`: an identifier for the property in its getter and setter.

var point = 0
    get() = field
    set(value) = when {
        value < 0 -> field = 0
        value > 100 -> field = 100
        else -> field = value
    }

+ Default getter and setter
+ The getter / setter of the data type of `val` / `var`
+ Computed properties

val passOrFail: Boolean
    get() = point >= 60

Sample code

import java.util.Scanner

class Score {
    var point = 0
        get() = field
        set(value) = when {
            value < 0 -> field = 0
            value > 100 -> field = 100
            else -> field = value
        }

    fun gradePoint() = when (point) {
        in 90..100 -> 4.3
        in 85..89 -> 4.0
        in 80..84 -> 3.7
        in 77..79 -> 3.3
        in 73..76 -> 3.0
        in 70..72 -> 2.7
        in 67..69 -> 2.3
        in 63..66 -> 2.0
        in 60..62 -> 1.7
        in 50..59 -> 1.0
        else -> 0.0
    }
    fun grade() = when (point) {
        in 90..100 -> "A+"
        in 85..89 -> "A"
        in 80..84 -> "A-"
        in 77..79 -> "B+"
        in 73..76 -> "B"
        in 70..72 -> "B-"
        in 67..69 -> "C+"
        in 63..66 -> "C"
        in 60..62 -> "C-"
        in 50..59 -> "D"
        in 1..49 -> "E"
        else -> "X"
    }
    fun isPass(isGraduate: Boolean = false): Boolean {
        return (!isGraduate && point >= 60) || point >= 70
    }
    fun isFail(isGraduate: Boolean = false) = !isPass(isGraduate)
}

fun main() {
    val scanner = Scanner(System.`in`)
    print("What is your score? ")
    val yourScore = Score()
    yourScore.point = scanner.nextInt()
    println("Your score is ${yourScore.point}")
    println("Your grade is ${yourScore.grade()}.")
    println("Your grade point is ${yourScore.gradePoint()}.")
    println("Pass as a graduate student: ${yourScore.isPass(true)}")
    println("Fail as an undergraduate student: ${yourScore.isFail(false)}")
}

## Visibility

+ Modify the visibility of properties, functions, getters, setters.
+ `public`
+ Default. Free for all.
+ `private`
+ Only visible in the class
+ `protected`
+ Same as `private` with visible in subclasses.
+ `internal`
+ Visible in the same module
+ Compiled together
+ In the same IntelliJ IDEA project

## Constructing Instances

### Primary Constructor

+ A constructor is like a function that initializes the instance.
+ The arguments of the primary constructor is defined after the class name.
+ The primary constructor `init` blocks.

import java.util.Scanner

class Score(_point: Int) {
    var point = 0
        get() = field
        set(value) = when {
            value < 0 -> field = 0
            value > 100 -> field = 100
            else -> field = value
        }
    init {
        point = _point
    }
    fun gradePoint() = when (point) {
        in 90..100 -> 4.3
        in 85..89 -> 4.0
        in 80..84 -> 3.7
        in 77..79 -> 3.3
        in 73..76 -> 3.0
        in 70..72 -> 2.7
        in 67..69 -> 2.3
        in 63..66 -> 2.0
        in 60..62 -> 1.7
        in 50..59 -> 1.0
        else -> 0.0
    }
    fun grade() = when (point) {
        in 90..100 -> "A+"
        in 85..89 -> "A"
        in 80..84 -> "A-"
        in 77..79 -> "B+"
        in 73..76 -> "B"
        in 70..72 -> "B-"
        in 67..69 -> "C+"
        in 63..66 -> "C"
        in 60..62 -> "C-"
        in 50..59 -> "D"
        in 1..49 -> "E"
        else -> "X"
    }
    fun isPass(isGraduate: Boolean = false): Boolean {
        return (!isGraduate && point >= 60) || point >= 70
    }
    fun isFail(isGraduate: Boolean = false) = !isPass(isGraduate)
}

fun main() {
    val scanner = Scanner(System.`in`)
    print("What is your score? ")
    val yourScore = Score(scanner.nextInt())
    println("Your score is ${yourScore.point}.")
    println("Your grade is ${yourScore.grade()}.")
    println("Your grade point is ${yourScore.gradePoint()}.")
    println("Pass as a graduate student: ${yourScore.isPass(true)}")
    println("Fail as an undergraduate student: ${yourScore.isFail(false)}")
}

#### Practice

+ Task 1
Change functions `Score.grade()` and `Score.gradePoint()` to properties.
+ Task 2
Change the getter of `point` to automatically adjust the score with the formula $f(x)=\lceil 10\sqrt{x}\rceil$