COMP3330 Chapter 5 Kotlin vs. Java Syntax PDF

Chapter 5. Kotlin vs. Java Syntax 2024-2025 COMP3330 Interactive Mobile Application Design and Programming Dr. T.W. Chim (E-mail: [email protected]) Department of Computer Science, The University of Hong Kong Agenda Data Types Functions & Functio...

Chapter 5. Kotlin vs. Java Syntax 2024-2025 COMP3330 Interactive Mobile Application Design and Programming Dr. T.W. Chim (E-mail: [email protected]) Department of Computer Science, The University of Hong Kong Agenda Data Types Functions & Function Calls Variables (Immutable vs. Mutable) Kotlin Arrays Null Safety in Kotlin String Interpolation Kotlin Classes 2 Data Types Recall that Java supports 8 primitive types, and each primitive type has the corresponding wrapper class. Primitive Wrapper Bit Depth Value Range Type Class boolean Boolean JVM-specific true or false char Character 16 bits 0 to 65535 byte Byte 8 bits -128 to 127 short Short 16 bits -32768 to 32767 int Integer 32 bits -2147483648 to 2147483647 long Long 64 bits -huge to huge float Float 32 bits varies double Double 64 bits varies 3 Data Types Kotlin doesn’t have primitive types. It uses classes as an object wrapper for primitives. Type Bit Depth Value Range Boolean JVM-specific true or false Char 16 bits 0 to 65535 Byte 8 bits -128 to 127 Short 16 bits -32768 to 32767 Int 32 bits -2147483648 to 2147483647 Long 64 bits -huge to huge Float 32 bits varies Double 64 bits varies String variable varies 4 Functions Java public int sum(int a, int b) { return a + b; } Kotlin public fun sum(a: Int, b: Int): Int { return a + b Key differences: - Keyword “fun” } - Parameter definition (colon notation) - Return type indication (colon notation) - Absence of semi-colon 5 Functions Interestingly, the following Kotlin function definitions are equivalent. public fun sum(a: Int, b: Int): Int { return a + b } public fun sum(a: Int, b: Int): Int = a + b public fun sum(a: Int, b: Int) = a + b 6 Function Calls Java public void calculate(int num1, int num2, int num3) { … }  calculate(1, 3, 5); Kotlin fun calculate(num1: Int, num2: Int, num3: Int) { … } Note that this is a void function  calculate(1, 3, 5) with no return value. 7 Function Calls In Kotlin, we can provide a default value for a function parameter. In the following, num2 will hold the provided value 3 (because value is provided) while num3 will hold the default value 4 (because value is not provided). fun calculate(num1: Int, num2: Int = 2, num3: Int = 4) { … }  calculate(1, 3) // 3rd parameter missing! Challenge: How to provide 1st and 3rd parameters while missing 2nd parameter? 8 Function Calls The following Kotlin function calls are possible. Parameter names can be used in function calls. fun calculate(num1: Int, num2: Int = 2, num3: Int = 4) { … }  calculate(1, 3) // num3 will be assigned the default value 4  calculate(1, num2 = 3) // num3 will be assigned the default value 4  calculate(1, num3 = 5) // num2 will be assigned the default value 2 9 Variables Java int a = 1; String b = “tim”; Kotlin (Immutable Version 1) Kotlin (Mutable Version 1) val a: Int = 1 var a: Int = 1 val b: String = “tim” var b: String = “tim” Kotlin (Immutable Version 2) Kotlin (Mutable Version 2) val a = 1 // Inferred type is Int var a = 1 // Inferred type is Int val b = “tim” // Inferred type is String var b = “tim” // Inferred type is String Value re-assignment to a and b will cause error! 10  Recall that in Java, we can create an array of Kotlin Arrays integers like: int[] num = {1, 2, 3, 4}; OR int[] num; num = new int; Using the arrayOf() function num = 1; num = 2; num = 3; num = 4; val num = arrayOf(1, 2, 3, 4) //implicit type declaration val num = arrayOf(1, 2, 3, 4) //explicit type declaration Traversing an array and accessing array elements for (i in 0..num.size-1) { print(" “ + num[i]) } 11 Kotlin Arrays Using the Array constructor Since Array is a class in Kotlin, we can also use the Array constructor to create an array. The constructor takes two parameters: The size of the array, and A function which accepts the index of a given element and returns the initial value of that element. val num = Array(3, {i-> i*1}) // equivalent to val num = arrayOf(0, 1, 2) / val num = arrayOf(0, 1, 2) 12 Null Safety in Kotlin In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that cannot (non- null references). A regular variable of type String cannot hold null by default and the following will cause compilation error. var str: String = “hku” str = null To allow nulls, we can declare a variable as nullable by writing ? after the type. var str: String? = “hku” str = null 13 Null Safety in Kotlin If we call a method or access a property on a non-null reference, it's guaranteed not to cause a Null Pointer Exception (NPE). The following is safe. var S1: String = “hku” val l = S1.length If we call a method or access a property on a nullable reference, it would not be safe. The compiler reports an error. var S2: String? = “hku” val l = S2.length // error: variable ‘S2’ can be null 14 Null Safety in Kotlin For nullable reference, we need to explicitly check whether the reference is null and handle the two options separately. var S2: String? = “hku” In Kotlin, it is possible to assign an if val l = if (S2 != null) S2.length else null condition to a variable! To simplify and to get rid of the if condition, we can make use of safe calls. l is assigned null if S2 is null. So the type of l is Int?. var S2: String? = “hku” val l = S2?.length 15 Null Safety in Kotlin Another example of safe calls: val a = "Computer" val b: String? = null println(b?.length) println(a?.length) // Unnecessary safe call as a is non-null Output: null 8 Note: The type of expressions a?.length and b?length is Int?. 16 Null Safety in Kotlin Safe calls can be used in chains. For example, Tim is an employee who may be assigned to a department (or not). That department may in turn have another employee as a department head (or not). To obtain the name of Tim's department head (if any), we write the following: tim?.department?.head?.name Such a chain returns null if any of the properties in it is null. 17 Null Safety in Kotlin vs. Java Java public ZipCode getZipCode(User user) { if (user != null) { if (user.address != null) { return user.address.zipCode; } } return null; } Kotlin Java: Several lines of code fun getZipCode(user: User?): ZipCode? { Kotlin: Single line of code return user?.address?.zipCode } or fun getZipCode(user: User?) = user?.address?.zipCode 18 String Interpolation The following three functions are equivalent. We want to compose a string with 3 variables inside. $ can be used to interpolate variables within a string. override fun toString(): String { return “Song{id=“ + id + “, title=‘” + title + “’, author=‘” + author + “’}” } override fun toString(): String { return “Song{id=$id, title=‘$title’, author=‘$author’}” } override fun toString() = “Song{id=$id, title=‘$title’, author=‘$author’}” 19  Like Java, the compiler will generate an additional constructor without parameters Kotlin Classes on the JVM. For this purpose, it’ll instantiate an object with default values. A Kotlin basic class declaration, with two properties and a primary constructor can be written as follows: class Person constructor(val name: String, val age: Int? = null) or class Person(val name: String, val age: Int? = null) // name and age are read-only here Like Java, we can create a new instance of a class. However, there is no “new” keyword in Kotlin. Instantiation is like constructor invocation. val person1 = Person(“Sam”, 20) val person2 = Person(“Priscilla”) // age is assigned null value 20 Kotlin Classes To do the same in Java, we need much more code! class Person { final String name; final Integer age; public Person(String name) { this.name = name; this.age = null; } If you can remember public Person(String name, Integer age) { Kotlin syntax well, a Kotlin program can be much this.name = name; shorter than its Java this.age = age; counterpart. } } 21 Kotlin Classes We cannot put any code in the primary constructor. If we want to execute some initialization code, we have to put them in an initializer block, which is prefixed with the “init” keyword. The initializer block is called after the primary constructor. We can also access class fields there. class Person(val name: String, val age: Int? = null) { init { println("Hello, I'm $name") } } val person1 = Person(“Sam”, 20) will produce the following on screen. Hello, I’m Sam 22 Kotlin Classes In a Kotlin class, we can declare one or more secondary constructors. Secondary constructors are prefixed with the constructor keyword. We can use a trailing comma class Car { when declaring class properties: val id: String val id: String, val type: String val type: String, constructor(id: String, type: String) { this.id = id this.type = type Sample usage: fun main(args: Array) { } val car1 = Car("1", “BYD") } val car2 = Car("2", “Isuzu") } 23 Kotlin Classes In a Kotlin class, a secondary constructor can invoke the primary constructor using this keyword. class Car(val id: String, val type: String) { constructor(id: String): this(id, "unknown") } 24 Kotlin Classes We can use a primary constructor of the superclass. class Employee(name: String, val salary: Int): Person(name) By doing this, we pass a name to the primary constructor of Person class. Additionally, we add a new field called salary in the Employee class. Note: All classes in Kotlin are final by default. We need to add the open keyword so that we can inherit from our Person class. 25  Kotlin Class Inheritance (Example) open class Polygon { open fun draw() { // some default polygon drawing method } } Java version: class Polygon { abstract class WildShape : Polygon() { void draw() { … // Classes that inherit WildShape need to provide } // their own draw method instead of using the } // default on Polygon abstract class WildShape extends Polygon abstract override fun draw() { } @override abstract void draw() { … } } 26 Kotlin Class Inheritance (Example) abstract class Polygon { abstract fun draw() } Java version: class Rectangle : Polygon() { abstract class Polygon { abstract void draw() { override fun draw() { … } // draw the rectangle } } class Rectangle extends Polygon { } @override abstract void draw() { … } } 27 We are with you! If you encounter any problems in understanding the materials in the lectures, please feel free to contact me or my TAs. We are always with you! We wish you enjoy learning Kotlin in this class. 28  Chapter 5. End 2024-2025 COMP3330 Interactive Mobile Application Design and Programming Dr. T.W. Chim (E-mail: [email protected]) Department of Computer Science, The University of Hong Kong