Kotlin while and do…while Loop

Kotlin Extension Function

In this article, you will learn to extend a class with new functionality using extension functions.

Suppose, you need to extend a class with new functionality. In most programming languages, you either derive a new class or use some kind of design pattern to do this.

However, in Koltin, you can also use extension function to extend a class with new functionality. Basically, an extension function is a member function of a class that is defined outside the class.

For example, you need to use a method to the String class that returns a new string with first and last character removed; this method is not already available in String class. You can use extension function to accomplish this task.

Example: Remove First and Last Character of String

fun String.removeFirstLastChar(): String =  this.substring(1, this.length - 1)

fun main(args: Array<String>) {
    val myString= "Hello Everyone"
    val result = myString.removeFirstLastChar()
    println("First character is: $result")
}

When you run the program, the output will be:

First character is: ello Everyon

Here, an extension function removeFirstLastChar() is added to the String class.

The class name is the receiver type (String class in our example). The this keyword inside the extension function refers the receiver object.

Kotlin extension function receiver type and obejct

If you need to integrate Kotlin on the top of Java project, you do not need to modify the whole code to Koltin. Just use extension functions to add functionalities.

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Kotlin Getters and Setters

In this article, you will learn to use getters and setters in Kotlin with the help of an example.

Before you learn about getters and setter, be sure to check Kotlin class and objects.

In programming, getters are used for getting value of the property. Similarly, setters are used for setting value of the property.

In Kotlin, getters and setters are optional and are auto-generated if you do not create them in your program.

How getters and setters work?

The following code in Kotlin

class Person {
    var name: String = "defaultValue"
}

is equivalent to

class Person {
    var name: String = "defaultValue"

    // getter
    get() = field

    // setter
    set(value) {
        field = value
    }
}

When you instantiate object of the Person class and initialize the name property, it is passed to the setters parameter value and sets field to value.

val p = Person()
p.name = "jack"

Now, when you access name property of the object, you will get field because of the code get() = field.

println("${p.name}")

Here’s an working example:

fun main(args: Array<String>) {

    val p = Person()
    p.name = "jack"
    println("${p.name}")
}

class Person {
    var name: String = "defaultValue"

    get() = field

    set(value) {
        field = value
    }
}

When you run the program, the output will be:

jack

This is how getters and setters work by default. However, you can change value of the property (modify value) using getters and setters.

Example: Changing value of the property

fun main(args: Array<String>) {

    val maria = Girl()
    maria.actualAge = 15
    maria.age = 15
    println("Maria: actual age = ${maria.actualAge}")
    println("Maria: pretended age = ${maria.age}")

    val angela = Girl()
    angela.actualAge = 35
    angela.age = 35
    println("Angela: actual age = ${angela.actualAge}")
    println("Angela: pretended age = ${angela.age}")
}

class Girl {
    var age: Int = 0
    get() = field
    set(value) {
        field = if (value < 18)
            18
        else if (value >= 18 && value <= 30)
            value
        else
            value-3
    }

    var actualAge: Int = 0
}

When you run the program, the output will be:

Maria: actual age = 15
Maria: pretended age = 18
Angela: actual age = 35
Angela: pretended age = 32

Here, the actualAge property works as expected.

However, there is additional logic is setters to modify value of the age property.

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Kotlin Class and Objects

In this article, you’ll be introduced to Object-oriented programming in Kotlin. You’ll learn what a class is, how to create objects and use it in your program.

Kotlin supports both functional and object-oriented programming.

Kotlin supports features such as higher-order functions, function types and lambdas which makes it a great choice for working in functional programming style. You will learn about these concept in later chapters. This article will focus on object-oriented style of programming in Kotlin.

Object-oriented Programming (OOP)

In object-oriented style of programming, you can divide a complex problem into smaller sets by creating objects.

These objects share two characteristics:

state
behavior

Let’s take few examples:

Lamp is an object
- It can be in on or off state.
- You can turn on and turn off lamp (behavior).
Bicycle is an object
- It has current gear, two wheels, number of gear etc. states.
- It has braking, accelerating, changing gears etc. behavior.

You will learn about detail features of an object-oriented programming like: data encapsulation, inheritance and polymorphism as we go on. This article will focus on the basics to keep things simple.

Kotlin Class

Before you create objects in Kotlin, you need to define a class.

A class is a blueprint for the object.

We can think of class as a sketch (prototype) of a house. It contains all the details about the floors, doors, windows etc. Based on these descriptions we build the house. House is the object.

Since, many houses can be made from the same description, we can create many objects from a class.

How to define a class in Kotlin?

To define a class in Kotlin, class keyword is used:

class ClassName {
    // property
    // member function
    ... .. ...
}

Here’s an example:

class Lamp {

    // property (data member)
    private var isOn: Boolean = false

    // member function
    fun turnOn() {
        isOn = true
    }

    // member function
    fun turnOff() {
        isOn = false
    }
}

Here, we defined a class named Lamp.

The class has one property isOn (defined in same way as variable), and two member functions turnOn() and turnOff().

In Kotlin, either the property must be initialized or must be declared abstract (Visit: Kotlin Abstract Class to learn more). In the above example, isOn property is initialized to false.

Classes, objects, properties, member function etc. can have visibility modifiers. For example, the isOn property is private. This means, the isOn property can be changed from only inside the Lamp class.

Other visibility modifiers are:

private – visible (can be accessed) from inside the class only.
public – visible everywhere.
protected – visible to the class and its subclass.
internal – any client inside the module can access them.

You will learn about protected and internal modifiers later in Kotlin visibility modifiers article.

If you do not specify the visibility modifier, it will be public by default.

In the above program, turnOn() and turnOff() member functions are public whereas, isOn property is private.

Kotlin Objects

When class is defined, only the specification for the object is defined; no memory or storage is allocated.

To access members defined within the class, you need to create objects. Let’s create objects of Lamp class.

class Lamp {

    // property (data member)
    private var isOn: Boolean = false

    // member function
    fun turnOn() {
        isOn = true
    }

    // member function
    fun turnOff() {
        isOn = false
    }
}

fun main(args: Array<String>) {

    val l1 = Lamp() // create l1 object of Lamp class
    val l2 = Lamp() // create l2 object of Lamp class
}

This program creates two objects l1 and l2 of class Lamp. The isOn property for both lamps l1 and l2 will be false.

How to access members?

You can access properties and member functions of a class by using . notation. For example,

l1.turnOn()

This statement calls turnOn() function for l1 object.

Let’s take another example:

l2.isOn = true

Here, we tried to assign true to isOn property of l2 object. Note that, isOn property is private, and if you try to access isOn from outside the class, an exception is thrown.

Example: Kotlin Class and Object

class Lamp {

    // property (data member)
    private var isOn: Boolean = false

    // member function
    fun turnOn() {
        isOn = true
    }

    // member function
    fun turnOff() {
        isOn = false
    }

    fun displayLightStatus(lamp: String) {
        if (isOn == true)
            println("$lamp lamp is on.")
        else
            println("$lamp lamp is off.")
    }
}

fun main(args: Array<String>) {

    val l1 = Lamp() // create l1 object of Lamp class
    val l2 = Lamp() // create l2 object of Lamp class

    l1.turnOn()
    l2.turnOff()

    l1.displayLightStatus("l1")
    l2.displayLightStatus("l2")
}

When you run the program, the output will be:

l1 Lamp is on.
l2 Lamp is off.

In the above program,

Lamp class is created.
The class has a property isOn and three member functions turnOn(), turnOff() and displayLightStatus().
Two objects l1 and l2 of Lamp class are created in the main() function.
Here, turnOn() function is called using l1 object: l1.turnOn(). This method sets isOn instance variable of l1 object to true.
And, turnOff() function is called using l2 object: l1.turnOff(). This method sets isOff instance variable of l2 object to false.
Then, displayLightStatus() function is called for l1 and l2 objects which prints appropriate message depending on whether isOn property is true or false.

Notice that, the isOn property is initialized to false inside the class. When an object of the class is created, isOn property for the object is initialized to false automatically. So, it’s not necessary for l2 object to call turnOff() to set isOn property to false.

For example:

class Lamp {

    // property (data member)
    private var isOn: Boolean = false

    // member function
    fun turnOn() {
        isOn = true
    }

    // member function
    fun turnOff() {
        isOn = false
    }

    fun displayLightStatus() {
        if (isOn == true)
            println("lamp is on.")
        else
            println("lamp is off.")
    }
}

fun main(args: Array<String>) {

    val lamp = Lamp()
    lamp.displayLightStatus()
}

When you run the program, the output will be:

lamp is off.

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Kotlin while and do…while Loop

Loop is used in programming to repeat a specific block of code. In this article, you will learn to create while and do…while loops in Kotlin programming.

Loop is used in programming to repeat a specific block of code until certain condition is met (test expression is false).

Loops are what makes computers interesting machines. Imagine you need to print a sentence 50 times on your screen. Well, you can do it by using print statement 50 times (without using loops). How about you need to print a sentence one million times? You need to use loops.

You will learn about two loops while and do..while in this article with the help of examples.

If you are familiar with while and do…while loops in Java, you are already familiar with these loops in Kotlin as well.

Kotlin while Loop

The syntax of while loop is:

while (testExpression) {
    // codes inside body of while loop
}

How while loop works?

The test expression inside the parenthesis is a Boolean expression.

If the test expression is evaluated to true,

statements inside the while loop are executed.
then, the test expression is evaluated again.

This process goes on until the test expression is evaluated to false.

If the test expression is evaluated to false,

while loop is terminated.

Flowchart of while Loop

Example: Kotlin while Loop

// Program to print line 5 times

fun main(args: Array<String>) {

    var i = 1

    while (i <= 5) {
        println("Line $i")
        ++i
    }
}

When you run the program, the output will be:

Line 1
Line 2
Line 3
Line 4
Line 5

Notice, ++i statement inside the while loop. After 5 iterations, variable i will be incremented to 6. Then, the test expression i <= 5 is evaluated to false and the loop terminates.

If the body of loop has only one statement, it’s not necessary to use curly braces { }.

Example: Compute sum of Natural Numbers

// Program to compute the sum of natural numbers from 1 to 100.
fun main(args: Array<String>) {

    var sum = 0
    var i = 100

    while (i != 0) {
        sum += i     // sum = sum + i;
        --i
    }
    println("sum = $sum")
}

When you run the program, the output will be:

sum = 5050

Here, the variable sum is initialized to 0 and i is initialized to 100. In each iteration of while loop, variable sum is assigned sum + i, and the value of i is decreased by 1 until i is equal to 0. For better visualization,

1st iteration: sum = 0+100 = 100, i = 99
2nd iteration: sum = 100+99 = 199, i = 98
3rd iteration: sum = 199+98 = 297, i = 97
... .. ...
... .. ...
99th iteration: sum = 5047+2 = 5049, i = 1
100th iteration: sum = 5049+1 = 5050, i = 0 (then loop terminates)

To learn more about test expression and how it is evaluated, visit comparison and logical operators.

Kotlin do…while Loop

The do...while loop is similar to while loop with one key difference. The body of do...while loop is executed once before the test expression is checked.

Its syntax is:

do {
   // codes inside body of do while loop
} while (testExpression);

How do…while loop works?

The codes inside the body of do construct is executed once (without checking the testExpression). Then, the test expression is checked.

If the test expression is evaluated to true, codes inside the body of the loop are executed, and test expression is evaluated again. This process goes on until the test expression is evaluated to false.

When the test expression is evaluated to false, do..while loop terminates.

Flowchart of do…while Loop

Example: Kotlin do…while Loop

The program below calculates the sum of numbers entered by the user until user enters 0.

To take input from the user, readline() function is used.

fun main(args: Array<String>) {

    var sum: Int = 0
    var input: String

    do {
        print("Enter an integer: ")
        input = readLine()!!
        sum += input.toInt()

    } while (input != "0")

    println("sum = $sum")
}

When you run the program, the output will be something like:

Enter an integer: 4
Enter an integer: 3
Enter an integer: 2
Enter an integer: -6
Enter an integer: 0
sum = 3

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Tag: Kotlin while and do…while Loop

Kotlin Extension Function

Kotlin Introduction

Kotlin Flow Control

Kotlin Functions

Kotlin OOP

Example: Remove First and Last Character of String

Kotlin Getters and Setters

Kotlin Introduction

Kotlin Flow Control

Kotlin Functions

Kotlin OOP

How getters and setters work?

Example: Changing value of the property

Kotlin Class and Objects

Kotlin Introduction

Kotlin Flow Control

Kotlin Functions

Kotlin OOP

Object-oriented Programming (OOP)

Kotlin Class

How to define a class in Kotlin?

Kotlin Objects

How to access members?

Example: Kotlin Class and Object

Kotlin while and do…while Loop

Kotlin Introduction

Kotlin Flow Control

Kotlin Functions

Kotlin OOP

Kotlin while Loop

How while loop works?

Flowchart of while Loop

Example: Kotlin while Loop

Example: Compute sum of Natural Numbers

Kotlin do…while Loop

How do…while loop works?

Flowchart of do…while Loop

Example: Kotlin do…while Loop