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.
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.
In this article, you will learn to create data classes in Kotlin. You will also learn about requirements that data class must fulfill, and their standard functionalities.
There may arise a situation where you need to create a class solely to hold data. In such cases, you can mark the class as data to create a data class. For example,
data class Person(val name: String, var age: Int)
For this class, the compiler automatically generates:
copy() function, equals() and hashCode() pair, and toString() form of the primary constructor
componentN() functions
Before talking about these features in detail, let’s talk about requirements that a data class must fulfill.
Kotlin Data Class Requirements
Here are the requirements:
The primary constructor must have at least one parameter.
The parameters of the primary constructor must be marked as either val (read-only) or var (read-write).
The class cannot be open, abstract, inner or sealed.
The class may extend other classes or implement interfaces. If you are using Kotlin version before 1.1, the class can only implement interfaces.
Example: Kotlin Data Class
data class User(val name: String, val age: Int)
fun main(args: Array<String>) {
val jack = User("jack", 29)
println("name = ${jack.name}")
println("age = ${jack.age}")
}
When you run the program, the output will be:
name = jack
age = 29
When you declare a data class, the compiler automatically generates several functions such as toString(), equals(), hashcode() etc behind the scenes. This helps to keep you code concise. Had you used Java, you would need to write a lot of boilerplate code.
Let’s use these functions:
Copying
For a data class, you can create a copy of an object with some of its properties different using copy() function. Here’s how it works:
data class User(val name: String, val age: Int)
fun main(args: Array<String>) {
val u1 = User("John", 29)
// using copy function to create an object
val u2 = u1.copy(name = "Randy")
println("u1: name = ${u1.name}, name = ${u1.age}")
println("u2: name = ${u2.name}, name = ${u2.age}")
}
When you run the program, the output will be:
u1: name = John, name = 29
u2: name = Randy, name = 29
toString() method
The toString() function returns a string representation of the object.
data class User(val name: String, val age: Int)
fun main(args: Array<String>) {
val u1 = User("John", 29)
println(u1.toString())
}
When you run the program, the output will be:
User(name=John, age=29)
hashCode() and equals()
The hasCode() method returns hash code for the object. If two objects are equal, hashCode() produces the same integer result.
The equals() returns true if two objects are equal (has same hashCode()). If objects are not equal, equals() returns false.
data class User(val name: String, val age: Int)
fun main(args: Array<String>) {
val u1 = User("John", 29)
val u2 = u1.copy()
val u3 = u1.copy(name = "Amanda")
println("u1 hashcode = ${u1.hashCode()}")
println("u2 hashcode = ${u2.hashCode()}")
println("u3 hashcode = ${u3.hashCode()}")
if (u1.equals(u2) == true)
println("u1 is equal to u2.")
else
println("u1 is not equal to u2.")
if (u1.equals(u3) == true)
println("u1 is equal to u3.")
else
println("u1 is not equal to u3.")
}
When you run the program, the output will be:
u1 hashcode = 71750738
u2 hashcode = 71750738
u3 hashcode = 771732263
u1 is equal to u2.
u1 is not equal to u3.
Destructuring Declarations
You can destructure an object into a number of variables using destructing declaration. For example:
data class User(val name: String, val age: Int, val gender: String)
fun main(args: Array<String>) {
val u1 = User("John", 29, "Male")
val (name, age, gender) = u1
println("name = $name")
println("age = $age")
println("gender = $gender")
}
When you run the program, the output will be:
name = John
age = 29
gender = Male
This was possible because the compiler generates componentN() functions all properties for a data class. For example:
data class User(val name: String, val age: Int, val gender: String)
fun main(args: Array<String>) {
val u1 = User("John", 29, "Male")
println(u1.component1()) // John
println(u1.component2()) // 29
println(u1.component3()) // "Male"
}
In this article, you will learn about constructors in Kotlin (both primary and secondary constructors) as well as initializer blocks with the help of examples.
A constructor is a concise way to initialize class properties.
It is a special member function that is called when an object is instantiated (created). However, how they work in Kotlin is slightly different.
In Kotlin, there are two constructors:
Primary constructor – concise way to initialize a class
Secondary constructor – allows you to put additional initialization logic
Primary Constructor
The primary constructor is part of the class header. Here’s an example:
class Person(val firstName: String, var age: Int) {
// class body
}
The block of code surrounded by parentheses is the primary constructor: (val firstName: String, var age: Int).
The constructor declared two properties: firstName (read-only property as it’s declared using keyword val) and age (read-write property as it is declared with keyword var).
Example: Primary Constructor
fun main(args: Array<String>) {
val person1 = Person("Joe", 25)
println("First Name = ${person1.firstName}")
println("Age = ${person1.age}")
}
class Person(val firstName: String, var age: Int) {
}
When you run the program, the output will be:
First Name = Joe
Age = 25
When the object of Person class is created, "Joe" and 25 values are passed as if Person is a function.
This initializes firstName and age properties of person1 object to "Joe" and 25 respectively.
There are other ways of using primary constructors.
Primary Constructor and Initializer Blocks
The primary constructor has a constrained syntax, and cannot contain any code.
To put the initilization code (not only code to initialize properties), initializer block is used. It is prefixed with init keyword. Let’s modify the above example with initializer block:
fun main(args: Array<String>) {
val person1 = Person("joe", 25)
}
class Person(fName: String, personAge: Int) {
val firstName: String
var age: Int
// initializer block
init {
firstName = fName.capitalize()
age = personAge
println("First Name = $firstName")
println("Age = $age")
}
}
When you run the program, the output will be:
First Name = Joe
Age = 25
Here, parameters fName and personAge inside the parenthesis accepts values "Joe" and 25 respectively when person1 object is created. However, fName and personAge are used without using var or val, and are not properties of the Person class.
The Person class has two properties firstName, and age are declared.
When person1 object is created, code inside initializer block is executed. The initializer block not only initializes its properties but also prints them.
Here is another way to perform the same task:
fun main(args: Array<String>) {
val person1 = Person("joe", 25)
}
class Person(fName: String, personAge: Int) {
val firstName = fName.capitalize()
var age = personAge
// initializer block
init {
println("First Name = $firstName")
println("Age = $age")
}
}
To distinguish the constructor parameter and property, different names are used (fName and firstName, and personAge and age). It’s more common to use _firstName and _age instead of completely different name for constructor parameters. For example:
class Person(_firstName: String, _age: Int) {
val firstName = _firstName.capitalize()
var age = _age
// initializer block
init {
... .. ...
}
}
Default Value in Primary Constructor
You can provide default value to constructor parameters (similar to providing default arguments to functions). For example:
fun main(args: Array<String>) {
println("person1 is instantiated")
val person1 = Person("joe", 25)
println("person2 is instantiated")
val person2 = Person("Jack")
println("person3 is instantiated")
val person3 = Person()
}
class Person(_firstName: String = "UNKNOWN", _age: Int = 0) {
val firstName = _firstName.capitalize()
var age = _age
// initializer block
init {
println("First Name = $firstName")
println("Age = $age\n")
}
}
When you run the program, the output will be:
First Name = Joe
Age = 25
person2 is instantiated
First Name = Jack
Age = 0
person3 is instantiated
First Name = UNKNOWN
Age = 0
Kotlin Secondary Constructor
In Kotlin, a class can also contain one or more secondary constructors. They are created using constructor keyword.
Secondary constructors are not that common in Kotlin. The most common use of secondary constructor comes up when you need to extend a class that provides multiple constructors that initialize the class in different ways. Be sure to check Kotlin Inheritance before you learn it.
Here’s how you can create a secondary constructor in Kotlin:
class Log {
constructor(data: String) {
// some code
}
constructor(data: String, numberOfData: Int) {
// some code
}
}
Here, the Log class has two secondary constructors, but no primary constructor.
fun main(args: Array<String>) {
val p1 = AuthLog("Bad Password")
}
open class Log {
var data: String = ""
var numberOfData = 0
constructor(_data: String) {
}
constructor(_data: String, _numberOfData: Int) {
data = _data
numberOfData = _numberOfData
println("$data: $numberOfData times")
}
}
class AuthLog: Log {
constructor(_data: String): this("From AuthLog -> " + _data, 10) {
}
constructor(_data: String, _numberOfData: Int): super(_data, _numberOfData) {
}
}
When you run the program, the output will be:
From AuthLog -> Bad Password: 10 times
Note: The secondary constructor must initialize the base class or delegate to another constructor (like in above example) if the class has no primary constructor.
In this tutorial, you will learn to use continue to skip the current iteration of a loop. Also, you will also learn about continue labels in this article.
Suppose you are working with loops. It is sometimes desirable to skip the current iteration of the loop.
In such case, continue is used. The continue construct skips the current iteration of the enclosing loop, and the control of the program jumps to the end of the loop body.
How continue works?
It is almost always used with if…else construct. For example,
while (testExpression1) {
// codes1
if (testExpression2) {
continue
}
// codes2
}
If the testExpression2 is evaluated to true, continue is executed which skips all the codes inside while loop after it for that iteration.
Example: Kotlin continue
fun main(args: Array<String>) {
for (i in 1..5) {
println("$i Always printed.")
if (i > 1 && i < 5) {
continue
}
println("$i Not always printed.")
}
}
When the value of i is greater than 1 and less than 5, continue is executed, which skips the execution of
println("$i Not always printed.")
statement.
However, the statement
println("$i Always printed.")
is executed in each iteration of the loop because this statement exists before the continue construct.
Example: Calculate Sum of Positive Numbers Only
The program below calculates the sum of maximum of 6 positive numbers entered by the user. If the user enters negative number or zero, it is skipped from calculation.
Visit Kotlin Basic Input Output to learn more on how to take input from the user.
fun main(args: Array<String>) {
var number: Int
var sum = 0
for (i in 1..6) {
print("Enter an integer: ")
number = readLine()!!.toInt()
if (number <= 0)
continue
sum += number
}
println("sum = $sum")
}
When you run the program, the output will be:
Enter an integer: 4
Enter an integer: 5
Enter an integer: -50
Enter an integer: 10
Enter an integer: 0
Enter an integer: 12
sum = 31
Kotlin Labeled continue
What you have learned till now is unlabeled form of continue, which skips current iteration of the nearest enclosing loop. continue can also be used to skip the iteration of the desired loop (can be outer loop) by using continue labels.
How labeled continue works?
Label in Kotlin starts with an identifier which is followed by @.
Here, outerloop@ is a label marked at outer while loop. Now, by using continue with the label (continue@outerloop in this case), you can skip the execution of codes of the specific loop for that iteration.
Example: labeled continue
fun main(args: Array<String>) {
here@ for (i in 1..5) {
for (j in 1..4) {
if (i == 3 || j == 2)
continue@here
println("i = $i; j = $j")
}
}
}
When you run the program, the output will be:
i = 1; j = 1
i = 2; j = 1
i = 4; j = 1
i = 5; j = 1
The use of labeled continue is often discouraged as it makes your code hard to understand. If you are in a situation where you have to use labeled continue, refactor your code and try to solve it in a different way to make it more readable.
There are 3 structural jump expressions in Kotlin: break, continue and return.
learngram.wordpress.com 