Control Statements in Kotlin

Control Statements in Kotlin :

1)If else does not have much difference.

2)For loop have a syntax change.

3)While and do while have minimal syntax change.

1)If else statement :

Java Code :

if(i>1){  

//code to be executed if condition1 is true  

}else if(i>2){  

//code to be executed if condition2 is true  

}  

else if(i>3){  

//code to be executed if condition3 is true  

}  

...  

else{  

//code to be executed if all the conditions are false  

}

Kotlin Code :

if (i > 1)

{

  //code to be executed if condition1 is true 

}

else if (i > 2)

{

  //code to be executed if condition2 is true 

}

else if (i > 3)

{

  //code to be executed if condition3 is true 

}

run({ 

  //code to be executed if all the conditions are false 

})

2)For Loop :

Java:

for(int i=1;i<=10;i++){  

        System.out.println(i);  

    }  

for(int i:arr){  

        System.out.println(i);  

    }  

Kotlin :

for (i in 1..10)

{

  println(i)

}

for (i in arr)

{

  System.out.println(i)

}

3)Switch Statement :

Java :

int number=20;  

    

switch(number){  

    case 10: System.out.println("10");break;  

    case 20: System.out.println("20");break;  

    case 30: System.out.println("30");break;  

    default:System.out.println("Not in 10, 20 or 30");  

    } 

Kotlin :

val number = 20

when (number) {

  10 -> println("10")

  20 -> println("20")

  30 -> println("30")

  else -> println("Not in 10, 20 or 30")

}

4)While Statement :

Java :

int i=1;

while(i<=10){

System.out.println(i);

i++;

} 

Kotlin :

val i = 1

while (i <= 10)

{

println(i)

i++

}

4)Do While Statement :

Java :

int i=1;

do{

System.out.println(i);

i++;

}while(i<=10);

}  

Kotlin :

val i = 1

do

{

println(i)

i++

}

while (i <= 10)

Kotlin Add two numbers example

Java Add two numbers example :

Simple kotlin example of adding two numbers :

Java Code :

class Simple{
public static void main(String[] args){
int a=10;
int b=10;
int c=a+b;
System.out.println(c);
}}

Kotlin Code :

class Simple {
  @JvmStatic fun main(args:Array<String>) {
    val a = 10
    val b = 10
    val c = a + b
    println(c)
  }
}

Class and Objects in Kotlin

Classes and Objects in Kotlin

1)Class :

Classes in Kotlin are declared using the keyword class{: .keyword }:

class Address { }

The class declaration consists of :

·      Class name.

·      Class header.(optional)

·      Class body (surrounded by curly braces).(oprtional)

Both the header and the body are optional; if the class has no body, curly braces can be omitted.

class Address

2)Constructors :

Kotlin have two types of constructors.

1)Primary constructor : 

The primary constructor is part of the class header: it is used after the class name.

class Place constructor(firstName: String) { }

constructor keyword can be omitted if constructor does not have any annotations or visibility modifiers.

class Place(placeName: String) { }

Primary constructor cannot contain any code and Initialization code can be placed in initializer blocks.

class Employee(name: String) {    

init {        

logger.info("Employee initialized with value ${name}")    

}

}

I

f the constructor has annotations or visibility modifiers, the constructor{: .keyword } keyword is required, and the modifiers go before it:

class Employee public @Inject constructor(name: String) { ... }

 2)Secondary constructor :

Used inside class which are prefixed with constructor{: .keyword }:

class Employee {    

constructor(parent: Employee)

{        

parent.children.add(this)    

}

}

If the class has a primary constructor, each secondary constructor needs to delegate to the primary constructor, either directly or indirectly through another secondary constructor(s). Delegation to another constructor of the same class is done using the this{: .keyword } keyword:

class Employee(val name: String) {    

constructor(name: String, parent: Employee) : this(name)

{        

parent.children.add(this)    

}

}

3)Creating instances or Objects of classes:

Kotlin doesn not have “new” keyword for creating object. Objects are created as shown below:

val employee = Employee() 

val student = Student("Joe Smith")

Some examples on class and objects :

Example1 :

Java code :

class Student{ 

int id;//field or data member or instance variable  

String name;  

 public static void main(String args[]){  

  Student s1=new Student();//creating an object of Student  

  System.out.println(s1.id);//accessing member through reference variable  

  System.out.println(s1.name);  

 }  

} 

Kotlin Code :

class Student {
  var id:Int = 0//field or data member or instance variable 
  var name:String
  companion object {
 @JvmStatic fun main(args:Array<String>) {
      val s1 = Student()//creating an object of Student 
      println(s1.id)//accessing member through reference variable 
      println(s1.name)
    }
  }
}

Example 2 :

Java code :

class Rectangle{ 

 int length;  

 int width;  

 void insert(int l, int w){  

  length=l;  

  width=w;  

 }  

 void calculateArea(){System.out.println(length*width);}  

}  

class TestRectangle1{  

 public static void main(String args[]){  

  Rectangle r1=new Rectangle();  

  Rectangle r2=new Rectangle();  

  r1.insert(11,5);  

  r2.insert(3,15);  

  r1.calculateArea();  

  r2.calculateArea();  

}  

} 

Kotlin Code :

class Rectangle {

  var length:Int = 0

  var width:Int = 0

  fun insert(l:Int, w:Int) {

    length = l

    width = w

  }

  fun calculateArea() {

    println(length * width)

  }

}

internal object TestRectangle1 {

  @JvmStatic fun main(args:Array<String>) {

    val r1 = Rectangle()

    val r2 = Rectangle()

    r1.insert(11, 5)

    r2.insert(3, 15)

    r1.calculateArea()

    r2.calculateArea()

  }

}

Variables and Data Types in Kotlin :

In this tutorial we will learn how to declare variables in Kotlin. We are having java comparison code for easy and quick learning.

In kotlin all data types are defined with keyword var. There is no final 

Example 1 :

Java Code :

class A{  

private int data=50;
public String mData = "Test";
procted float test1 = 1.23;
private double test2 = 2.33; //instance variable  
public static final int FILES_TO_DOWNLOAD = 100 //static variable 

void method(){  
int n=90;//local variable  
}  

}//end of class 

Converted Kotlin Code : 

class A {

  private val data = 50
  var mData = "Test"
  var test1 = 1.23f
  private val test2 = 2.33//instance variable 

  fun method() {
    val n = 90//local variable 
  }

  companion object {
    val FILES_TO_DOWNLOAD = 100//static variable
  }

}//end of class

From Java To Kotlin - Your Cheat Sheet For Java To Kotlin

From Java To Kotlin - Your Cheat Sheet For Java To Kotlin

Java

System.out.print("Amit Shekhar");
System.out.println("Amit Shekhar");

Kotlin

print("Amit Shekhar")
println("Amit Shekhar")

---

Java

String name = "Amit Shekhar";
final String name = "Amit Shekhar";

Kotlin

var name = "Amit Shekhar"
val name = "Amit Shekhar"

---

Java

String otherName;
otherName = null;

Kotlin

var otherName : String?
otherName = null

---

Java

if (text != null) {
  int length = text.length();
}

Kotlin

text?.let {
    val length = text.length
}

---

Java

String firstName = "Amit";
String lastName = "Shekhar";
String message = "My name is: " + firstName + " " + lastName;

Kotlin

val firstName = "Amit"
val lastName = "Shekhar"
val message = "My name is: $firstName $lastName"

---

Java

String text = "First Line\n" +
              "Second Line\n" +
              "Third Line";

Kotlin

val text = """
        |First Line
        |Second Line
        |Third Line
        """.trimMargin()

---

Java

String text = x > 5 ? "x > 5" : "x <= 5";

Kotlin

val text = if (x > 5)
              "x > 5"
           else "x <= 5"

---

Java

if (object instanceof Car) {
}
Car car = (Car) object;

Kotlin

if (object is Car) {
}
var car = object as Car

---

Java

if (object instanceof Car) {
   Car car = (Car) object;
}

Kotlin

if (object is Car) {
   var car = object // smart casting
}

---

Java

if (score >= 0 && score <= 300) { }

Kotlin

if (score in 0..300) { }

---

Java

int score = // some score;
String grade;
switch (score) {
 case 10:
 case 9:
  grade = "Excellent";
  break;
 case 8:
 case 7:
 case 6:
  grade = "Good";
  break;
 case 5:
 case 4:
  grade = "Ok";
  break;
 case 3:
 case 2:
 case 1:
  grade = "Fail";
  break;
 default:
     grade = "Fail";    
}

Kotlin

var score = // some score
var grade = when (score) {
 9, 10 -> "Excellent" 
 in 6..8 -> "Good"
 4, 5 -> "Ok"
 in 1..3 -> "Fail"
 else -> "Fail"
}

---

Java

for (int i = 1; i <= 10 ; i++) { }

for (int i = 1; i < 10 ; i++) { }

for (int i = 10; i >= 0 ; i--) { }

for (int i = 1; i <= 10 ; i+=2) { }

for (int i = 10; i >= 0 ; i-=2) { }

for (String item : collection) { }

for (Map.Entry<String, String> entry: map.entrySet()) { }

Kotlin

for (i in 1..10) { }

for (i in 1 until 10) { }

for (i in 10 downTo 0) { }

for (i in 1..10 step 2) { }

for (i in 10 downTo 1 step 2) { }

for (item in collection) { }

for ((key, value) in map) { }

---

Java

final List<Integer> listOfNumber = Arrays.asList(1, 2, 3, 4);

final Map<Integer, String> keyValue = new HashMap<Integer, String>();
map.put(1, "Amit");
map.put(2, "Ali");
map.put(3, "Mindorks");

// Java 9
final List<Integer> listOfNumber = List.of(1, 2, 3, 4);

final Map<Integer, String> keyValue = Map.of(1, "Amit",
                                             2, "Ali",
                                             3, "Mindorks");

Kotlin

val listOfNumber = listOf(1, 2, 3, 4)
val keyValue = mapOf(1 to "Amit",
                     2 to "Ali",
                     3 to "Mindorks")

---

Java

// Java 7 and below
for (Car car : cars) {
  System.out.println(car.speed);
}

// Java 8+
cars.forEach(car -> System.out.println(car.speed));

// Java 7 and below
for (Car car : cars) {
  if (car.speed > 100) {
    System.out.println(car.speed);
  }
}

// Java 8+
cars.stream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed));

Kotlin

cars.forEach {
    println(it.speed)
}

cars.filter { it.speed > 100 }
      .forEach { println(it.speed)}

---

Java

void doSomething() {
   // logic here
}

Kotlin

fun doSomething() {
   // logic here
}

---

Java

void doSomething(int... numbers) {
   // logic here
}

Kotlin

fun doSomething(vararg numbers: Int) {
   // logic here
}

---

Java

int getScore() {
   // logic here
   return score;
}

Kotlin

fun getScore(): Int {
   // logic here
   return score
}

// as a single-expression function

fun getScore(): Int = score

---

Java

int getScore(int value) {
    // logic here
    return 2 * value;
}

Kotlin

fun getScore(value: Int): Int {
   // logic here
   return 2 * value
}

// as a single-expression function

fun getScore(value: Int): Int = 2 * value

---

Java

public class Utils {

    private Utils() { 
      // This utility class is not publicly instantiable 
    }
    
    public static int getScore(int value) {
        return 2 * value;
    }
    
}

Kotlin

class Utils private constructor() {

    companion object {
    
        fun getScore(value: Int): Int {
            return 2 * value
        }
        
    }
}

// other way is also there

object Utils {

    fun getScore(value: Int): Int {
        return 2 * value
    }

}

---

Java

public class Developer {

    private String name;
    private int age;

    public Developer(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Developer developer = (Developer) o;

        if (age != developer.age) return false;
        return name != null ? name.equals(developer.name) : developer.name == null;

    }

    @Override
    public int hashCode() {
        int result = name != null ? name.hashCode() : 0;
        result = 31 * result + age;
        return result;
    }

    @Override
    public String toString() {
        return "Developer{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

Kotlin

data class Developer(var name: String, var age: Int)

---

Java

public class Utils {

    private Utils() { 
      // This utility class is not publicly instantiable 
    }
    
    public static int triple(int value) {
        return 3 * value;
    }
    
}

int result = Utils.triple(3);

Kotlin

fun Int.triple(): Int {
  return this * 3
}

var result = 3.triple()