java-> 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

final String name = null;
String otherName;
otherName = null;

Kotlin

val name : String? = null
var otherName : String?
otherName = null

Java

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

Kotlin

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+=2) { }

for (String item : collection) { }

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

Kotlin

for (i in 1..10) { }

for (i in 1..10 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");

Kotlin

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

Java

for (Car car : cars) {
  System.out.println(car.speed);
}

for (Car car : cars) {
  if(cars.speed > 100) {
    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

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

Kotlin

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

Java

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

Kotlin

fun getScore(value: Int): Int {
   // logic here
   return 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(val name: String, val 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()