Kotlin for Java Developers

From Java To Kotlin 

Java
System.out.print("Umer");
System.out.println("Umer");

Kotlin
print("Umer")
println("Umer")

Java
String name = "Umer";
final String name = "Umer";

Kotlin
var name = "Umer"
val name = "Umer"

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 = "Umer";
String lastName = "farooq";
String message = "My name is: " + firstName + " " + lastName;
Kotlin
val firstName = "Umer"
val lastName = "farooq"
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, "Umer");
map.put(2, "Ali");
map.put(3, "usman");

// 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 "Umer",
                     2 to "Ali",
                     3 to "usman")

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()