Kotlin is a statically typed programming language that runs on Java virtual machine and also can be compiled to JavaScript source code. It is designed to interoperate with Java code, meaning Kotlin code can be called from Java and Java code from Kotlin.
Variables
Java
final int x; final int y = 1; int z = 2;
Kotlin
val x: Int val y = 1 var z = 2
Nullable Property
Java
final String name = null;
String lastName;
lastName = null
Kotlin
val name: String? = null
var lastName: String?
lastName = null
var firstName: String
// Compilation error!!
firstName = null
Null Safety
Java
if (name != null) {
int length = name.length
}Kotlin
val length = name?.length
Multi Line String
Java
String address = "FirstLine\n" +
"Second Line\n" +
"Third Line";
Kotlin
val address = """
|FirstLine
|SecondLine
|ThirdLine
""".trimMargin()
Ternary Operator
Java
String result = x > 5 ? "x > 5" : "x<=5";
Kotlin
val result = if (x > 5)
"x > 5"
else "x <=5"
Switch/When
Java
final int x = 3;
final String result;
switch (x) {
case 0:
case 11:
result = "0 or 11"
break;
case 1:
case 2:
//...
case 10:
result = " from 1 to 10";
break;
default
result = " not within range 0-11"Kotlin
val x = 3
val result = when (x) {
0,11 -> "0 or 11"
in 1..10 -> " from 1 to 10"
else -> "not within range 0-11"
}
Collections
Java
final List<Integer> numbers = Arrays.asList(1, 2, 3);
final Map<Integer, String> map = new HashMap<Integer, String>();
map.put(1, "One");
map.put(2, "Two);
map.put(3, "Three);
Kotlin
val numbers = listof(1, 2, 3)
val map = mapOf(1 to "One",
2 to "Two",
3 to "Three")
Functions
Vararg
Java
public int sum(int... numbers ) {}Kotlin
fun sum(vararg x: It) {}Main
Java
public class MyClass {
public static void main(String[] args) {
}
}Kotlin
fun main(args: Array<String>){
}Optional Arguments
Java
openFile("file.txt", true);
public static File openFile(String filename, boolean readonly);Kotlin
openFile("file.txt")
fun openFile(filename: String, readOnly: Boolean = true)Classes
Constructor Call
Java
final User user = new User("Tom");Kotlin
val user = User("Tom")Final Class
Java
public final class User {
}Kotlin
class User
Open Class
Java
public class User {
}Kotlin
open class User
Final Attributes
Java
final class User {
private final String name;
public User(String name) {
this.name = name;
}
public String getName() {
return name;
}
}Kotlin
class User(val name: String)
Primary Constructor
Java
final class User {
private String name;
public User(String name) {
this.name = name;
}
public String getName() {
return name;
} public void setName(String name) {
this.name = name;
}
}
Kotlin
class User(var name: String)
Optional arguments in Constructors
Java
final class User {
private String name;
private String lastName;
public User(String name) {
this(name, "");
} public User(String name, String lastName) {
this.name = name;
this.lastName = lastName;
}
// Getter and setters
}
Kotlin
class User(var name: String, var lastName: String = "")
Properties
Java
public class User {
private String id = "00x";
public String getId() {
return id;
} public void setId(String id) {
if(id !=null && !id.isEmpty()){
this.id = id;
}
}
}
Kotlin
class User {
var id: String = "00x"
set(value) {
if(value.isNotEmpty()) field = value
}
}Abstract Class
Java
public abstract class Animal {
public abstract void sound();
}
public class Dog extends Animal {
@override
public void sound() {
System.out.println("Woof");
}
}
Kotlin
abstract class Animal {
abstract fun sound(): Unit
}
class Dog : Animal() {
override fun sound(): Unit {
println("Woof")
}
}Singleton
Java
public class Animal {
private static final Animal instance = new Animal();
public static Animal getInstance(){
return instance;
}
}Kotlin
object Animal {
}Extensions
Java
public class ByteArrayUtils {
public static String toHexString(byte[] data) {
}
}
final byte[] dummyData = new byte[10];
final String hexValue = ByteArrayUtils.toHexString(dummyData);Kotlin
fun ByteArray.toHex() : String {
}
val dummyData = byteArrayOf()
val hexValue = dummyData.toHex()Interface
Java
public interface Animal {
void eat();
}
public class Dog implements Animal {
@Override
public void eat() {
}
}Kotlin
interface Animal {
fun eat()
}
class Dog : Animal {
override fun eat() {
}
}