Print to Console
Java
System.out.print("Amit Shekhar");
System.out.println("Amit Shekhar");
Kotlin
print("Amit Shekhar")
println("Amit Shekhar")
Constants and Variables
Java
String name = "Amit Shekhar";
final String name = "Amit Shekhar";
Kotlin
var name = "Amit Shekhar"
val name = "Amit Shekhar"
Assigning the null value
Java
String otherName;
otherName = null;
Kotlin
var otherName : String?
otherName = null
Verify if value is null
Java
if (text != null) {
int length = text.length();
}
Kotlin
text?.let {
val length = text.length
}
// or simply
val length = text?.length
Verify if value is NotNull OR NotEmpty
Java
String sampleString = "Shekhar";
if (!sampleString.isEmpty()) {
myTextView.setText(sampleString);
}
if(sampleString!=null && !sampleString.isEmpty()){
myTextView.setText(sampleString);
}
Kotlin
var sampleString ="Shekhar"
if(sampleString.isNotEmpty()){ //the feature of kotlin extension function
myTextView.text=sampleString
}
if(!sampleString.isNullOrEmpty()){
myTextView.text=sampleString
}
Concatenation of strings
Java
String firstName = "Amit";
String lastName = "Shekhar";
String message = "My name is: " + firstName + " " + lastName;
Kotlin
var firstName = "Amit"
var lastName = "Shekhar"
var message = "My name is: $firstName $lastName"
New line in string
Java
String text = "First Line\n" +
"Second Line\n" +
"Third Line";
Kotlin
val text = """
|First Line
|Second Line
|Third Line
""".trimMargin()
Substring
Java
String str = "Java to Kotlin Guide";
String substr = "";
//print java
substr = str.substring(0, 4);
System.out.println("substring = " + substr);
//print kotlin
substr = str.substring(8, 14);
System.out.println("substring = " + substr);
Kotlin
var str = "Java to Kotlin Guide"
var substr = ""
//print java
substr = str.substring(0..3) //
println("substring $substr")
//print kotlin
substr = str.substring(8..13)
println("substring $substr")
Ternary Operations
Java
String text = x > 5 ? "x > 5" : "x <= 5";
String message = null;
log(message != null ? message : "");
Kotlin
val text = if (x > 5) "x > 5" else "x <= 5"
val message: String? = null
log(message ?: "")
Bitwise Operators
Java
final int andResult = a & b;
final int orResult = a | b;
final int xorResult = a ^ b;
final int rightShift = a >> 2;
final int leftShift = a << 2;
final int unsignedRightShift = a >>> 2;
Kotlin
val andResult = a and b
val orResult = a or b
val xorResult = a xor b
val rightShift = a shr 2
val leftShift = a shl 2
val unsignedRightShift = a ushr 2
Check the type and casting
Java
if (object instanceof Car) {
Car car = (Car) object;
}
Kotlin
if (object is Car) {
var car = object as Car
}
// if object is null
var car = object as? Car // var car = object as Car?
Check the type and casting (implicit)
Java
if (object instanceof Car) {
Car car = (Car) object;
}
Kotlin
if (object is Car) {
var car = object // smart casting
}
// if object is null
if (object is Car?) {
var car = object // smart casting, car will be null
}
Multiple conditions
Java
if (score >= 0 && score <= 300)
Kotlin
if (score in 0..300)
Multiple Conditions (Switch case)
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"
else -> "Fail"
}
For-loops
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 0 step 2)
for (item in collection)
for ((key, value) in map)
Collections
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, "Anand");
map.put(3, "Messi");
// Java 9
final List<Integer> listOfNumber = List.of(1, 2, 3, 4);
final Map<Integer, String> keyValue = Map.of(1, "Amit",
2, "Anand",
3, "Messi");
Kotlin
val listOfNumber = listOf(1, 2, 3, 4)
val keyValue = mapOf(1 to "Amit",
2 to "Anand",
3 to "Messi")
for each
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));
cars.parallelStream().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)}
// kotlin 1.1+
cars.stream().filter { it.speed > 100 }.forEach { println(it.speed)}
cars.parallelStream().filter { it.speed > 100 }.forEach { println(it.speed)}
Splitting arrays
java
String[] splits = "param=car".split("=");
String param = splits[0];
String value = splits[1];
kotlin
val (param, value) = "param=car".split("=")
Defining methods
Java
void doSomething() {
// logic here
}
Kotlin
fun doSomething() {
// logic here
}
Default values for method parameters
Java
double calculateCost(int quantity, double pricePerItem) {
return pricePerItem * quantity;
}
double calculateCost(int quantity) {
// default price is 20.5
return 20.5 * quantity;
}
Kotlin
fun calculateCost(quantity: Int, pricePerItem: Double = 20.5) = quantity * pricePerItem
calculateCost(10, 25.0) // 250
calculateCost(10) // 205
Variable number of arguments
Java
void doSomething(int... numbers) {
// logic here
}
Kotlin
fun doSomething(vararg numbers: Int) {
// logic here
}
Defining methods with return
Java
int getScore() {
// logic here
return score;
}
Kotlin
fun getScore(): Int {
// logic here
return score
}
// as a single-expression function
fun getScore(): Int = score
// even simpler (type will be determined automatically)
fun getScore() = score // return-type is Int
Returning result of an operation
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
// even simpler (type will be determined automatically)
fun getScore(value: Int) = 2 * value // return-type is int
Constructors
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
}
}
}
// another way
object Utils {
fun getScore(value: Int): Int {
return 2 * value
}
}
Getters and Setters
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)
Cloning or copying
Java
public class Developer implements Cloneable {
private String name;
private int age;
public Developer(String name, int age) {
this.name = name;
this.age = age;
}
@Override
protected Object clone() throws CloneNotSupportedException {
return (Developer)super.clone();
}
}
// cloning or copying
Developer dev = new Developer("Messi", 30);
try {
Developer dev2 = (Developer) dev.clone();
} catch (CloneNotSupportedException e) {
// handle exception
}
Kotlin
data class Developer(var name: String, var age: Int)
// cloning or copying
val dev = Developer("Messi", 30)
val dev2 = dev.copy()
// in case you only want to copy selected properties
val dev2 = dev.copy(age = 25)
Class methods
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);
Generics
Java
// Example #1
interface SomeInterface<T> {
void doSomething(T data);
}
class SomeClass implements SomeInterface<String> {
@Override
public void doSomething(String data) {
// some logic
}
}
// Example #2
interface SomeInterface<T extends Collection<?>> {
void doSomething(T data);
}
class SomeClass implements SomeInterface<List<String>> {
@Override
public void doSomething(List<String> data) {
// some logic
}
}
interface SomeInterface<T> {
fun doSomething(data: T)
}
class SomeClass: SomeInterface<String> {
override fun doSomething(data: String) {
// some logic
}
}
interface SomeInterface<T: Collection<*>> {
fun doSomething(data: T)
}
class SomeClass: SomeInterface<List<String>> {
override fun doSomething(data: List<String>) {
// some logic
}
}
Kotlin
fun Int.triple(): Int {
return this * 3
}
var result = 3.triple()
Defining uninitialized objects
Java
Person person;
Kotlin
internal lateinit var person: Person
enum
Java
public enum Direction {
NORTH(1),
SOUTH(2),
WEST(3),
EAST(4);
int direction;
Direction(int direction) {
this.direction = direction;
}
public int getDirection() {
return direction;
}
}
Kotlin
enum class Direction(val direction: Int) {
NORTH(1),
SOUTH(2),
WEST(3),
EAST(4);
}
Sorting List
Java
List<Profile> profiles = loadProfiles(context);
Collections.sort(profiles, new Comparator<Profile>() {
@Override
public int compare(Profile profile1, Profile profile2) {
if (profile1.getAge() > profile2.getAge()) return 1;
if (profile1.getAge() < profile2.getAge()) return -1;
return 0;
}
});
Kotlin
val profile = loadProfiles(context)
profile.sortedWith(Comparator({ profile1, profile2 ->
if (profile1.age > profile2.age) return@Comparator 1
if (profile1.age < profile2.age) return@Comparator -1
return@Comparator 0
}))
Anonymous Class
Java
AsyncTask<Void, Void, Profile> task = new AsyncTask<Void, Void, Profile>() {
@Override
protected Profile doInBackground(Void... voids) {
// fetch profile from API or DB
return null;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
// do something
}
};
Kotlin
val task = object : AsyncTask<Void, Void, Profile>() {
override fun doInBackground(vararg voids: Void): Profile? {
// fetch profile from API or DB
return null
}
override fun onPreExecute() {
super.onPreExecute()
// do something
}
}
Initialization block
Java
public class User {
{ //Initialization block
System.out.println("Init block");
}
}
Kotlin
class User {
init { // Initialization block
println("Init block")
}
}
