Cheat Sheet for Java to Kotlin

[Mengsreang-Chhoeung]

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, "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")

---

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("Mindorks", 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("Mindorks", 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")
        }
    }

---

Important things to know in Kotlin

• What is the equivalent of Java static methods in Kotlin?
• What is the difference between "const" and "val"?
• Learn Kotlin - lateinit vs lazy
• Learn Kotlin - apply vs with
• Learn Kotlin - Data Class
• Learn Kotlin - Destructuring Declarations
• Learn Kotlin - Extension Functions
• Learn Kotlin - Sealed Classes
• Understanding Higher-Order Functions and Lambdas in Kotlin
• Understanding inline, noinline, and crossinline in Kotlin
• Mastering Kotlin Coroutines In Android - Step By Step Guide
• Using Scoped Functions in Kotlin - let, run, with, also, apply
• What are Reified Types in Kotlin?

For more details, please check out this fork:

👉 From Java To Kotlin