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From Java To Kotlin

From Java To Kotlin - Your Cheat Sheet For Java To Kotlin

About me

Hi, I am Amit Shekhar, Co-Founder @ Outcome School • IIT 2010-14 • I have taught and mentored many developers, and their efforts landed them high-paying tech jobs, helped many tech companies in solving their unique problems, and created many open-source libraries being used by top companies. I am passionate about sharing knowledge through open-source, blogs, and videos.

You can connect with me on:

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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)

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
    }
}

Kotlin

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
    }
}

Extension function

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()

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

Mastering Kotlin Coroutines - Mastering Kotlin Coroutines
Dispatchers in Kotlin Coroutines - Dispatchers in Kotlin Coroutines
coroutineScope vs supervisorScope - coroutineScope vs supervisorScope
CoroutineContext in Kotlin - CoroutineContext in Kotlin
What is Flow API in Kotlin? - What is Flow API in Kotlin?
Long-running tasks in parallel with Kotlin Flow - Long-running tasks in parallel with Kotlin Flow
Retry Operator in Kotlin Flow - Retry Operator in Kotlin Flow
Callback to Coroutines in Kotlin - Callback to Coroutines in Kotlin
Retrofit with Kotlin Flow - Retrofit with Kotlin Flow
Room Database with Kotlin Flow - Room Database with Kotlin Flow
Remove duplicates from an array - Remove duplicates from an array in Kotlin
JvmStatic Annotation in Kotlin - JvmStatic Annotation in Kotlin
JvmOverloads Annotation in Kotlin - JvmOverloads Annotation in Kotlin
JvmField Annotation in Kotlin - JvmField Annotation in Kotlin
inline function in Kotlin - inline function in Kotlin
noinline in Kotlin - noinline in Kotlin
crossinline in Kotlin - crossinline in Kotlin
lateinit vs lazy in Kotlin - lateinit vs lazy in Kotlin
init block in Kotlin - init block in Kotlin
Retrofit with Kotlin Coroutines - Retrofit with Kotlin Coroutines
Advantage of using const in Kotlin - Advantage of using const in Kotlin
AssociateBy - List to Map in Kotlin - Kotlin Collection Functions - associateBy that converts a list into a map
partition - filtering function in Kotlin - partition - filtering function in Kotlin
Infix notation in Kotlin - Infix notation in Kotlin
Open keyword in Kotlin - Open keyword in Kotlin
Companion object in Kotlin - Companion object in Kotlin
Extension function in Kotlin - Extension function in Kotlin
data class in Kotlin - data class in Kotlin
How does the Kotlin Multiplatform work? - How does the Kotlin Multiplatform work?

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License

   Copyright (C) 2024 Amit Shekhar

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.

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