From Java To Kotlin#

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

Print to Console#

Java

1
System.out.print("Hoang Cao Luu");
2
System.out.println("Hoang Cao Luu");

Kotlin

1
print("Hoang Cao Luu")
2
println("Hoang Cao Luu")

Constants and Variables#

Java

1
String name = "Hoang Cao Luu";
2
final String name = "Hoang Cao Luu";

Kotlin

1
var name = "Hoang Cao Luu"
2
val name = "Hoang Cao Luu"

Assigning the null value#

Java

1
List<Student> students = null;
2

3
String otherName;
4
otherName = null;

Kotlin

1
var students : List<Student>? = null
2

3
var otherName : String?
4
otherName = null

Verify if value is null#

Java

1
if (text != null) {
2
  int length = text.length();
3
}

Kotlin

1
text?.let {
2
    val length = text.length
3
}
4
// or simply
5
val length = text?.length

Verify if value is NotNull OR NotEmpty#

Java

1
String sampleString = "Shekhar";
2

3
if (!sampleString.isEmpty()) {
4
    myTextView.setText(sampleString);
5
}
6
if(sampleString!=null && !sampleString.isEmpty()){
7
    myTextView.setText(sampleString);
8
}

Java 11 with isBlank: returns true if the string is empty or contains only white space codepoints.

1
String sampleString = "Shekhar";
2

3
if (!sampleString.isBlank()) {
4
    myTextView.setText(sampleString);
5
}

Kotlin

1
var sampleString ="Shekhar"
2
if(sampleString.isNotEmpty()){  //the feature of kotlin extension function
3
    myTextView.text=sampleString
4
}
5
if(!sampleString.isNullOrEmpty()){
6
   myTextView.text=sampleString
7
}

Concatenation of strings#

Java

1
String firstName = "Amit";
2
String lastName = "Shekhar";
3
String message = "My name is: " + firstName + " " + lastName;
4

5
//Use string builder for better performance
6
String message = new StringBuilder("My name is: ")
7
    .append(firstName)
8
    .append(" ")
9
    .append(lastName)
10
    .toString();

Kotlin

1
var firstName = "Amit"
2
var lastName = "Shekhar"
3
var message = "My name is: $firstName $lastName"
4

5
//Or Use string builder for better performance
6
var message = StringBuilder("My name is: ")
7
    .append(firstName)
8
    .append(" ")
9
    .append(lastName)
10
    .toString()

New line in string#

Java

1
String text = "First Line\n" +
2
              "Second Line\n" +
3
              "Third Line";

Java 13 with Text Blocks

1
String text = """
2
        First Line
3
        Second Line
4
        Third Line
5
        """;

Kotlin

1
val text = """
2
        |First Line
3
        |Second Line
4
        |Third Line
5
        """.trimMargin()

Substring#

Java

1
String str = "Java to Kotlin Guide";
2
String substr = "";
3

4
//print java
5
substr = str.substring(0, 4);
6
System.out.println("substring = " + substr);
7

8
//print kotlin
9
substr = str.substring(8, 14);
10
System.out.println("substring = " + substr);

Kotlin

1
var str = "Java to Kotlin Guide"
2
var substr = ""
3

4
//print java
5
substr = str.substring(0..3) //
6
println("substring $substr")
7

8
//print kotlin
9
substr = str.substring(8..13)
10
println("substring $substr")

Ternary Operations#

Java

1
String text = x > 5 ? "x > 5" : "x <= 5";
2

3
String message = null;
4
log(message != null ? message : "");

Kotlin

1
val text = if (x > 5) "x > 5" else "x <= 5"
2

3
val message: String? = null
4
log(message ?: "")

Bitwise Operators#

Java

1
final int andResult  = a & b;
2
final int orResult   = a | b;
3
final int xorResult  = a ^ b;
4
final int rightShift = a >> 2;
5
final int leftShift  = a << 2;
6
final int unsignedRightShift = a >>> 2;

Kotlin

1
val andResult  = a and b
2
val orResult   = a or b
3
val xorResult  = a xor b
4
val rightShift = a shr 2
5
val leftShift  = a shl 2
6
val unsignedRightShift = a ushr 2

Check the type and casting#

Java

1
if (object instanceof Car) {
2
  Car car = (Car) object;
3
}

Kotlin

1
if (object is Car) {
2
var car = object as Car
3
}
4

5
// if object is null
6
var car = object as? Car // var car = object as Car?

Check the type and casting (implicit)#

Java

1
if (object instanceof Car) {
2
   Car car = (Car) object;
3
}

Kotlin

1
if (object is Car) {
2
   var car = object // smart casting
3
}
4

5
// if object is null
6
if (object is Car?) {
7
   var car = object // smart casting, car will be null
8
}

Multiple conditions#

Java

1
if (score >= 0 && score <= 300) { }

Kotlin

1
if (score in 0..300) { }

Multiple Conditions (Switch case)#

Java

1
int score = // some score;
2
String grade;
3
switch (score) {
4
  case 10:
5
  case 9:
6
    grade = "Excellent";
7
    break;
8
  case 8:
9
  case 7:
10
  case 6:
11
    grade = "Good";
12
    break;
13
  case 5:
14
  case 4:
15
    grade = "OK";
16
    break;
17
  case 3:
18
  case 2:
19
  case 1:
20
    grade = "Fail";
21
    break;
22
  default:
23
      grade = "Fail";
24
}

Kotlin

1
var score = // some score
2
var grade = when (score) {
3
  9, 10 -> "Excellent"
4
  in 6..8 -> "Good"
5
  4, 5 -> "OK"
6
  else -> "Fail"
7
}

For-loops#

Java

1
for (int i = 1; i <= 10 ; i++) { }
2

3
for (int i = 1; i < 10 ; i++) { }
4

5
for (int i = 10; i >= 0 ; i--) { }
6

7
for (int i = 1; i <= 10 ; i+=2) { }
8

9
for (int i = 10; i >= 0 ; i-=2) { }
10

11
for (String item : collection) { }
12

13
for (Map.Entry<String, String> entry: map.entrySet()) { }

Kotlin

1
for (i in 1..10) { }
2

3
for (i in 1 until 10) { }
4

5
for (i in 10 downTo 0) { }
6

7
for (i in 1..10 step 2) { }
8

9
for (i in 10 downTo 0 step 2) { }
10

11
for (item in collection) { }
12

13
for ((key, value) in map) { }

Collections#

Java

1
final List<Integer> listOfNumber = Arrays.asList(1, 2, 3, 4);
2

3
final Map<Integer, String> keyValue = new HashMap<Integer, String>();
4
map.put(1, "Amit");
5
map.put(2, "Anand");
6
map.put(3, "Messi");
7

8
// Java 9
9
final List<Integer> listOfNumber = List.of(1, 2, 3, 4);
10

11
final Map<Integer, String> keyValue = Map.of(1, "Amit",
12
                                             2, "Anand",
13
                                             3, "Messi");

Kotlin

1
val listOfNumber = listOf(1, 2, 3, 4)
2
val keyValue = mapOf(1 to "Amit",
3
                     2 to "Anand",
4
                     3 to "Messi")

for each#

Java

1
// Java 7 and below
2
for (Car car : cars) {
3
  System.out.println(car.speed);
4
}
5

6
// Java 8+
7
cars.forEach(car -> System.out.println(car.speed));
8

9
// Java 7 and below
10
for (Car car : cars) {
11
  if (car.speed > 100) {
12
    System.out.println(car.speed);
13
  }
14
}
15

16
// Java 8+
17
cars.stream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed));
18
cars.parallelStream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed));

Kotlin

1
cars.forEach {
2
    println(it.speed)
3
}
4

5
cars.filter { it.speed > 100 }
6
      .forEach { println(it.speed)}
7

8
// kotlin 1.1+
9
cars.stream().filter { it.speed > 100 }.forEach { println(it.speed)}
10
cars.parallelStream().filter { it.speed > 100 }.forEach { println(it.speed)}

Splitting arrays#

java

1
String[] splits = "param=car".split("=");
2
String param = splits[0];
3
String value = splits[1];

kotlin

1
val (param, value) = "param=car".split("=")

Defining methods#

Java

1
void doSomething() {
2
   // logic here
3
}

Kotlin

1
fun doSomething() {
2
   // logic here
3
}

Default values for method parameters#

Java

1
double calculateCost(int quantity, double pricePerItem) {
2
    return pricePerItem * quantity;
3
}
4

5
double calculateCost(int quantity) {
6
    // default price is 20.5
7
    return 20.5 * quantity;
8
}

Kotlin

1
fun calculateCost(quantity: Int, pricePerItem: Double = 20.5) = quantity * pricePerItem
2

3
calculateCost(10, 25.0) // 250
4
calculateCost(10) // 205

Variable number of arguments#

Java

1
void doSomething(int... numbers) {
2
   // logic here
3
}

Kotlin

1
fun doSomething(vararg numbers: Int) {
2
   // logic here
3
}

Defining methods with return#

Java

1
int getScore() {
2
   // logic here
3
   return score;
4
}

Kotlin

1
fun getScore(): Int {
2
   // logic here
3
   return score
4
}
5

6
// as a single-expression function
7

8
fun getScore(): Int = score
9

10
// even simpler (type will be determined automatically)
11

12
fun getScore() = score // return-type is Int

Single-expression functions#

Document #

Returning result of an operation

Java

1
int getScore(int value) {
2
    // logic here
3
    return 2 * value;
4
}

Kotlin

1
fun getScore(value: Int): Int {
2
   // logic here
3
   return 2 * value
4
}
5

6
// as a single-expression function
7
fun getScore(value: Int): Int = 2 * value
8

9
// even simpler (type will be determined automatically)
10

11
fun getScore(value: Int) = 2 * value // return-type is int

1
@PostMapping("/login")
2
fun login(
3
    @RequestBody @Valid userLoginDTO: AuthPort.UserLoginDTO
4
): ResponseEntity<ApiResponse<AuthPort.LoginResponse>>
5
    = ResponseEntity.ok(
6
        ApiResponse.builder<AuthPort.LoginResponse>()
7
            .message(localizationUtils.getLocalizedMessage(MessageKey.LOGIN_SUCCESSFULLY))
8
            .statusCode(HttpStatus.OK.value())
9
            .isSuccess(true)
10
            .data(
11
                authService.login(
12
                    userLoginDTO.email,
13
                    userLoginDTO.password
14
                )
15
            )
16
            .build()
17
    );

Constructors#

Java

1
public class Utils {
2

3
    private Utils() {
4
      // This utility class is not publicly instantiable
5
    }
6

7
    public static int getScore(int value) {
8
        return 2 * value;
9
    }
10

11
}

Kotlin

1
class Utils private constructor() {
2

3
    companion object {
4

5
        fun getScore(value: Int): Int {
6
            return 2 * value
7
        }
8

9
    }
10
}
11

12
// another way
13

14
object Utils {
15

16
    fun getScore(value: Int): Int {
17
        return 2 * value
18
    }
19

20
}

Getters and Setters#

Java

1
public class Developer {
2

3
    private String name;
4
    private int age;
5

6
    public Developer(String name, int age) {
7
        this.name = name;
8
        this.age = age;
9
    }
10

11
    public String getName() {
12
        return name;
13
    }
14

15
    public void setName(String name) {
16
        this.name = name;
17
    }
18

19
    public int getAge() {
20
        return age;
21
    }
22

23
    public void setAge(int age) {
24
        this.age = age;
25
    }
26

27
    @Override
28
    public boolean equals(Object o) {
29
        if (this == o) return true;
30
        if (o == null || getClass() != o.getClass()) return false;
31

32
        Developer developer = (Developer) o;
33

34
        if (age != developer.age) return false;
35
        return name != null ? name.equals(developer.name) : developer.name == null;
36

37
    }
38

39
    @Override
40
    public int hashCode() {
41
        int result = name != null ? name.hashCode() : 0;
42
        result = 31 * result + age;
43
        return result;
44
    }
45

46
    @Override
47
    public String toString() {
48
        return "Developer{" +
49
                "name='" + name + '\'' +
50
                ", age=" + age +
51
                '}';
52
    }
53
}

With Lombok annotation

1
@Data
2
@AllArgsConstructor
3
@EqualsAndHashCode
4
@ToString
5
public class Developer {
6
    private String name;
7
    private int age;
8
}

Kotlin

1
data class Developer(var name: String, var age: Int)

Abstract class#

Java

1
@Data
2
@AllArgsConstructor
3
public abstract class Animal {
4
    private String name;
5
    public abstract void makeSound();
6
}

Kotlin

1
abstract class Animal(var name: String) {
2

3
    abstract fun makeSound()
4

5
}

Inheritance (from abstract class)#

Java

1
@Data
2
@AllArgsConstructor
3
public abstract class Animal {
4
    protected String name;
5
    public abstract void makeSound();
6
}
7

8
public class Dog extends Animal {
9

10
    public Dog(String name) {
11
        super(name);
12
    }
13

14
    @Override
15
    public void makeSound() {
16
        System.out.println("Bark");
17
    }
18
}
19

20
// Avoid to override the method
21
public abstract class Fish extends Animal{
22
    private int weight;
23
}

Kotlin

1
abstract class Animal(protected var name: String) {
2

3
    abstract fun makeSound()
4

5
}
6

7
class Dog(name: String) : Animal(name) {
8

9
    override fun makeSound() {
10
        println("Bark")
11
    }
12

13
}
14

15
// Avoid to override the method
16
abstract class Fish(name: String, var weight: Int) : Animal(name)

Cloning or copying#

Java

1
public class Developer implements Cloneable {
2

3
    private String name;
4
    private int age;
5

6
    public Developer(String name, int age) {
7
        this.name = name;
8
        this.age = age;
9
    }
10

11
    @Override
12
    protected Object clone() throws CloneNotSupportedException {
13
        return (Developer)super.clone();
14
    }
15
}
16

17
// cloning or copying
18
Developer dev = new Developer("Messi", 30);
19
try {
20
    Developer dev2 = (Developer) dev.clone();
21
} catch (CloneNotSupportedException e) {
22
    // handle exception
23
}

Kotlin

1
data class Developer(var name: String, var age: Int)
2

3
// cloning or copying
4
val dev = Developer("Messi", 30)
5
val dev2 = dev.copy()
6
// in case you only want to copy selected properties
7
val dev2 = dev.copy(age = 25)

Generics#

Java

1
// Example #1
2
interface SomeInterface<T> {
3
    void doSomething(T data);
4
}
5

6
class SomeClass implements SomeInterface<String> {
7
    @Override
8
    public void doSomething(String data) {
9
        // some logic
10
    }
11
}
12

13
// Example #2
14
interface SomeInterface<T extends Collection<?>> {
15
    void doSomething(T data);
16
}
17

18
class SomeClass implements SomeInterface<List<String>> {
19

20
    @Override
21
    public void doSomething(List<String> data) {
22
        // some logic
23
    }
24
}

Kotlin

1
interface SomeInterface<T> {
2
    fun doSomething(data: T)
3
}
4

5
class SomeClass: SomeInterface<String> {
6
    override fun doSomething(data: String) {
7
        // some logic
8
    }
9
}
10

11
interface SomeInterface<T: Collection<*>> {
12
    fun doSomething(data: T)
13
}
14

15
class SomeClass: SomeInterface<List<String>> {
16
    override fun doSomething(data: List<String>) {
17
        // some logic
18
    }
19
}

Extension function#

Java

1
public class Utils {
2

3
    private Utils() {
4
      // This utility class is not publicly instantiable
5
    }
6

7
    public static int triple(int value) {
8
        return 3 * value;
9
    }
10

11
}
12

13
int result = Utils.triple(3);

Kotlin

1
fun Int.triple(): Int {
2
  return this * 3
3
}
4

5
var result = 3.triple()

Defining uninitialized objects#

Java

1
Person person;

Kotlin

1
internal lateinit var person: Person

Enum#

Java

1
public enum Direction {
2
    NORTH(1),
3
    SOUTH(2),
4
    WEST(3),
5
    EAST(4);
6

7
    int direction;
8

9
    Direction(int direction) {
10
        this.direction = direction;
11
    }
12

13
    public int getDirection() {
14
        return direction;
15
    }
16
}

With Lombok annotation

1
@Getter
2
@AllArgsConstructor
3
public enum Direction {
4
    NORTH(1),
5
    SOUTH(2),
6
    WEST(3),
7
    EAST(4);
8

9
    private final int direction;
10
}

Kotlin

1
enum class Direction(val direction: Int) {
2
    NORTH(1),
3
    SOUTH(2),
4
    WEST(3),
5
    EAST(4);
6
}

Sorting List#

Java

1
List<Profile> profiles = loadProfiles(context);
2
Collections.sort(profiles, new Comparator<Profile>() {
3
    @Override
4
    public int compare(Profile profile1, Profile profile2) {
5
        if (profile1.getAge() > profile2.getAge()) return 1;
6
        if (profile1.getAge() < profile2.getAge()) return -1;
7
        return 0;
8
    }
9
});

Kotlin

1
val profile = loadProfiles(context)
2
profile.sortedWith(Comparator({ profile1, profile2 ->
3
    if (profile1.age > profile2.age) return@Comparator 1
4
    if (profile1.age < profile2.age) return@Comparator -1
5
    return@Comparator 0
6
}))

Anonymous Class#

Java

1
 AsyncTask<Void, Void, Profile> task = new AsyncTask<Void, Void, Profile>() {
2
    @Override
3
    protected Profile doInBackground(Void... voids) {
4
        // fetch profile from API or DB
5
        return null;
6
    }
7

8
    @Override
9
    protected void onPreExecute() {
10
        super.onPreExecute();
11
        // do something
12
    }
13
};

Kotlin

1
val task = object : AsyncTask<Void, Void, Profile>() {
2
    override fun doInBackground(vararg voids: Void): Profile? {
3
        // fetch profile from API or DB
4
        return null
5
    }
6

7
    override fun onPreExecute() {
8
        super.onPreExecute()
9
        // do something
10
    }
11
}

Initialization block#

Java

1
public class User {
2
    {  //Initialization block
3
        System.out.println("Init block");
4
    }
5
}

Kotlin

1
class User {
2
    init { // Initialization block
3
        println("Init block")
4
    }
5
}

Record#

Java

1
public interface AuthPort {
2
    record UserLoginDTO (
3
        @JsonProperty("email") @Email(message = "Email is not valid") @NotBlank(message = "Email is required")
4
        String email,
5

6
        @JsonProperty("password") @NotBlank(message = "Password is required")
7
        String password
8
    ){}
9
}

Kotlin

1
interface AuthPort {
2
    data class UserLoginDTO (
3
        @JsonProperty("email") @Email(message = "Email is not valid") @NotBlank(message = "Email is required")
4
        val email: String,
5

6
        @JsonProperty("password") @NotBlank(message = "Password is required")
7
        val password: String
8
    )
9
}

From Java To Kotlin#

Print to Console#

Constants and Variables#

Assigning the null value#

Verify if value is null#

Verify if value is NotNull OR NotEmpty#

Concatenation of strings#

New line in string#

Substring#

Ternary Operations#

Bitwise Operators#

Check the type and casting#

Check the type and casting (implicit)#

Multiple conditions#

Multiple Conditions (Switch case)#

For-loops#

Collections#

for each#

Splitting arrays#

Defining methods#

Default values for method parameters#

Variable number of arguments#

Defining methods with return#

Single-expression functions#

Document#

Constructors#

Getters and Setters#

Abstract class#

Inheritance (from abstract class)#

Cloning or copying#

Generics#

Extension function#

Defining uninitialized objects#

Enum#

Sorting List#

Anonymous Class#

Initialization block#

Record#

Document #