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From Java To Kotlin

by ahmedin Kotlin 0 Comment(s)

From Java To Kotlin – Your Cheat Sheet For Java To Kotlin.

Java


System.out.print("Ahmed Ben Fadhel");
System.out.println("Ahmed Ben Fadhel");

Kotlin


print("Ahmed Ben Fadhel")
println("Ahmed Ben Fadhel")

Java


String name = "Ahmed Ben Fadhel";
final String name = "Ahmed Ben Fadhel";

Kotlin


var name = "Ahmed Ben Fadhel"
val name = "Ahmed Ben Fadhel"

Java


String otherName;
otherName = null;

Kotlin


var otherName : String?
otherName = null

Java


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

Kotlin


text?.let {
    val length = text.length
}
// or simple
val length = text?.length

Java


String firstName = "Amit";
String lastName = "Shekhar";
String message = "My name is: " + firstName + " " + lastName;

Kotlin


val firstName = "Amit"
val lastName = "Shekhar"
val message = "My name is: $firstName $lastName"

Java


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

Kotlin


val text = """
        |First Line
        |Second Line
        |Third Line
        """.trimMargin()

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 ?: "")

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;

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

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?

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
}

Java


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

Kotlin


if (score in 0..300) { }

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"
	in 1..3 -> "Fail"
	else -> "Fail"
}

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 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 1 step 2) { }

for (item in collection) { }

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

Java


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

final Map keyValue = new HashMap();
map.put(1, "Amit");
map.put(2, "Ali");
map.put(3, "Mindorks");

// Java 9
final List listOfNumber = List.of(1, 2, 3, 4);

final Map 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")

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));

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)}

Java


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

Kotlin


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

Java


void doSomething() {
   // logic here
}

Kotlin


fun doSomething() {
   // logic here
}

Java


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

Kotlin


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

Java


int getScore() {
   // logic here
   return score;
}

Kotlin


fun getScore(): Int {
   // logic here
   return score
}

// as a single-expression function

fun getScore(): Int = score

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

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
        }
        
    }
}

// other way is also there

object Utils {

    fun getScore(value: Int): Int {
        return 2 * value
    }

}

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)

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)

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()

Java


Person person;

Kotlin


internal lateinit var person: Person

Source

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