Kotlin == Java++

1. Hello world

Java

public class HelloWorldApp {

    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

Kotlin

fun main() {
    println("Hello World!")
}

2. Variables

a) Declaration

Java

    public final double PI = 3.14;
    public final double CALUCLATED_PI = calculatePi();

    public void type() {
        String stringValue = "Value";
        final String STRING_CONSTANT = "Const value";

        var value = "Value";
        final var CONSTANT = "Const value";
    }

    private double calculatePi() {
        return 3.14;
    }

Kotlin

const val PI = 3.14
val CALCULATED_PI = calculatePi()

fun type() {
    var stringValue: String = "Value"
    val STRING_CONSTANT: String = "Const value"

    var value = "Value"
    val CONSTANT = "Const value"

    // value = 128 <- error
}

fun calculatePi(): Double {
    return 3.14
}

b) Optional

Java

Optional.ofNullable(name)
    .map(String::toUpperCase)
    .orElse("UNKNOWN");

Kotlin

name?.toUpperCase ?: "UNKNOWN"

3. Function

a) Single-expression functions

Java

Not implemented

String changeMyName(String name) {
    return Optional.ofNullable(name).map(String::toUpperCase).orElse("UNKNOWN");
}

Kotlin

fun changeMyName(name: String?) = name?.toUpperCase() ?: "UNKNOWN"

b) Default arguments

Java

Not implemented

changeMyName("Patryk");
changeMyName("Patryk", "My default value");

...

String changeMyName(String name, String defaultName) {
    return Optional.ofNullable(name)
            .map(String::toUpperCase)
            .orElse(defaultName);
}

String changeMyName(String name) {
    return getMyName(name, "UNKNOWN");
}

Kotlin

changeMyName("Patryk")
changeMyName("Patryk", "My default value")

...

fun changeMyName(name: String?, defaultName: String = "UNKNOWN") = name?.toUpperCase() ?: defaultName

c) Named arguments

Java

Not implemented

Kotlin

changeMyName(defaultName = "My default value", name = "Patryk")

...

fun changeMyName(name: String?, defaultName: String = "UNKNOWN") = name?.toUpperCase() ?: defaultName

d) Extension function

Java

Not implemented

StringUtil.myCapitalize("patryk");

...

class StringUtil {

    static String myCapitalize(String text) {
        return text.substring(0, 1).toUpperCase() + text.substring(1);
    }
}

Kotlin

"patryk".myCapitalize()

...

fun String.myCapitalize() = this.substring(0, 1).toUpperCase() + this.substring(1)

e) Infix notation

Java

Not implemented

Kotlin

val date = 2 JANUARY 2018

...

infix fun Int.JANUARY(year: Int) = Date(year - 1900, Calendar.JANUARY, this)

f) Method names with spaces

Java

Not implemented

Kotlin

@Test
fun `Should return true if the user is active`() {
//    ...
}

3. Class

a) Data class

Java

public class Person {

    private String name;
    private String surname;

    public Person(String name, String surname) {
        this.name = name;
        this.surname = surname;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getSurname() {
        return surname;
    }

    public void setSurname(String surname) {
        this.surname = surname;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Person person = (Person) o;
        return Objects.equals(name, person.name) &&
                Objects.equals(surname, person.surname);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, surname);
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", surname='" + surname + '\'' +
                '}';
    }

}

Kotlin

data class Person(val name: String, val surname: String)

b) Singleton

Java

public class SynchronizedSingleton {

    private static SynchronizedSingleton instance = null;

    private SynchronizedSingleton() {
    }

    private synchronized static void createInstance() {
        if (instance == null) {
            instance = new SynchronizedSingleton();
        }
    }

    public static SynchronizedSingleton getInstance() {
        if (instance == null) {
            createInstance();
        }
        return instance;
    }

}

Kotlin

object SynchronizedSingleton {

}

4. String

Java

String string = name + " " + surname;

Kotlin

val string = "$name $surname"

5. Delegate

a) Lazy

Java

String calculatedValue = null;

...

String getCalculatedValue() {
    if (calculatedValue == null) {
        System.put.prinltn("in progress ...";)
        calculatedValue = takeFromDatabase(); 
    }
    return calculatedValue;
}

Kotlin

val calculatedValue: Int by lazy {
    println("in progress ...")
    takeFromDatabase()
}

b) Observable

Java

Integer observable = null;

...

void setObservable(Integer n) {
    System.out.println("Old: " + observable + " new: " + n);
    observable = n;
}

Kotlin

var observable: Int by observable(0) { property, oldValue, newValue ->
    println("Old: $oldValue and new: $newValue")
}

5. Range

Java

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

Kotlin

for (i in 1..5) {
    println(i)
}

6. When

Java

Not implemented

Kotlin

fun showType(objectToCheck: Any): String {
    return when (objectToCheck) {
        0 -> "Number: zero"
        in 10..99 -> "Two digits number"
        is String -> "String: ${objectToCheck.length}" // smart casting
        else -> "Unknown type"
    }
}

7. Collection

Java

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 7);
List<Integer> evenNumbers = list.stream()
        .filter(number -> number % 2 == 0)
        .collect(Collectors.toList());

Kotlin

val list = listOf(1, 2, 3, 4, 5, 7)
val evenNumbers = list.filter { it % 2 == 0 }