This tutorial will share the main similarities and differences between Kotlin and Java. I hope you can use your existing Java knowledge to get started with Kotlin development quickly.

Print log

Java

System.out.print("hello world");
System.out.println("hello world");
Copy the code

Kotlin

print("hello world")
println("hello world")
Copy the code

Define variables and constants

Java

String name = "hello world";
final String name = "hello world";
Copy the code

Kotlin

var name = "hello world"
val name = "hello world"
Copy the code

A null statement

Java

String otherName;
otherName = null;
Copy the code

Kotlin

var otherName : String?
otherName = null
Copy the code

Empty judgment

Java

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

Kotlin

text? .let { val length = text.length } // or simply val length = text? .lengthCopy the code

String splicing

Java

String firstName = "Android";
String lastName = "Architect";
String message = "My name is: " + firstName + "" + lastName;
Copy the code

Kotlin

val firstName = "Android"
val lastName = "Architect"
val message = "My name is: $firstName $lastName"
Copy the code

A newline

Java

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

Kotlin

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

Ternary expression

Java

String text = x > 5 ? "x > 5" : "x <= 5";
Copy the code

Kotlin

val text = if (x > 5)
              "x > 5"
           else "x <= 5"
Copy the code

The operator

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;
Copy the code

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
Copy the code

Type determination and conversion (declarative)

Java

Car car = (Car) object;
Copy the code

Kotlin

var car = object as Car
Copy the code

Type judgment and conversion (implicit)

Java

if (object instanceof Car) {
   Car car = (Car) object;
}
Copy the code

Kotlin

if(object is Car) {var Car = objectCopy the code

Multiple conditions

Java

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

Kotlin

if (score in 0..300) { }
Copy the code

More flexible case statements

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";
}
Copy the code

Kotlin

var score = // some score
var grade = when (score) {
    9, 10 -> "Excellent"
    in6.. 8 - >"Good"4, 5 - >"OK"
    in1.. 3 - >"Fail"
    else -> "Fail"
}
Copy the code

The for loop

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()) { }
Copy the code

Kotlin

for (i in 1..10) { }

for (i in 1 until 10) { }

for (i in 10 downTo 0) { }

for (i in1.. 10 step 2) { }for (i in 10 downTo 0 step 2) { }

for (item in collection) { }

for ((key, value) in map) { }
Copy the code

More convenient collection operation

Java

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

final Map<Integer, String> keyValue = new HashMap<Integer, String>();
map.put(1, "Android");
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, "Android",
                                             2, "Ali",
                                             3, "Mindorks");
Copy the code

Kotlin

val listOfNumber = listOf(1, 2, 3, 4)
val keyValue = mapOf(1 to "Android",
                     2 to "Ali",
                     3 to "Mindorks")
Copy the code

traverse

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));
Copy the code

Kotlin

cars.forEach {
    println(it.speed)
}

cars.filter { it.speed > 100 }
      .forEach { println(it.speed)}
Copy the code

Method definition

Java

void doSomething() {
   // logic here
}

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

Kotlin

fun doSomething() {
   // logic here
}

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

A method with a return value

Java

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

Kotlin

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

// as a single-expression function

fun getScore(): Int = score
Copy the code

Termineless symbol

Java

int getScore(int value) {
    // logic here
    return 2 * value;
}
Copy the code

Kotlin

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

// as a single-expression function

fun getScore(value: Int): Int = 2 * value
Copy the code

The constructor constructor

Java

public class Utils {

    private Utils() {
      // This utility class is not publicly instantiable
    }

    public static int getScore(int value) {
        return2 * value; }}Copy the code

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
    }

}
Copy the code

Get Set constructor

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; }}Copy the code

Kotlin

data class Developer(val name: String, val age: Int)Copy the code