From Java to Kotlin
CLASSES
Constructor Call
Java
final File file = new File("file.txt");
Kotlin
val file = File("file.txt")
Class
Java
public final class User {
}
Kotlin
class User
Open Class
Java
public class User {
}
Kotlin
open class User
Final Attributes
Java
final class User {
     private final String name;

     public User(String name) {
         this.name = name;
     }

     public String getName() {
         return name;
     }
 }
Kotlin
class User(val name: String)
Primary Constructor
Java
final class User {
     private String name;

     public User(String name) {
         this.name = name;
     }

     public String getName() {
         return name;
     }

     public void setName(String name) {
         this.name = name;
     }
 }
Kotlin
class User(var name: String)
Optional Arguments in Constructors
Java
final class User {
     private String name;
     private String lastName;

     public User(String name) {
         this(name, "");
     }

     public User(String name, String lastName) {
         this.name = name;
         this.lastName = lastName;
     }

     // And Getters & Setters
 }
Kotlin
class User(var name: String, var lastName: String = "")
Properties
Java
public class Document {
    private String id = "00x";

     public String getId() {
         return id;
     }

     public void setId(String id) {
         if(id != null && !id.isEmpty()) {
             this.id = id;
         }
     }
 }
Kotlin
class Document{
    var id : String = "00x"
        set(value) {
            if(value.isNotEmpty())  field = value
        }
}
FUNCTIONS
Abstract Class
Java
public abstract class Document{
   public abstract int calculateSize();
}

public class Photo extends Document{
    @Override
    public int calculateSize() {

    }
}
Kotlin
abstract class Document {
    abstract fun calculateSize(): Int
}

class Photo : Document() {
    override fun calculateSize(): Int {

    }
}
Singleton
Java
public class Document {
   private static final Document INSTANCE = new Document();

   public static Document getInstance(){
       return INSTANCE;
   }

 }
 
Kotlin
object Document {

}
Extensions
Java
public class ByteArrayUtils {
      public static String toHexString(byte[] data) {

      }
  }

  final byte[] dummyData = new byte[10];
  final String hexValue = ByteArrayUtils.toHexString(dummyData);
Kotlin
fun ByteArray.toHex() : String {

}

val dummyData = byteArrayOf()
val hexValue = dummyData.toHex()
Inner Class
Java
public class Documment {

    class InnerClass {

    }

}
Kotlin
class Document {
    internal inner class InnerClass
}
Nested Class
Java
public class Documment {

    public static class InnerClass {

    }

}
Kotlin
class Document {

    class InnerClass

}
Interface
Java
public interface Printable {
    void print();
}

public class Document implements Printable {
    @Override
    public void print() {

    }
}
Kotlin
interface Printable{
    fun print()
}

class Document : Printable{
    override fun print() {

    }
}