What are the objects in Kotlin and how are they compiled?

Objects in Kotlin are used to create singletons. They are declared with the keyword object.

object Singleton {
    fun doSomething() {
        println("Hello from singleton!")
    }
}

When compiled, the Kotlin Singleton object is transformed into a Java class Singleton with a private static field INSTANCE and a static initialization block.

// Example of generated Java code
public final class Singleton {
   public static final Singleton INSTANCE; // Static field for the single instance

   public final void doSomething() { // Object method
      System.out.println("Hello from singleton!");
   }

   private Singleton() { // Private constructor to prevent external instantiation
   }

   static { // Static initialization block
      INSTANCE = new Singleton(); // Creating the single instance
   }
}

Calling the method Singleton.doSomething() in Kotlin results in the compiler generating a call to the static method Singleton.INSTANCE.doSomething() in Java.

Besides regular objects, Kotlin has:

• Companion object: Used to define static members inside a class. They are accessed via the class name. Each class can have at most one companion object.

  class MyClass {
      companion object {
          const val MY_CONSTANT = "some value"
          fun create(): MyClass {
              return MyClass()
          }
      }
  }
  

  Compiled into static members of the class in Java.

• Object expressions: Used to create anonymous objects (similar to anonymous classes in Java).

  val listener = object : MyInterface {
      override fun onClick() {
          println("Clicked!")
      }
  }
  

  Compiled into an anonymous class in Java.