Kotlin explain me about Backing Fields

Question

I have been looking at Kotlin official tutorial. I came across the topic called Backing Fields

It says,

Classes in Kotlin cannot have fields. However, sometimes it is necessary to have a backing field when using custom accessors. For these purposes, Kotlin provides an automatic backing field which can be accessed using the field identifier:

var counter = 0 // the initializer value is written directly to the backing field
    set(value) {
        if (value >= 0) field = value
    }

I got the above from this official link

My question is, is the "field" pointing to counter variable ?

Can someone please provide me an example for the backing field or describe me in an understanding word ?

Answer 1

Consider this class

class SomeClass {
    var counter: Int = 0
        set(value) {
            if (value >= 0) field = value
        }
}

In Android Studio go to Main menu -> Tools -> Kotlin -> Show Kotlin Bytecode and click Decompile in the Kotlin bytecode panel.

What you see is the equivalent code in Java.

public final class SomeClass {
   private int counter;

   public final int getCounter() {
      return this.counter;
   }

   public final void setCounter(int value) {
      if(value >= 0) {
         this.counter = value;
      }
   }
}