Explain the Java code generated from using @JvmOverloads on Kotlin method

Question

I tried using @JvmOverloads to created overloaded methods for Kotlin function. Just out of curiosity I tried to look at the bytecode and decompiled Java code. But, I could not make much sense of the Java code.

Here, is a simple Kotlin code

class MyClass {

        @JvmOverloads
        fun doSomething(a : Int = 200, b : Int = 300){

        }
    }

Below is the decompiled java code :

public final class MyClass {
   @JvmOverloads
   public final void doSomething(int a, int b) {
   }

   // $FF: synthetic method
   // $FF: bridge method
   @JvmOverloads
   public static void doSomething$default(MyClass var0, int var1, int var2, int var3, Object var4) {
      if((var3 & 1) != 0) { // What does this do ?
         var1 = 200;
      }

      if((var3 & 2) != 0) { //Where did var3 come from ? 
         var2 = 300;
      }

      var0.doSomething(var1, var2);
   }

   @JvmOverloads
   public final void doSomething(int a) {
      doSomething$default(this, a, 0, 2, (Object)null);
   }

   @JvmOverloads
   public final void doSomething() {
      doSomething$default(this, 0, 0, 3, (Object)null);
   }
}

Please explain or point me to the right resources which could help me learn more about the same. Thanks.

Answer 1

var3 is a bit mask indicating which parameters need to be substituted with default values. If bit 0 is set, the first parameter ( var1 , or a in the source code) is substituted with the default value 200. If bit 1 is set, var2 takes the default value 300. The 1-parameter overload passes 2 as the bit mask (so only bit 1 is set), and the 0-parameter overload passes 3 (bits 0 and 1 are set).