So, now that we are somewhat familiar with observables, let's modify the even-odd program in a reactive way. Here is the code for doing so:

    fun main(args: Array<String>) { 
      var subject:Subject<Int> = PublishSubject.create() 
 
      subject.map({ isEven(it) }).subscribe({println
      ("The number is ${(if (it) "Even" else "Odd")}" )}) 
 
      subject.onNext(4) 
      subject.onNext(9) 
    } 

Here is the output:

In this program, we have used subject and map, which we will cover in the later chapters. Here, it is just to show how easy it is in reactive programming to notify the changes. If you look at the program closely, then you'll also find that the code is modular and functional. When we notify subject with a number, it calls the method in map, then it calls the method in subscribe with the return value of the map method. The map method checks if the number is even and returns true or false accordingly; in the subscribe method, we are receiving that value and printing even or odd accordingly. The subject.onNext method is the way through which we message the new value to the subject, so it can process it.