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I am making a timer in Android using RxJava. I need to make a timer in RxJava to emit an observable every second. I have tried the following but with no luck. Any thoughts on what I am doing wrong?

Observable.interval(1000L, TimeUnit.MILLISECONDS)
          .timeInterval()
          .observeOn(AndroidSchedulers.mainThread())
          .subscribe({Log.d(LOG_TAG, "&&&& on timer") })
Your code seems not to be called. Check whether it is executed and when. As of working with Observable, it is completely correct. 
For example, I put your snippet inside onCreate(...) of my MainActivity:
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    Observable.interval(1000L, TimeUnit.MILLISECONDS)
            .timeInterval()
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe { Log.d("tag", "&&&& on timer") }
    // ...
And it works:
Also, probably you don't need .timeInterval() because Observable.interval(...) itself emits sequential numbers within the specified rate, and .timeInterval() just transforms it to emit the time intervals elapsed between the emissions.
                @KaranSharma , you can wrap it with disposable meaning you can write something like disposable = Observable.interval().... and then in onDestroy of Activity use disposable.dispose()
– kukroid
                Nov 27, 2019 at 2:39
In your subscribe() you don't consume the longTimeInterval object that's returned by the timeInterval() operator.
Correct version:
.subscribe(longTimeInterval -> {
     Log.d(LOG_TAG, "&&&& on timer"); 
Also I think you don't need the timeInterval() operator at all. Observable.interval() will emit an observable every second in your case, which I guess is what you want. timeInterval() transforms that to an observable that holds the exact time difference between two events occur, I doubt you'll need that.
                Kotlin doesn't require the lamda parameter declaration in case of single parameter, so that's not the problem. .subscribe { Log.d(...) } is correct RxJava usage in Kotlin.
– hotkey
                Feb 28, 2016 at 22:09
In Kotlin & RxJava 2.0.2 simply define an Observable Interval with an initialDelay 0 and period 1 which will emit list item each second. 
val list = IntRange(0, 9).toList()
val disposable = Observable
    .interval(0,1, TimeUnit.SECONDS)
    .map { i -> list[i.toInt()] }
    .take(list.size.toLong())
    .subscribe {
       println("item: $it")
Thread.sleep(11000)
        
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