distinctUntilChanged()

The distinctUntilChanged() function will ignore duplicate consecutive emissions. It is a helpful way to ignore repetitions until they change. If the same value is being emitted repeatedly, all the duplicates will be ignored until a new value is emitted. Duplicates of the next value will be ignored until it changes again, and so on. Observe the output for the following code to see this behavior in action:

import io.reactivex.Observable;

public class Launcher {
      public static void main(String[] args) {

        Observable.just(1, 1, 1, 2, 2, 3, 3, 2, 1, 1)
          .distinctUntilChanged()
          .subscribe(i -> System.out.println("RECEIVED: " + i));
      }
}

The output of the preceding code snippet is as follows:

RECEIVED: 1
RECEIVED: 2
RECEIVED: 3
RECEIVED: 2
RECEIVED: 1

We first receive an emission of 1, which is allowed forward. But the next two 1 are ignored because they are consecutive duplicates. When it switches to 2, that initial 2 is emitted, but the following duplicate is ignored. A 3 is emitted and its following duplicate is ignored as well. Finally, we switch back to a 2 that emits and then a 1 whose duplicate is ignored.

Just like distinct(), you can provide an optional argument for a key through a lambda mapping. In the following code snippet, we execute the distinctUntilChanged() operation with strings keyed on their lengths:

import io.reactivex.Observable;

public class Launcher {
      public static void main(String[] args) {

        Observable.just("Alpha", "Beta", "Zeta", "Eta", "Gamma", 
"Delta")
          .distinctUntilChanged(String::length)
          .subscribe(i -> System.out.println("RECEIVED: " + i));
      }
}

The output of the preceding code snippet is as follows:

RECEIVED: Alpha
RECEIVED: Beta
RECEIVED: Eta
RECEIVED: Gamma

Note that Zeta was skipped because it comes right after Beta, which also is four characters. Delta is ignored as well because it follows Gamma, which is five characters as well.