startWith()

For a given Observable<T>, the startWith() operator allows you to insert a T emission that precedes all the other emissions. For instance, if we have an Observable<String>that emits items on a menu we want to print, we can use startWith() to append a title header first:

import io.reactivex.Observable;

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

        Observable<String> menu =
          Observable.just("Coffee", "Tea", "Espresso", "Latte");

        //print menu
        menu.startWith("COFFEE SHOP MENU")
          .subscribe(System.out::println);

      }
}

The output of the preceding code snippet is as follows:

COFFEE SHOP MENU
Coffee
Tea
Espresso
Latte

If you want to start with more than one emission, use startWithArray() to accept varargs parameters. If we want to add a divider between our header and menu items, we can start with both the header and divider as emissions:

import io.reactivex.Observable;

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

        Observable<String> menu =
          Observable.just("Coffee", "Tea", "Espresso", "Latte");

        //print menu
        menu.startWithArray("COFFEE SHOP MENU","----------------")
          .subscribe(System.out::println);

     }
}

The output of the preceding code snippet is as follows:

COFFEE SHOP MENU
----------------
Coffee
Tea
Espresso
Latte

The startWith() operator is helpful for cases like this, where we want to seed an initial value or precede our emissions with one or more emissions.