While working with filtering operators and/or working on complex requirements, it may occur that we encounter an empty producer (see the following code block):

    fun main(args: Array<String>) { 
      Observable.range(0,10)//(1) 
       .filter{it>15}//(2) 
       .subscribe({ 
         println("Received $it") 
      }) 
    } 

Here, on comment (1), we will create Observable of range 0 to 10; however, on comment (2), we will filter it for emission value >15. So, basically, we will end up with an empty Observable.

The defaultIfEmpty operator helps us deal with such situations. The preceding example, with defaultIfEmpty looks like this:

    fun main(args: Array<String>) { 
      Observable.range(0,10)//(1) 
       .filter{it>15}//(2) 
       .defaultIfEmpty(15)//(3) 
       .subscribe({ 
           println("Received $it") 
        }) 
    } 

This is the same program, but, just on comment (3), we added the defaultIfEmpty operator.

The output looks like the following screenshot:

The output shows that, although Observable doesn't contain any number above 10, defaultIfEmpty adds 15 to the Observable as it's empty after filtering.