Flux represents an asynchronous sequence of 0 to n items.

We will again borrow a diagram from the Project Reactor documentation, which explains how a Flux object emits items:

The preceding diagram illustrates the following process:

• At least six values have been produced
• An operator is applied to the produced values
• The result is delivered

In the following example, we will first convert each produced value to uppercase, in order to deliver the values:

@Test
public void givenAListOfCapitalizedStrings_WhenThe
FlatMapConvertsToUpperCaseTheStrings_ThenTheStringsAre
InUpperCase() throws Exception 
{
  List<String> namesCapitalized = Arrays.asList("John", 
  "Steve", "Rene");
  Iterator<String> namesCapitalizedIterator = namesCapitalized.
  iterator();
  Flux<String> fluxWithNamesCapitalized = Flux.fromIterable
  (namesCapitalized);
  Flux<String> fluxWithNamesInUpperCase = fluxWithNamesCapitalized
  .map(name -> name.toUpperCase());
  fluxWithNamesInUpperCase.subscribe 
  (
    nameInUpperCase -> 
    {
      String expectedString =namesCapitalizedIterator.
      next().toUpperCase();                
      Assert.assertEquals(expectedString, nameInUpperCase);
    }
  );
}