The main goal of this project is to show how to use collectingAndThen from collectors API.

• Reference: Exploring Collectors by Venkat Subramaniam

public static<T,A,R,RR> 
Collector<T,A,RR> 
collectingAndThen(Collector<T,A,R> downstream,
                  Function<R,RR> finisher)

• <T>: the type of the input elements
• <A>: intermediate accumulation type of the downstream collector
• <R>: result type of the downstream collector
• <RR>: result type of the resulting collector
• downstream - a collector
• finisher - a function to be applied to the final result of the downstream collector
• return - a collector which performs the action of the downstream collector, followed by an additional finishing step

All tests are in CollectingAndThenTest class

1. collect stream values to ImmutableList from guava

   ImmutableList<Integer> collect = stream
               .collect(Collectors.collectingAndThen(
                   Collectors.toList(), 
                   ImmutableList::copyOf));    
   

2. in a stream of integers find a max positive value or return -1

   Integer max = stream
           .filter(x -> x >= 0)
           .collect(Collectors.collectingAndThen(
                   Collectors.maxBy(Comparator.naturalOrder()),
                   x -> x.orElse(-1)));    
   

3. collect stream to list and check if result list is not empty

   Boolean isEmpty = stream
                     // other operations
                     .collect(Collectors.collectingAndThen(
                             Collectors.toList(),
                             List::isEmpty)));    
   

4. find name of the oldest person

   String nameOfTheOldest = persons.stream()
           .collect(Collectors.collectingAndThen(
                   Collectors.maxBy(Comparator.comparing(Person::getAge)),
                   x -> x.map(Person::getName).orElse("")
           ));