java11-collectors-collectingAndThen

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

• Reference: Exploring Collectors by Venkat Subramaniam

preface

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

project description

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("")
           ));