Options to flatMap a list of Optionals in Java8

flatmap_optionals.java

// Ways to transform List<Optional<T>> to List<T> (excluding empty ones)

// 1. Using filter()
List<String> filteredList = listOfOptionals.stream()
  .filter(Optional::isPresent)
  .map(Optional::get)
  .collect(Collectors.toList());

// 2. Using flatMap()
List<String> filteredList = listOfOptionals.stream()
  .flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty())
  .collect(Collectors.toList());

// 3. Java 9's Opional::stream
List<String> filteredList = listOfOptionals.stream()
  .flatMap(Optional::stream)
  .collect(Collectors.toList());

// 4. Create a custom collector (my favorate in Java 8 if you need to do this a lot)

// PresentCollector.java
package com.example.Collector;

import java.util.ArrayList;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collector;

public final class PresentCollector {
    public static <T> Collector<Optional<T>, List<T>, List<T>> toList() {
        return Collector.of(ArrayList::new, PresentCollector::accumulate, PresentCollector::combine);
    }

    private static <T> void accumulate(List<T> a, Optional<T> v) {
        v.ifPresent(a::add);
    }

    private static <T> List<T> combine(List<T> x, List<T> y) {
        x.addAll(y);
        return x;
    }

    private PresentCollector() {
    }
}

// Usage
List<String> filteredList = listOfOptionals.collect(PresentCollector.toList());