Advanced.java

package com.lithium.java8.chapter3;

import java.util.function.BiFunction;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Stream;

public class Advanced {

    public static void main(String[] args) {
        Stream.of(1, 2, 3, 10, 20)
              .reduce(Stream.empty(), filteringAccumulator(x -> x % 2 == 0), Stream::concat)
              .reduce(Stream.empty(), mappingAccumulator(x -> x * 2), Stream::concat)
              .forEach(System.out::println);
    }

    static <T>  BiFunction<Stream<T>, T, Stream<T>> filteringAccumulator(Predicate<T> filter) {
        return (stream, element) ->  {
            if (filter.test(element)) {
                return appendToStream(stream, element);
            } else {
                return stream;
            }
        };
    }

    static <T, R>  BiFunction<Stream<R>, T, Stream<R>> mappingAccumulator(Function<T, R> mapper) {
        return (stream, element) -> appendToStream(stream, mapper.apply(element));
    }

    static <R> Stream<R> appendToStream(Stream<R> stream, R element) {
        return Stream.concat(stream, Stream.of(element));
    }

}