Java8 - 関数の合成 1 .. andThen

Java8のラムダは関数の合成に対する操作も用意されている．
そのうちの一つがandThen()である．以下の例は引換券のIDを入力として本のタイトルを返す．

package org.tanuneko;

import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import static java.util.stream.Collectors.*;

public class AndThenSample {

    private static List<ClaimTicket> tickets;

    private static void init() {

        tickets = new ArrayList<ClaimTicket>();
        tickets.add( new ClaimTicket( 1, new Book( Janre.Sports, "All about Payton Manning") ) );
        tickets.add( new ClaimTicket( 2, new Book( Janre.Sports, "Andy - how to win at playoff") ) );
        tickets.add( new ClaimTicket( 3, new Book( Janre.Education, "ABC") ) );
        tickets.add( new ClaimTicket( 101, new Book( Janre.Education, "Waldorf education - how it works") ) );

    }

    public static void main( String args[] ) {

        init();
        Function<ClaimTicket,Book> getBookByClaimId = ClaimTicket::getBook;
        Function<Book,String> getTitle = Book::getTitle;
        Function<ClaimTicket,String> getTitleOfClaimedBook = getBookByClaimId.andThen(getTitle);
        System.out.println( tickets.stream().filter(x->x.getId()<100)
                .map(getTitleOfClaimedBook)
                .collect( Collectors.reducing((x,y)-> x + "," + y))
                .toString() );
    }

}

enum Janre {
    Education,
    Sports
}

class Book {

    private Janre janre;
    private String title;

    public Book( Janre janre, String title ) {
        this.janre = janre;
        this.title = title;
    }

    public Janre getJanre() { return janre; }
    public String getTitle() { return title; }
}

class ClaimTicket {

    private int id;
    private Book book;

    public ClaimTicket( final int id, final Book book ) {
        this.id = id;
        this.book = book;
    }

    public int getId() { return id; }
    public Book getBook() { return book; }
}