Java8 - オブジェクトへのmapper適用

以下の例はJava8のmapを用いて，

1. カスタムオブジェクトItemのマスタリスト中から興味のある品だけを取り出し
2. 更に10000円以上のものだけを選び
3. かつ価格を安いものから昇順に並べる

例である．

package org.tanuneko;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Reducer {

    public static void main( String args[] ) {

        // since this is lazy evaluation, WareHouse::findItembyId is not called for all entries in item list held by WareHouse
        List<Integer> interests = Arrays.asList( 1,2,3,4,5,6 );
        List<Item> items = interests.stream().map(WareHouse::findItembyId)
                            .filter((x)->x.getPrice()>10000)
                            .sorted( (x,y)->x.getPrice() - y.getPrice() )
                            .collect(Collectors.toList());
        items.stream().forEach(System.out::println);
 
     }

}

class WareHouse {

    static final List<Item> items = genItems();

    public static Item findItembyId( int id )  {
        // better to use orElse()
        return items.stream().filter(x->x.getId()==id).findFirst().get();

    }

    private static List<Item> genItems() {
        List<Item> items = new ArrayList<>();
        items.add( new Item( 1, 500, "Basket" ) );
        items.add( new Item( 2,1200, "Rack" ) );
        items.add( new Item( 3, 700, "Small Table" ) );
        items.add( new Item( 4, 50000, "Granite Table" ) );
        items.add( new Item( 5, 15000, "Closet" ) );
        items.add( new Item( 6, 400, "Grout" ) );
        items.add( new Item( 7, 28000, "Small Couch" ) );
        return items;
    }
}

Items.java

package org.tanuneko;

public class Item {

    private int id;
    private int price;
    private String name;

    public Item( int id, int price, String name ) {
        this.id = id;
        this.price = price;
        this.name = name;
    }

    public int getId() { return id; }
    public int getPrice() { return price; }
    public String getName() { return name; }

    @Override
    public String toString() {
        return "Item{" +
                "id=" + id +
                ", price=" + price +
                ", name='" + name + '\'' +
                '}';
    }
}

Item{id=5, price=15000, name='Closet'}
Item{id=4, price=50000, name='Granite Table'}