通过枚举值从 ArrayList 中删除特定元素

Question

I have to remove elements from an ArrayList, but I could not make it work. I have to collect the oldest book name per each genre.

List<BookItem> list = new ArrayList<>();
list.add(new BookItem("TitleName1", 1950, "NOVEL"));
list.add(new BookItem("TitleName2", 1970, "HORROR"));
list.add(new BookItem("TitleName3", 1954, "NOVEL"));
list.add(new BookItem("TitleName4", 1989, "HORROR"));
list.add(new BookItem("TitleName5", 1990, "HUMOR"));
list.add(new BookItem("TitleName6", 1995, "HUMOR"));

For example, in case above the desired result would be "TitleName1,TitleName2,TitleName5".

I tried the following code to achieve that, but I did not receive the desired result. BookGenre is of enum type.

public List<BookItem> getOldestBookPerGenre(List<BookItem> bookItems){
    List<BookItem> genreList = new ArrayList<>();
    List<BookItem> returnList = new ArrayList<>();
    for(int i=0; i < BookGenre.values().length; i++){
        for(BookItem item: bookItems) {
            if (item.getBookGenre().ordinal() == i) {
                genreList.add(item);
            }
        }
        if (genreList.size() > 0) {
            genreList.sort(compareByYear);
            returnList.add(genreList.get(0));
        }
        genreList.clear();
    }
    return returnList;
}

Answer 1

To get the oldest element from the ArrayList you can use Java streams ( stream() , filter(...) , min(...) , etc).

It isn't exactly your script but you can easily see how the logic works and therefore adapt it with your code.

This example searchs for the oldest NOVEL book:

import java.util.*;
import java.util.Arrays;
import java.util.List;

public class test
{
    public static void main(String[] args)
    {
        List<BookItem> list = new ArrayList<>();
        list.add(new BookItem("TitleName1", 1950, "NOVEL"));
        list.add(new BookItem("TitleName2", 1970, "HORROR"));
        list.add(new BookItem("TitleName3", 1954, "NOVEL"));
        list.add(new BookItem("TitleName4", 1989, "HORROR"));
        list.add(new BookItem("TitleName5", 1990, "HUMOR"));
        list.add(new BookItem("TitleName6", 1995, "HUMOR"));

        BookItem oldestNovel = list.stream()
            .filter(element -> element.genre.equals("NOVEL"))
            .min((el1, el2) -> { if(el1.year == el2.year) return 0; return el1.year < el2.year ? -1 : 1; })
            .get();

        System.out.println(oldestNovel.title);
    }

    static class BookItem
    {
        String title = "";
        int year = 0;
        String genre = "";
        public BookItem(String title, int year, String genre)
        {
            this.title = title;
            this.year  = year;  
            this.genre = genre;
        }
    }
}

Output:

TitleName1

Answer 2

It is easy with java stream, but as you say you can't use Java8 here you have another solution:

public List<BookItem> getOldestBookPerGenre(List<BookItem> bookItems) {
    Map<String, BookItem> oldestBookByGenre = new HashMap<>();

    for (BookItem bookItem : bookItems) {
        oldestBookByGenre.putIfAbsent(bookItem.getGenre(), bookItem);
        if (Integer.parseInt(bookItem.getYear()) < Integer.parseInt(oldestBookByGenre.get(bookItem.getGenre()).getYear())) {
            oldestBookByGenre.put(bookItem.getGenre(), bookItem);
        }
    }

    return new ArrayList<>(oldestBookByGenre.values());
}

Output list, given the question input:

BookItem{title='TitleName2', year='1970', novel='HORROR'}
BookItem{title='TitleName1', year='1950', novel='NOVEL'}
BookItem{title='TitleName5', year='1990', novel='HUMOR'}