使用 lambda 表达式的枚举覆盖方法

Question

我有两个解决方案来让枚举形成一个比较器库：

public enum SongComparator {

    BY_TITLE(Comparator.comparing(Song::getTitle)),
    BY_ARTIST(Comparator.comparing(Song::getArtist)),
    BY_DURATION(Comparator.comparing(Song::getDuration));

    private Comparator<Song> comp;

    private SongComparator(Comparator<Song> comp) {
        this.comp = comp;
    }

    public Comparator<Song> get() {
        return comp;
    }
}

还有……

public enum SongComparator2 implements Comparator<Song> {

    BY_TITLE {
        public int compare(Song s1, Song s2) {
            return s1.getTitle().compareTo(s2.getTitle());
        };
    },
    BY_ARTIST {
        public int compare(Song s1, Song s2) {
            return s1.getArtist().compareTo(s2.getArtist());
        };
    },
    BY_DURATION {
        public int compare(Song s1, Song s2) {
            return Integer.compare(s1.getDuration(), s2.getDuration());
        };
    };

}

如果我想在需要 Comparator 的地方替换一个枚举值，在第一个解决方案中我必须说 SongComparator.BY_TITLE.get();而在第二个解决方案中，我只能说 SongComparator2.BY_TITLE。

从这个意义上说，第二种方法更好，但是，我不喜欢为每个枚举值编写 public int compare... 等，而是希望像第一种方法一样使用 Comparator.comparing。有没有办法做到这一点？

我几乎想说：BY_TITLE { return Comparator.comparing(Song::getTitle); };

Answer 1

有什么问题：

public enum SongComparator implements Comparator<Song> {

    BY_TITLE(Comparator.comparing(Song::getTitle)),
    BY_ARTIST(Comparator.comparing(Song::getArtist)),
    BY_DURATION(Comparator.comparing(Song::getDuration));

    private Comparator<Song> comp;

    private SongComparator(Comparator<Song> comp) {
        this.comp = comp;
    }

    public int compare(Song s1, Song s2) {
        return comp.compare(s1, s2);
    };

}