使用JDK 8轻松进行细粒度排序

Java的8的推出流和有用的静态 / 默认的方法比较接口可以很容易地根据个人的领域两个对象比较“值，而不需要实现一个比较（T，T）在其对象的类方法被比较。

我将使用一个简单的Song类来帮助演示这一点，接下来显示其Song.java代码清单。

Song.java

package dustin.examples.jdk8;/*** Simple class encapsulating details related to a song* and intended to be used for demonstration of JDK 8.*/
public class Song
{/** Song title. */private final String title;/** Album on which song was originally included. */private final String album;/** Song's artist. */private final String artist;/** Year song was released. */private final int year;/*** Constructor accepting this instance's title, artist, and release year.** @param newTitle Title of song.* @param newAlbum Album on which song was originally included.* @param newArtist Artist behind this song.* @param newYear Year song was released.*/public Song(final String newTitle, final String newAlbum,final String newArtist, final int newYear){title = newTitle;album = newAlbum;artist = newArtist;year = newYear;}public String getTitle(){return title;}public String getAlbum(){return album;}public String getArtist(){return artist;}public int getYear(){return year;}@Overridepublic String toString(){return "'" + title + "' (" + year + ") from '" + album + "' by " + artist;}
}

刚刚显示了清单的Song类缺少一个compare方法，但是我们仍然可以很容易地在JDK 8中比较该类的实例。根据刚刚显示的Song的类定义，以下代码可用于根据歌曲发行年份，歌手和专辑的顺序对歌曲实例List进行排序。

按年份，艺术家和专辑对歌曲排序（按此顺序）

/*** Returns a sorted version of the provided List of Songs that is* sorted first by year of song's release, then sorted by artist,* and then sorted by album.** @param songsToSort Songs to be sorted.* @return Songs sorted, in this order, by year, artist, and album.*/
private static List<Song> sortedSongsByYearArtistAlbum(final List<Song> songsToSort)
{return songsToSort.stream().sorted(Comparator.comparingInt(Song::getYear).thenComparing(Song::getArtist).thenComparing(Song::getAlbum)).collect(Collectors.toList());
}

如果我静态地导入 Comparator和Collectors ，则上面的代码清单将稍微冗长一些，但是将这些接口和类名称包括在清单中仍然很简洁，对于该主题的入门博客文章可能更有用。

在上面的代码清单中， static default方法Comparator.comparingInt和Comparator.thenComparing用于按年份，然后是艺术家，最后是唱片，对与基础List关联的Song流进行排序。该代码具有很高的可读性，并且可以基于任意单独的访问器方法进行对象比较（以及对这些实例进行排序），而无需显式指定的Comparator （用于每个比较的访问器结果的自然排序顺序）。请注意，如果需要显式Comparator ，则可以通过接受Comparator的同名重载方法将其提供给这些static default方法。

下一个代码清单是整个演示类。它包括刚刚显示的方法，还显示了由未排序的歌曲List构成的人为示例。

FineGrainSortingDemo.java

package dustin.examples.jdk8;import static java.lang.System.out;import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;/*** Demonstration of easy fine-grained sorting in JDK 8 via* stream support for sorting and Comparator's static and* default method implementations.*/
public class FineGrainSortingDemo
{/*** Construct List of {@code Song}s.* * @return Instances of {@code Song}.*/private static List<Song> generateSongs(){final ArrayList<Song> songs = new ArrayList<>();songs.add(new Song("Photograph","Pyromania","Def Leppard",1983));songs.add(new Song("Hysteria","Hysteria","Def Leppard",1987));songs.add(new Song("Shout","Songs from the Big Chair","Tears for Fears",1984));songs.add(new Song("Everybody Wants to Rule the World","Songs from the Big Chair","Tears for Fears",1985));songs.add(new Song("Head Over Heels","Songs from the Big Chair","Tears for Fears",1985));songs.add(new Song("Enter Sandman","Metallica","Metallica",1991));songs.add(new Song("Money for Nothing","Brothers in Arms","Dire Straits",1985));songs.add(new Song("Don't You (Forget About Me)","A Brass Band in African Chimes","Simple Minds",1985));return songs;}/*** Returns a sorted version of the provided List of Songs that is* sorted first by year of song's release, then sorted by artist,* and then sorted by album.** @param songsToSort Songs to be sorted.* @return Songs sorted, in this order, by year, artist, and album.*/private static List<Song> sortedSongsByYearArtistAlbum(final List<Song> songsToSort){return songsToSort.stream().sorted(Comparator.comparingInt(Song::getYear).thenComparing(Song::getArtist).thenComparing(Song::getAlbum)).collect(Collectors.toList());}/*** Demonstrate fine-grained sorting in JDK 8.** @param arguments Command-line arguments; none expected.*/public static void main(final String[] arguments){final List<Song> songs = generateSongs();final List<Song> sortedSongs = sortedSongsByYearArtistAlbum(songs);out.println("Original Songs:");songs.stream().forEach(song -> out.println("\t" + song));out.println("Sorted Songs");sortedSongs.forEach(song -> out.println("\t" + song));}
}

接下来显示运行上述代码的输出，并在使用排序代码后列出新排序的Song 。值得注意的是，此stream.sorted()操作不会更改原始List （它作用于流而不是List ）。

Original Songs:'Photograph' (1983) from 'Pyromania' by Def Leppard'Hysteria' (1987) from 'Hysteria' by Def Leppard'Shout' (1984) from 'Songs from the Big Chair' by Tears for Fears'Everybody Wants to Rule the World' (1985) from 'Songs from the Big Chair' by Tears for Fears'Head Over Heels' (1985) from 'Songs from the Big Chair' by Tears for Fears'Enter Sandman' (1991) from 'Metallica' by Metallica'Money for Nothing' (1985) from 'Brothers in Arms' by Dire Straits'Don't You (Forget About Me)' (1985) from 'A Brass Band in African Chimes' by Simple Minds
Sorted Songs'Photograph' (1983) from 'Pyromania' by Def Leppard'Shout' (1984) from 'Songs from the Big Chair' by Tears for Fears'Money for Nothing' (1985) from 'Brothers in Arms' by Dire Straits'Don't You (Forget About Me)' (1985) from 'A Brass Band in African Chimes' by Simple Minds'Everybody Wants to Rule the World' (1985) from 'Songs from the Big Chair' by Tears for Fears'Head Over Heels' (1985) from 'Songs from the Big Chair' by Tears for Fears'Hysteria' (1987) from 'Hysteria' by Def Leppard'Enter Sandman' (1991) from 'Metallica' by Metallica

JDK 8在接口中引入了流以及默认方法和静态方法（在这种情况下，尤其是在Comparator上），可以轻松地按期望的顺序逐个字段地比较两个对象，而无需使用任何显式的Comparator而是预先构建了static default方法。 Comparator界面（如果要比较的字段具有所需的自然顺序）。