Java 8 中的List排序
- 按字母顺序排序字符串列表
List cities = Arrays.asList(
"Milan",
"london",
"San Francisco",
"Tokyo",
"New Delhi"
);
System.out.println(cities);
//[Milan, london, San Francisco, Tokyo, New Delhi]
cities.sort(String.CASE_INSENSITIVE_ORDER);
System.out.println(cities);
//[london, Milan, New Delhi, San Francisco, Tokyo]
cities.sort(Comparator.naturalOrder());
System.out.println(cities);
//[Milan, New Delhi, San Francisco, Tokyo, london]
Java 7中，我们使用Collections.sort对List进行排序，Collections.sort方法参数为List，Java 8中新增了一个集合方法List.sort，方法接受参数Comparator
- 排序整数集合
List numbers = Arrays.asList(6, 2, 1, 4, 9);
System.out.println(numbers); //[6, 2, 1, 4, 9]
numbers.sort(Comparator.naturalOrder());
System.out.println(numbers); //[1, 2, 4, 6, 9]
- 按某个字符串属性列排序
List movies = Arrays.asList(
new Movie("Lord of the rings"),
new Movie("Back to the future"),
new Movie("Carlito's way"),
new Movie("Pulp fiction"));
movies.sort(Comparator.comparing(Movie::getTitle));
movies.forEach(System.out::println);
结果输出
Movie{title='Back to the future'}
Movie{title='Carlito's way'}
Movie{title='Lord of the rings'}
Movie{title='Pulp fiction'}
我们注意到，给sort方法传递的并非Comparator类型，但结果仍然正确排序，原因就在于我们比较的属性列是String，而String已经实现了Comparable接口，查看Comparator.comparing()的定义实现，我们可以看到其其调用的就是比较关键字的compareTo方法
return (Comparator & Serializable)
(c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
- 按Double类型排序
List movies = Arrays.asList(
new Movie("Lord of the rings", 8.8),
new Movie("Back to the future", 8.5),
new Movie("Carlito's way", 7.9),
new Movie("Pulp fiction", 8.9));
movies.sort(Comparator.comparingDouble(Movie::getRating)
.reversed());
movies.forEach(System.out::println);
本质上，Comparator.comparingDouble()调用的还是Double.compare()的方法，同样类似的还有comparingInt()和comparingLong()方法
- 自定义比较规则
比如，对于我们之前提到的Movie,这里我们需要按星标排序，那么我们可以自定义Comparator实现类
List movies = Arrays.asList(
new Movie("Lord of the rings", 8.8, true),
new Movie("Back to the future", 8.5, false),
new Movie("Carlito's way", 7.9, true),
new Movie("Pulp fiction", 8.9, false));
movies.sort(new Comparator() {
@Override
public int compare(Movie m1, Movie m2) {
if(m1.getStarred() == m2.getStarred()){
return 0;
}
return m1.getStarred() ? -1 : 1;
}
});
movies.forEach(System.out::println);
得到输出结果如下：
List movies = Arrays.asList(
new Movie("Lord of the rings", 8.8, true),
new Movie("Back to the future", 8.5, false),
new Movie("Carlito's way", 7.9, true),
new Movie("Pulp fiction", 8.9, false));
movies.sort(new Comparator() {
@Override
public int compare(Movie m1, Movie m2) {
if(m1.getStarred() == m2.getStarred()){
return 0;
}
return m1.getStarred() ? -1 : 1;
}
});
movies.forEach(System.out::println);
当然，我们也可以Lambda表达式以匿名类简写如下：
movies.sort((m1, m2) -> {
if(m1.getStarred() == m2.getStarred()){
return 0;
}
return m1.getStarred() ? -1 : 1;
});
或者通过Comparator.comparing()方法
movies.sort(Comparator.comparing(Movie::getStarred, (star1, star2) -> {
if(star1 == star2){
return 0;
}
return star1 ? -1 : 1;
}));
- 按
chain of Comparator排序集合
还是上面的例子，如果我们需要首先按电影星标，然后再按评分高低排序，那么我们可以通过如下方式实现。
List movies = Arrays.asList(
new Movie("Lord of the rings", 8.8, true),
new Movie("Back to the future", 8.5, false),
new Movie("Carlito's way", 7.9, true),
new Movie("Pulp fiction", 8.9, false));
movies.sort(Comparator.comparing(Movie::getStarred)
.reversed()
.thenComparing(Comparator.comparing(Movie::getRating)
.reversed())
);
movies.forEach(System.out::println);
我们可以得到排序结果如下：
Movie{starred=true, title='Lord of the rings', rating=8.8}
Movie{starred=true, title='Carlito's way', rating=7.9}
Movie{starred=false, title='Pulp fiction', rating=8.9}
Movie{starred=false, title='Back to the future', rating=8.5}
原文链接：
https://reversecoding.net/java-8-comparator-how-to-sort-a-list/