Java8的Lambda和排序
對數組和集合進行排序是Java 8 lambda令人驚奇的一個應用,我們可以實作一個Comparators來實作各種排序。看下面案例:
static class Person {
final String firstName;
final String lastName;
Person(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
@Override
public String toString() {
return "Person{" +
"firstName='" + firstName + '\'' +
", lastName='" + lastName + '\'' +
'}';
}
}
Person的資料有:
List people =
Arrays.asList(
new Person("Jane", "Henderson"),
new Person("Michael", "White"),
new Person("Henry", "Brighton"),
new Person("Hannah", "Plowman"),
new Person("William", "Henderson")
);
我們希望根據名字排序(last name),然後再根據姓(first name)排序。
在Java 7之前,我們通常是通過實作一個Comparator:
people.sort(new Comparator() {
@Override
public int compare(Person o1, Person o2) {
int result = o1.lastName.compareTo(o2.lastName);
if (result == 0)
result = o1.firstName.compareTo(o2.firstName);
return result;
}
});
people.forEach(System.out::println);
而在Java 8中,我們可以使用lambda替代匿名函數,如下:
Comparator c = (p, o) -> p.lastName.compareTo(o.lastName);
c = c.thenComparing((p, o) -> p.firstName.compareTo(o.firstName));
people.sort(c);
people.forEach(System.out::println);
在這裡,Lambda表達式(p, o) -> p.lastName.compareTo(o.lastName)替代之前的匿名函數new Comparator() {}
因為Java編譯器并不能推遲對Lambda表達式類型判斷,比如延遲到将comparator傳遞到sort()方法,是以使得我們進行鍊式Comparator書寫有點困難,比如我們想寫成如下:
XXX.thenComparing((p, o) -> p.lastName.compareTo(o.lastName))
.thenComparing((p, o) -> p.firstName.compareTo(o.firstName))
換句話說,類型判斷是從左到右,而不是從右到左,我們可以通過建立一個泛型來進行類型推斷:
class Utils {
static Comparator compare() {
return (e1, e2) -> 0;
}
}
通過上面的compare()方法,我們可以書寫流暢的comparator鍊:
people.sort(
Utils.compare()
.thenComparing((p, o) ->
p.lastName.compareTo(o.lastName))
.thenComparing((p, o) ->
p.firstName.compareTo(o.firstName))
);
people.forEach(System.out::println);
本文譯自Java8 Friday系列,源碼Gihub