Arrays.sort(T[], Comparator < ? super T > c) 方法用于对象数组按用户自定义规则排序.
官方Java文档只是简要描述此方法的作用,并未进行详细的介绍,本文将深入解析此方法。
1. 简单示例
sort方法的使用非常的简单明了,下面的例子中,先定义一个比较Dog大小的Comparator,然后将其实例对象作为参数传给sort方法,通过此示例,你应该能够快速掌握Arrays.sort()的使用方法。
- import java.util.Arrays;
- import java.util.Comparator;
- class Dog{
- int size;
- public Dog(int s){
- size = s;
- }
- }
- class DogSizeComparator implements Comparator<Dog>{
- @Override
- public int compare(Dog o1, Dog o2) {
- return o1.size - o2.size;
- }
- }
- public class ArraySort {
- public static void main(String[] args) {
- Dog d1 = new Dog(2);
- Dog d2 = new Dog(1);
- Dog d3 = new Dog(3);
- Dog[] dogArray = {d1, d2, d3};
- printDogs(dogArray);
- Arrays.sort(dogArray, new DogSizeComparator());
- printDogs(dogArray);
- }
- public static void printDogs(Dog[] dogs){
- for(Dog d: dogs)
- System.out.print(d.size + " " );
- System.out.println();
- }
- }
输出为:
- 2 1 3
- 1 2 3
2. 使用策略模式
这是策略模式(Strategy pattern)的一个完美又简洁的示例,值得一提的是为什么这种场景下适合使用策略模式.
总体来说,策略模式允许在程序执行时选择不同的算法.比如在排序时,传入不同的比较器(Comparator),就采用不同的算法.
根据上面的例子,假设你想要根据Dog的重量来进行排序,可以像下面这样,创建一个新的比较器来进行排序:
- class Dog{
- int size;
- int weight;
- public Dog(int s, int w){
- size = s;
- weight = w;
- }
- }
- class DogSizeComparator implements Comparator<Dog>{
- @Override
- public int compare(Dog o1, Dog o2) {
- return o1.size - o2.size;
- }
- }
- class DogWeightComparator implements Comparator<Dog>{
- @Override
- public int compare(Dog o1, Dog o2) {
- return o1.weight - o2.weight;
- }
- }
- public class ArraySort {
- public static void main(String[] args) {
- Dog d1 = new Dog(2, 50);
- Dog d2 = new Dog(1, 30);
- Dog d3 = new Dog(3, 40);
- Dog[] dogArray = {d1, d2, d3};
- printDogs(dogArray);
- Arrays.sort(dogArray, new DogSizeComparator());
- printDogs(dogArray);
- Arrays.sort(dogArray, new DogWeightComparator());
- printDogs(dogArray);
- }
- public static void printDogs(Dog[] dogs){
- for(Dog d: dogs)
- System.out.print("size="+d.size + " weight=" + d.weight + " ");
- System.out.println();
- }
- }
执行结果:
- size=2 weight=50 size=1 weight=30 size=3 weight=40
- size=1 weight=30 size=2 weight=50 size=3 weight=40
- size=1 weight=30 size=3 weight=40 size=2 weight=50
Comparator 是一个接口,所以sort方法中可以传入任意实现了此接口的类的实例,这就是策略模式的主要思想.
3. 为何使用"super"
如果使用 "Comparator < T > c" 那是很简单易懂的,但是sort的第2个参数里面的 < ? super T > 意味着比较器所接受的类型可以是T或者它的超类. 为什么是超类呢? 答案是: 这允许使用同一个比较器对不同的子类对象进行比较.在下面的示例中很明显地演示了这一点:
- import java.util.Arrays;
- import java.util.Comparator;
- class Animal{
- int size;
- }
- class Dog extends Animal{
- public Dog(int s){
- size = s;
- }
- }
- class Cat extends Animal{
- public Cat(int s){
- size = s;
- }
- }
- class AnimalSizeComparator implements Comparator<Animal>{
- @Override
- public int compare(Animal o1, Animal o2) {
- return o1.size - o2.size;
- }
- //in this way, all sub classes of Animal can use this comparator.
- }
- public class ArraySort {
- public static void main(String[] args) {
- Dog d1 = new Dog(2);
- Dog d2 = new Dog(1);
- Dog d3 = new Dog(3);
- Dog[] dogArray = {d1, d2, d3};
- printDogs(dogArray);
- Arrays.sort(dogArray, new AnimalSizeComparator());
- printDogs(dogArray);
- System.out.println();
- //when you have an array of Cat, same Comparator can be used.
- Cat c1 = new Cat(2);
- Cat c2 = new Cat(1);
- Cat c3 = new Cat(3);
- Cat[] catArray = {c1, c2, c3};
- printDogs(catArray);
- Arrays.sort(catArray, new AnimalSizeComparator());
- printDogs(catArray);
- }
- public static void printDogs(Animal[] animals){
- for(Animal a: animals)
- System.out.print("size="+a.size + " ");
- System.out.println();
- }
- }
输出结果:
- size=2 size=1 size=3
- size=1 size=2 size=3
- size=2 size=1 size=3
- size=1 size=2 size=3
4. 小结
与Arrays.sort()相关的信息总结如下:
-
- 通用: super 类
- 策略设计模式(strategy pattern);
- 归并排序(merge sort): 时间复杂度 n*log(n);
- Java.util.Collections#sort(List < T > list, Comparator < ? super T > c)与Arrays.sort 使用类似的思想.
http://blog.csdn.net/wisgood/article/details/16541013
