以下内容转自:

  原文链接: programcreek 翻译: ImportNew.com刘志军
  译文链接: http://www.importnew.com/8952.html

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Arrays.sort(T[], Comparator < ? super T > c) 是用来对用户自定义的对象数组排序功能的。Java 官方文档简单描述了它的作用,但不足以让我们深刻理解。在这篇文章中,我将顺着一下关键点息做个比较深入的理解。

1、简单实例:如何使用Arrays.sort()

通过阅读下面代码,你能快速正确了解这个方法的用途。Comparator(比较器)用于根据Dogs的size比较其大小,并作为sort方法的参数。

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:

2 1 3
1 2 3

2、策略模式的使用

这是运用策略模式的一个很好的场景,为什么策略模式对于这种场景非常适用?简单来说,策略模式使不同的算法在运行时得以选择。在这个例子中,通过传递不同的Comparator,可以选择不同的算法。基于上例,现在假设你有一个Comparator,用weight来代替size来比较Dogs。你可以简单创建一个新的Comprator如下:

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();
}
}

输出:

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

3、为什么使用“super”

很显然,如果”Comparator<T>c”作为参数,但是第二个参数是”Comparator< ? super T > c”,使用<? super T>意味着类型可以是T或者是它的超类。为什么允许超类型呢?答案是:这种方式允许所有子类使用同一个comparator。看看下面这个例子一目了然。

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:

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、总结

总的来说,从Arrays.sort()中你应该了解到:

  1. generic(范型)——super
  2. 策略模式
  3. 归并排序——nlog(n)时间复杂度
  4. java.util.Collections.sort(List<T>list, Comparator<?super T> c)类似于Arrays.sort
04-16 05:20