Java ArrayList排序的3种方法
1. 使用Collections.sort()方法进行排序
ArrayList 存储着以字符串形式存在的国名（country name），为了对这个 ArrayList 进行排序，你需要调用 Collections.sort()方法，传递由国名构成的 ArrayList 对象。这种方法将按照自然顺序（按字母升序）对元素（国名）进行排序。代码如下：
public class SortArrayListAscendingDescending {
private ArrayList arrayList;
public SortArrayListAscendingDescending(ArrayList arrayList) {
this.arrayList = arrayList;
}
public ArrayList getArrayList() {
return this.arrayList;
}
public ArrayList sortAscending() {
Collections.sort(this.arrayList);
return this.arrayList;
}
public ArrayList sortDescending() {
Collections.sort(this.arrayList, Collections.reverseOrder());
return this.arrayList;
}
}
在SortArrayListAscendingDescending类中，我们在构造器中初始化了一个 ArrayList 对象。在 sortAscending()方法中，我们调用了 Collections.sort()方法，并传递这个初始化的 ArrayList对象为参数，返回排序后的 ArrayList。在 sortDescending()方法中，我们调用重载的 Collections.sort()方法让其按照降序对元素排序，这个版本的 Collections.sort()接收ArrayList对象作为第一个参数，一个由 Collections.reverseOrder()方法返回的 Comparator 对象作为第二个参数。
public class SortArrayListAscendingDescendingTest {
public void testSortAscendingDescending() throws Exception {
ArrayList countryList = new ArrayList();
countryList.add("France");
countryList.add("USA");
countryList.add("India");
countryList.add("Spain");
countryList.add("England");
SortArrayListAscendingDescending sortArrayList = new SortArrayListAscendingDescending(countryList);
ArrayList unsortedArrayList = sortArrayList.getArrayList();
System.out.println("Unsorted ArrayList: " + unsortedArrayList);
ArrayList sortedArrayListAscending = sortArrayList.sortAscending();
System.out.println("Sorted ArrayList in Ascending Order : " +
sortedArrayListAscending);
ArrayList sortedArrayListDescending = sortArrayList.sortDescending();
System.out.println("Sorted ArrayList in Descending Order: " +
sortedArrayListDescending);
}
}
在SortArrayListAscendingDescendingTest测试代码中，我们创建一个 ArrayList 对象，并添加了 5 个字符串对象代表 5 个国家的名字。然后我们调用 getArrayList()、sortAscending()和 sortDescending()方法，并打印这些方法返回的 ArrayList 对象。
显示结果：
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T E S T S
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Running guru.springframework.blog.sortarraylist.ascendingdescending.SortArrayListAscendingDescendingTest
Unsorted ArrayList: [France, USA, India, Spain, England]
Sorted ArrayList in Ascending Order : [England, France, India, Spain, USA]
Sorted ArrayList in Descending Order: [USA, Spain, India, France, England]
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.001 sec - in guru.springframework.blog.sortarraylis
2. 使用Comparable排序
让我们来举一个例子，JobCandidate 类的对象保存在 ArrayList 中并准备对其进行排序。JobCandidate 类有三个成员变量：字符串类型的姓名和性别、整型的年龄。我们想要对保存在 ArrayList 中的 JobCandidate 对象按照年龄进行排序。因此我们要让 JobCandidate 类实现 Comparable 接口并重写 compareTo()方法。
public class JobCandidate implements Comparable {
private String name;
private String gender;
private int age;
public JobCandidate(String name, String gender, int age) {
this.name = name;
this.gender = gender;
this.age = age;
}
public String getName() {
return name;
}
public String getGender() {
return gender;
}
public int getAge() {
return age;
}
@Override
public int compareTo(JobCandidate candidate) {
return ((this.getAge() < candidate.getAge()) ? (-1)
: ((this.getAge() == candidate.getAge())
? 0 : 1));
}
@Override
public String toString() {
return " Name: " + this.name + ", Gender: " + this.gender + ", age:" +
this.age;
}
}
JobCandidate 类被重写的 compareTo()方法中，我们实现了基于年龄的比较逻辑。
public class JobCandidateSorter {
ArrayList jobCandidate = new ArrayList<>();
public JobCandidateSorter(ArrayList jobCandidate) {
this.jobCandidate = jobCandidate;
}
public ArrayList getSortedJobCandidateByAge() {
Collections.sort(jobCandidate);
return jobCandidate;
}
}
在 JobCandidateSorter 类中，我们初始化了一个 ArrayList 对象，委托方将通过构造函数实例化 JobCandidateSorter 。然后我们编写了 getSortedJobCandidateByAge()方法，在这个方法中，我们调用 Collections.sort()并传递已经初始化了的 ArrayList 为参数，最后返回排序后的 ArrayList。
public class JobCandidateSorterTest {
public void testGetSortedJobCandidateByAge() throws Exception {
JobCandidate jobCandidate1 = new JobCandidate("Mark Smith", "Male", 26);
JobCandidate jobCandidate2 = new JobCandidate("Sandy Hunt", "Female", 23);
JobCandidate jobCandidate3 = new JobCandidate("Betty Clark", "Female",
20);
JobCandidate jobCandidate4 = new JobCandidate("Andrew Styne", "Male", 24);
ArrayList jobCandidateList = new ArrayList();
jobCandidateList.add(jobCandidate1);
jobCandidateList.add(jobCandidate2);
jobCandidateList.add(jobCandidate3);
jobCandidateList.add(jobCandidate4);
JobCandidateSorter jobCandidateSorter = new JobCandidateSorter(jobCandidateList);
ArrayList sortedJobCandidate = jobCandidateSorter.getSortedJobCandidateByAge();
System.out.println("-----Sorted JobCandidate by age: Ascending-----");
for (JobCandidate jobCandidate : sortedJobCandidate) {
System.out.println(jobCandidate);
}
}
}
JobCandidateSorterTest类中，我们创建了四个 JobCandidate 对象并把它们添加到 ArrayList，然后传递这个 ArrayList 到构造函数来实例化 JobCandidateSorter 类。最后，我们调用 JobCandidateSorter 类的 getSortedJobCandidateByAge()方法，并打印这个方法返回的排序后的 ArrayList。测试的输出结果如下：
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T E S T S
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Running guru.springframework.blog.sortarraylist.comparable.JobCandidateSorterTest
-----Sorted JobCandidate by age: Ascending-----
Name: Betty Clark, Gender: Female, age:20
Name: Sandy Hunt, Gender: Female, age:23
Name: Andrew Styne, Gender: Male, age:24
Name: Mark Smith, Gender: Male, age:26
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.003 sec
- in guru.springframework.blog.sortarraylist.comparable.JobCandidateSorterTest
3. 使用 Comparator 排序
比如我有一个Person类,它的实例对象存储在ArrayList数组中,现在要把ArrayList数组中的Person对象按照年龄排序.代码如下：
Person类
public class Person {
String name;
int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Mycomparator类
public class Mycomparator implements Comparator {
public int compare(Object o1, Object o2) {
Person p1 = (Person) o1;
Person p2 = (Person) o2;
if (p1.age < p2.age) return 1;
else return 0;
}
}
ListSort类
public class ListSort {
public static void main(String[] args) {
ArrayList list = new ArrayList();
list.add(new Person("lcl", 28));
list.add(new Person("fx", 23));
list.add(new Person("wqx", 29));
Comparator comp = new Mycomparator();
Collections.sort(list, comp);
for (int i = 0; i < list.size(); i) {
Person p = (Person) list.get(i);
System.out.println(p.getName());
}
}
}
当然，如果你的List包装的是基本类型或者String，则只要 Collections.sort(list);即可
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