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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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