java中的Colletions类主要实现列表List的排序功能。根据函数参数的传递，具体的排序可以分为 ：
1. 自然排序（natural ordering）。
函数原型：sort(List<T> list)
说明：参数是要参与排序列表的List对象
实例说明：参与排序的列表的元素Student必须实现Comparable接口的
public int compareTo(Object o) 方法，在里面写对比的原则。
然后调用Colletions.sort(排序对象的列表)
请看如下示例：
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
class ArrayListTest{
public static void printElements(Collection c){
Iterator it = c.iterator();
while(it.hasNext()){
System.out.println(it.next());
}
}
public static void main(String[] args){
ArrayList<Student> al = new ArrayList<Student>();
al.add(new Student(2,"aora"));
al.add(new Student(1,"longyu"));
al.add(new Student(3,"goso"));
Collections.sort(al);
printElements(al);
}
}
class Student implements Comparable{
int num;
String name;
Student(int num,String name){
this.num = num;
this.name = name;
}
public int compareTo(Object o) {
Student s = (Student)o;
return num > s.num ? 1 : (num == s.num ? 0 : -1);
};
public String toString(){
return "num = " + this.num + ",name = " + this.name;
}
}
2. 实现比较器（Comparator）接口。
函数原型：sort(List<T> list, Comparator<? super T> c)
说明：第一个参数同左，第二个参数是构建对比规则的对比器Comparator。
实例说明（如下）：在参与排序的列表的元素Student中写一个内部类作为
Student的对比器，这个对比器要实现Comparator接口的public int compare(Object o1,
Object o2)方法，然后调用Colletions.sort(排序对象的列表，对比器)
请看如下示例：
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.Comparator;
class ArrayListTest{
public static void printElements(Collection c){
8 Iterator it = c.iterator();
while(it.hasNext()){
10 System.out.println(it.next());
}
}
public static void main(String[] args){
ArrayList<Student> al = new ArrayList<Student>();
al.add(new Student(2,"qingan"));
al.add(new Student(1,"longyu"));
al.add(new Student(3,"goso"));
al.add(new Student(2,"aora"));
19 Collections.sort(al,new Student.studentComparator());
20 printElements(al);
}
}
class Student{
int num;
String name;
Student(int num,String name){
this.num = num;
this.name = name;
}
static class studentComparator implements Comparator{
public int compare(Object o1,Object o2){
Student s1 = (Student)o1;
Student s2 = (Student)o2;
34 int result = s1.num > s2.num ? 1 : (s1.num == s2.num ? 0 : -1);
// 注意：此处在对比num相同时，再按照name的首字母比较。
if(result == 0){
37 result = s1.name.compareTo(s2.name);
38 }
return result;
}
}
public String toString(){
return "num = " + this.num + ",name = " + this.name;
}
}
（转http://viver120.blog.163.com/blog/static/60072482013010111228695/）