static
<T extends Comparable <? super T>>
void
sort ( List <T> list)
 Sorts the specified list into ascending order,
according to the natural ordering of its elements.
static
<T> void
sort ( List <T> list, Comparator <? super T> c)
 Sorts the specified list according to the order
 induced by the specified comparator.

API如上

具体使用方法:

第一种方法:容器内要排序的类必须时下Comparable接口,Comparable接口来自java.lang包
必须实现下面这个方法:
intcompareTo(T o)
Compares this object with the specified object for order.

例子:
import java.util.*;
public class Main{
    public static void main(String args[]){
        ArrayList<Student> al=new ArrayList<Student>();
        al.add(new Student(2,"aa"));
        al.add(new Student(1,"bb"));
        al.add(new Student(3,"dd"));
        al.add(new Student(3,"cc"));
        Collections.sort(al);
        Iterator it=al.iterator();
        while(it.hasNext()){
            System.out.println(it.next());
        }
    }
}
class Student implements Comparable{
    int id;
    String name;
    Student(int id,String name){
        this.id=id;
        this.name=name;
    }
    public int compareTo(Object o){
        Student s=(Student)o;
        int result=(id>s.id)?1:((id==s.id)?0:-1);
        if(0==result){
            result=name.compareTo(s.name);
        }
        return result;
    }
    public String toString(){
        return "id="+this.id+",name="+this.name;
       
    }
}

第二种方法:使用静态内部类实现Comparator接口,Comparator接口位于java.util包下
 intcompare(T o1, T o2)
Compares its two arguments for order.
 booleanequals(Object obj)
Indicates whether some other object is "equal to" this Comparator.


只需实现compare方法就行,equals方法在obeject类就会有,而实体类继承自object类,就必然会有equals方法,所以不需实现

例子
import java.util.*;
public class Main{
    public static void main(String args[]){
        ArrayList<Student> al=new ArrayList<Student>();
        al.add(new Student(2,"aa"));
        al.add(new Student(1,"bb"));
        al.add(new Student(3,"dd"));
        al.add(new Student(3,"cc"));
        Collections.sort(al,new Student.StudentComparator());
        Iterator it=al.iterator();
        while(it.hasNext()){
            System.out.println(it.next());
        }
    }
}
class Student {
    int id;
    String name;
    Student(int id,String name){
        this.id=id;
        this.name=name;
    }
    public String toString(){
        return "id="+this.id+",name="+this.name;
       
    }
    static class StudentComparator implements Comparator{
        public int compare(Object o1,Object o2){
            Student s1=(Student)o1;
            Student s2=(Student)o2;
            int result=(s1.id>s2.id)?1:((s1.id==s2.id)?0:-1);
            if(0==result){
                result=s1.name.compareTo(s2.name);
               
            }
            return result;
        }
    }
}