Collections.sort方法对list排序的两种方式

http://blog.csdn.net/treeroot/article/details/99613

Collections.sort( )分为两部分，一部分为排序规则，一部分为排序算法 。

规则用来判断对象，算法则考虑如何进行排序

对于自定义对象，sort()不知道规则，所以无法比较，这种情况下一定要定义排序规则。方式有两种：

第一种，java.lang下面的一个接口：Comparable。可以让自定义对象实现一个Comparable接口，这个接口只有一个方法comparableTo（Object o）

其规则是当前对象与o对象进行比较，返回一个int值，系统根据此值进行排序。

如当前对象>o对象，则返回值>0；

如当前对象=o对象，则返回值=0；

如当前对象

import java.util.*;

class User implements Comparable
{
    private String name;
    private Integer order;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getOrder() {
        return order;
    }
    public void setOrder(Integer order) {
        this.order = order;
    }
    public int compareTo(User arg0) {
        return this.getOrder().compareTo(arg0.getOrder());
    }
}

public class Test{

    public static void main(String[] args) {
        User user1 = new User();
        user1.setName("zhangsan");
        user1.setOrder(10);
        User user2 = new User();
        user2.setName("lisi");
        user2.setOrder(6);
        User user3 = new User();
        user3.setName("wangwu");
        user3.setOrder(11);
        User user4 = new User();
        user4.setName("zhaoqi");
        user4.setOrder(2);
        List list = new ArrayList();
        list.add(user1);
        list.add(user2);
        list.add(user3);
        list.add(user4);
        Collections.sort(list);
        for(User u : list){
            System.out.println(u.getName());
        }
    }
}

import java.util.*;

class User implements Comparable
{
    private String name;
    private Integer order;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getOrder() {
        return order;
    }
    public void setOrder(Integer order) {
        this.order = order;
    }
    public int compareTo(User arg0) {
        return this.getOrder().compareTo(arg0.getOrder());
    }
}

public class Test{

    public static void main(String[] args) {
		User user1 = new User();
        user1.setName("zhangsan");
        user1.setOrder(10);
        User user2 = new User();
        user2.setName("lisi");
        user2.setOrder(6);
		User user3 = new User();
		user3.setName("wangwu");
        user3.setOrder(11);
		User user4 = new User();
		user4.setName("zhaoqi");
        user4.setOrder(2);
        List list = new ArrayList();
        list.add(user1);
		list.add(user2);
		list.add(user3);
		list.add(user4);
        Collections.sort(list);
        for(User u : list){
            System.out.println(u.getName());
        }
    }
}
      

第二种方式，java.util下有一个Comparator（比较器）。它拥有compare( )方法，用来比较两个对象。

import java.util.*;
class User { //此处无需实现Comparable接口
    private String name;
    private Integer order;
    public User(){};
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getOrder() {
        return order;
    }
    public void setOrder(Integer order) {
        this.order = order;
    }
}

public class Test2{
    public static void main(String[] args) {
        User user1 = new User();
        user1.setName("zhangsan");
        user1.setOrder(10);
        User user2 = new User();
        user2.setName("lisi");
        user2.setOrder(6);
        User user3 = new User();
        user3.setName("wangwu");
        user3.setOrder(11);
        User user4 = new User();
        user4.setName("zhaoqi");
        user4.setOrder(2);

        Listlist = new ArrayList();
        list.add(user2);
        list.add(user1);
        list.add(user3);
        list.add(user4);

        Collections.sort(list,new Comparator(){
            public int compare(User arg0, User arg1) {
                return arg0.getOrder().compareTo(arg1.getOrder());
            }
        });
        for(User u : list){
            System.out.println(u.getName());
        }
    }
}

import java.util.*;
class User { //此处无需实现Comparable接口
    private String name;
    private Integer order;
	public User(){};
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getOrder() {
        return order;
    }
    public void setOrder(Integer order) {
        this.order = order;
    }
}

public class Test2{
    public static void main(String[] args) {
        User user1 = new User();
        user1.setName("zhangsan");
        user1.setOrder(10);
        User user2 = new User();
        user2.setName("lisi");
        user2.setOrder(6);
		User user3 = new User();
		user3.setName("wangwu");
        user3.setOrder(11);
		User user4 = new User();
		user4.setName("zhaoqi");
        user4.setOrder(2);
	
        Listlist = new ArrayList();
        list.add(user2);
        list.add(user1);
		list.add(user3);
		list.add(user4);
        
        Collections.sort(list,new Comparator(){
            public int compare(User arg0, User arg1) {
                return arg0.getOrder().compareTo(arg1.getOrder());
            }
        });
        for(User u : list){
            System.out.println(u.getName());
        }
    }
}

前者代码结构简单，但是只能根据固定的属性排序，后者灵活，可以临时指定排序项，但是代码不够简洁。

多字段比较：

Collections.sort(list,new Comparator (){
    public int compare(User arg0, User arg1) {
//            第一次比较专业
        int i = arg0.getOrder().compareTo(arg1.getOrder());
//            如果专业相同则进行第二次比较
        if(i==0){
//            第二次比较学制
            int j=arg0.getXXX().compareTo(arg1.getXXX());
//                如果学制相同则返回按年龄排序
            if(j==0){
                return arg0.getCCC().compareTo(arg1.getCCC());
            }
            return j;
        }
        return i;
    }
});