如何对ArrayList中对象按照该对象某属性排序
public static void display (Collection c){ Iterator it = c.iterator (); while (it.hasNext()){ Object o = it.next(); System.out.println(o); } } public static void main(String[] args) { Student stu1 = new Student (1,"zhangsan","male",18,"cs"); Student stu2 = new Student (2,"lisi","female",19,"cs"); Student stu3 = new Student (3,"wangwu","male",20,"cs"); Student stu4 = new Student (4,"zhaoliu","female",21,"cs"); Student stu5 = new Student (5,"xiaoming","male",22,"cs"); ArrayList<Student> List = new ArrayList<Student>(); List.add(stu1); List.add(stu2); List.add(stu3); List.add(stu4); List.add(stu5); display(List);加排序功能，打印时：输出学生对象的时候，需要先按照年龄排序，如果年龄相同，则按照姓名排序，如果姓名也相同，则按照学号排序。
让 Student 实现Comparable接口，或是实例化一个比较器
import java.util.ArrayList;import java.util.Collections;import java.util.Comparator;public class ComparableTest { public static void main(String[] args) { Comparator<Student> comparator = new Comparator<Student>(){ public int compare(Student s1, Student s2) { //先排年龄 if(s1.age!=s2.age){ return s1.age-s2.age; } else{ //年龄相同则按姓名排序 if(!s1.name.equals(s2.name)){ return s1.name.compareTo(s2.name); } else{ //姓名也相同则按学号排序 return s1.id-s2.id; } } } }; Student stu1 = new Student (1,"zhangsan","male",28,"cs"); Student stu2 = new Student (2,"lisi","female",19,"cs"); Student stu3 = new Student (3,"wangwu","male",22,"cs"); Student stu4 = new Student (4,"zhaoliu","female",17,"cs"); Student stu5 = new Student (5,"jiaoming","male",22,"cs"); ArrayList<Student> List = new ArrayList<Student>(); List.add(stu1); List.add(stu2); List.add(stu3); List.add(stu4); List.add(stu5); //这里就会自动根据规则进行排序 Collections.sort(List,comparator); display(List); } static void display(ArrayList<Student> lst){ for(Student s:lst) System.out.println(s); }}class Student{ int age; int id; String gender; String name; String cs; Student(int id,String name,String gender,int age,String cs){ this.age=age; this.name=name; this.gender=gender; this.id=id; this.cs=cs; } public String toString(){ return id+" "+name+" "+gender+" "+age+" "+cs; }}
HashMap重写比较器
package test;import java.util.ArrayList;import java.util.Collections;import java.util.Comparator;import java.util.HashMap;import java.util.Map;public class HashMapSort {public void test(){HashMap<String, Integer> map = new HashMap<String, Integer>();map.put("d", 2);map.put("c", 1);map.put("b", 1);map.put("a", 3);ArrayList<Map.Entry<String, Integer>> infoIds =new ArrayList<Map.Entry<String, Integer>>(map.entrySet());System.out.println("排序前=================");for (int i = 0; i < infoIds.size(); i++) { String id = infoIds.get(i).toString(); System.out.println(id);}//d 2//c 1//b 1//a 3System.out.println("排序后==============");Collections.sort(infoIds, new Comparator<Map.Entry<String, Integer>>() { public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2) { return (o2.getValue() - o1.getValue()); //return (o1.getKey()).toString().compareTo(o2.getKey()); }}); //排序后for (int i = 0; i < infoIds.size(); i++) { String id = infoIds.get(i).toString(); System.out.println(id);}}public static void main(String[] args){HashMapSort so=new HashMapSort();so.test();}}
