将数据集合对某一字段进行排序

List<Student> list = new ArrayList<>();
list.add(new Student(1,25,"关羽"));
list.add(new Student(2,25,"张飞"));
list.add(new Student(3,18,"刘备"));
list.add(new Student(4,32,"袁绍"));
list.add(new Student(5,36,"赵云"));
list.add(new Student(6,16,"曹操"));
System.out.println("排序前:");
for (Student student : list) {
System.out.println(student.toString());
}
//使用默认排序
List<Student> list2 = new ArrayList<>();
list2.addAll(list); //创建一个新的集合将list全部添加进来
Collections.sort(list2,new Comparator<Student>() { //将list2进行排序从写方法 o2-o1返回的是从大到小
           @Override
           public int compare(Student o1, Student o2) {
               return o2.getAge()-o1.getAge();
           }
});
System.out.println("默认排序后:");
for (Student student : list2) {
System.out.println(student.toString());
}
Map<Integer, String> lmap = new LinkedHashMap<>(); //linkedHashMap将重复的key去掉
       for (Student tran : list2) { //将对比的年龄当做key存放在map中这样子就会把相同的年龄都算作一个
           Integer key =tran.getAge();
           lmap.put(key, "0");
       }
       int m=0;
       for (Integer key : lmap.keySet()) { //循环key值 key值由大到小进行排序排名也会被添加进value中
           lmap.put(key, (++m)+"");
       }
       for (Student tran : list) {
           for (Integer key : lmap.keySet()) {
               if(tran.getAge()==key) { //循环list如何年龄一样的话就将名次set进去
                   tran.setH(Integer.parseInt(lmap.get(key)));
               }
           }
       }
       System.out.println("按照年龄从大到小排序后:");
   for (Student student : list) {
   System.out.println(student.toString());
   }

//student类

private int id;
   private int age;
   private String name;
   private int h;


   public int getH() {
           return h;
       }
       public void setH(int h) {
           this.h = h;
       }
       public Student(int id, int age, String name) {
   this.id = id;
   this.age = age;
   this.name = name;
   }

       public int getId() {
           return id;
       }
       public void setId(int id) {
           this.id = id;
       }
       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;
       }
       @Override
       public String toString() {
           return "Student [id=" + id + ", age=" + age + ", name=" + name + ", h=" + h + "]";
       }

将数据集合对某一字段进行排序

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