根据list中对象的属性去重和排序小结(必看篇)

//去重public class User {    private int id;    private String name;    private int age;    public User(){}    public User(int id, String name, int age) {      super();      this.id = id;      this.name = name;      this.age = age;    }    public int getId() {      return id;    }    public void setId(int id) {      this.id = id;    }    public String getName() {      return name;    }    public void setName(String name) {      this.name = name;    }    public int getAge() {      return age;    }    public void setAge(int age) {      this.age = age;    }    @Override    public String toString() {      return "User [id=" + id + ", name=" + name + ", age=" + age + "]";    }       }public class ListTest {  //list有序可重复、set无序不可重复、mapkey不允许重复，key相同的后面的value会把前面的覆盖掉  //List存放的数据，默认是按照放入时的顺序存放的，比如依次放入A、B、C，则取得时候，则也是A、B、C的顺序  public static void main(String[] args) {    List<User> list = new ArrayList<>();    list.add(new User(1,"a",20));    list.add(new User(1,"a",20));    list.add(new User(2,"a",20));    list.add(new User(3,"b",20));    list.add(new User(1,"c",20));    list.add(new User(4,"d",20));    list.add(new User(2,"e",20));    list.add(new User(1,"a",20));        list = mySort(list);    for (User user : list) {      System.out.println(user.toString());    }           }    public static List<User> mySort(List<User> list){      HashMap<Integer,User> tempMap = new HashMap<>();      for (User user : list) {        int key = user.getId();// containsKey(Object key) 该方法判断Map集合对象中是否包含指定的键名。如果Map集合中包含指定的键名，则返回true，否则返回false// containsValue(Object value)  value：要查询的Map集合的指定键值对象.如果Map集合中包含指定的键值，则返回true，否则返回false        if(tempMap.containsKey(key)){          User tempUser = new User(key,                       tempMap.get(key).getName() + user.getName(),                       tempMap.get(key).getAge());//user.getAge();//HashMap是不允许key重复的，所以如果有key重复的话，那么前面的value会被后面的value覆盖                    tempMap.put(key, tempUser);        }else{          tempMap.put(key, user);        }      }      List<User> tempList = new ArrayList<>();      for(int key : tempMap.keySet()){        tempList.add(tempMap.get(key));      }      return tempList;    }  }//排序=============================================public class Student {  private int age;   private String name;   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 [age=" + age + ", name=" + name + "]";  }   }public class ListSort {  public static void main(String[] args) {     List<Student> list = new ArrayList<Student>();            //创建3个学生对象，年龄分别是20、19、21，并将他们依次放入List中       Student s1 = new Student();       s1.setAge(20);       s1.setName("葛大");      Student s2 = new Student();       s2.setAge(19);       s2.setName("张杰");      Student s3 = new Student();       s3.setAge(21);       s3.setName("宝爷");      list.add(s1);       list.add(s2);       list.add(s3);              System.out.println("排序前："+list);             Collections.sort(list, new Comparator<Student>(){                     public int compare(Student o1, Student o2) {                    //按照学生的年龄进行升序排列 ;<是降序//          //          return o1.getAge()-o2.getAge();//升序//          return o2.getAge()-o1.getAge();//降序           return o1.getName().compareTo(o2.getName()) ;// 按照姓名升序//          return o2.getName().compareTo(o1.getName()) ;// 按照姓名降序        }       });        System.out.println("排序后："+list);     }   }