如何覆盖 compareTo (Java)

Question

I'm a beginner in programming and I have two classes. First class is:

public class User implements Comparable<User>

with field int age , constructor and overrided method of interface Comparable:

 @Override
    public int compareTo(User user) {
        return user.age >= age ? -1 : 0;
    }

Second class is public class SortUser with a method to make a Set collection from a List:

public Set<User> sort(List<User> list) {
        Set<User> result = new TreeSet<>();
        for (User user : list) {
            result.add(user);
        }
        return result;
    }

It seems to me that all User objects in a Set should be sorted, but when I made a List with 3 User objects...

 User a = new User(1);
 User b = new User(2);
 User c = new User(3);
 List<User> list = new ArrayList<>();
 list.add(c);
 list.add(a);
 list.add(b);

(Now the list's order is: 312 ) ...and created a Set ( TreeSet ) from that list:

SortUser sortUser = new SortUser();
Set<User> set = sortUser.sort(list);

At the end I have a set with that order: 13 , it means that only two objects are in the set . What is going wrong?

Answer 1

As I see you have wrong implementation of compare method. Could you update it to?

@Override
public int compareTo(User user) {
  return Integer.compare(age, user.age);
}

Answer 2

What you're doing with the TreeSet is unnecessary. I'm not sure they're guaranteed to have certain ordering when iterated.

Just replace your sort method with

Collections.sort(list)

And my guess as to why an element is being dropped is your compareTo method never returns a 1 in any case, so elements are always considered to be less than or equal to other elements, which is probably screwing with the TreeSet .

Answer 3

Please follow below methodology

In case of string.

    public static Comparator<Employee> NameComparator = new Comparator<Employee>() {
    @Override
    public int compare(Employee e1, Employee e2) {
        return e1.getName().compareTo(e2.getName());
    }
};

In case of Integer values

public static Comparator<Employee> SalaryComparator = new Comparator<Employee>() {

    @Override
    public int compare(Employee e1, Employee e2) {
        return (int) (e1.getSalary() - e2.getSalary());
    }
};

Answer 4

User class

 public class User implements Comparable<User>{
  int age;
  User(int age){age=age;}
  @Override
  public int compareTo(User user) {
    return this.age >= age ? -1 : 0;
  }
 }

prepare list

   User a = new User(1);
   User b = new User(2);
   User c = new User(3);
   List<User> list = new ArrayList<>();
  list.add(c);
  list.add(a);
  list.add(b);

for sorting

 Set<User> list1 = new TreeSet(list);

Answer 5

class Scratch {
    public static void main(String[] args) {
        List<User> list = new ArrayList<>();
        list.add(new User(3));
        list.add(new User(1));
        list.add(new User(2));
        Collections.sort(list);
        list.forEach(el -> System.out.println(el.age));
    }
}

class User implements Comparable<User> {
    int age;

    User(int age) {
        this.age = age;
    }

    @Override
    public int compareTo(User user) {
        return this.age >= user.age ? -1 : 0;
    }
}

Answer 6

You might want to try this:

@Override
public int compareTo(User user) {
  if (this == user){
     return 0;
  }
  if (user != null){
     return this.age.compareTo(user.getAge())
  }
}