package com.example.demo;

import java.util.Date;

public class User implements Comparable<User> {
  private long id;
  private String email;
  private Date createdOn;

  // other getters and setters omitted

  public Date getCreatedOn() {
    return createdOn;
  }

  public void setCreatedOn(Date createdOn) {
    this.createdOn = createdOn;
  }

  @Override
  public int compareTo(User u) {
    if (getCreatedOn() == null || u.getCreatedOn() == null) {
      return 0;
    }
    return getCreatedOn().compareTo(u.getCreatedOn());
  }
}

Collections.sort(users);
Collections.reverse(users);