Как найти Max Date в списке <Object>?

Ответ 1

Поскольку вы запрашиваете lambdas, вы можете использовать следующий синтаксис с Java 8:

Date maxDate = list.stream().map(u -> u.date).max(Date::compareTo).get();

или, если у вас есть получатель для даты:

Date maxDate = list.stream().map(User::getDate).max(Date::compareTo).get();

Ответ 2

Comparator<User> cmp = new Comparator<User>() {
    @Override
    public int compare(User user1, User user2) {
        return user1.date.compareTo(user2.date);
    }
};

Collections.max(list, cmp);

Ответ 3

Небольшое улучшение от принятого ответа состоит в том, чтобы выполнить нулевую проверку, а также получить полный объект.

public class DateComparator {

public static void main(String[] args) throws ParseException {
    SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");

    List<Employee> employees = new ArrayList<>();
    employees.add(new Employee(1, "name1", addDays(new Date(), 1)));
    employees.add(new Employee(2, "name2", addDays(new Date(), 3)));
    employees.add(new Employee(3, "name3", addDays(new Date(), 6)));
    employees.add(new Employee(4, "name4", null));
    employees.add(new Employee(5, "name5", addDays(new Date(), 4)));
    employees.add(new Employee(6, "name6", addDays(new Date(), 5)));
    System.out.println(employees);
    Date maxDate = employees.stream().filter(emp -> emp.getJoiningDate() != null).map(Employee::getJoiningDate).max(Date::compareTo).get();
    System.out.println(format.format(maxDate));
    //Comparator<Employee> comparator = (p1, p2) -> p1.getJoiningDate().compareTo(p2.getJoiningDate());
    Comparator<Employee> comparator = Comparator.comparing(Employee::getJoiningDate);
    Employee maxDatedEmploye = employees.stream().filter(emp -> emp.getJoiningDate() != null).max(comparator).get();
    System.out.println(" maxDatedEmploye : " + maxDatedEmploye);

    Employee minDatedEmployee = employees.stream().filter(emp -> emp.getJoiningDate() != null).min(comparator).get();
    System.out.println(" minDatedEmployee : " + minDatedEmployee);

}

public static Date addDays(Date date, int days) {
    Calendar cal = Calendar.getInstance();
    cal.setTime(date);
    cal.add(Calendar.DATE, days); // minus number would decrement the days
    return cal.getTime();
}
}

Вы получите ниже результаты:

 [Employee [empId=1, empName=name1, joiningDate=Wed Mar 21 13:33:09 EDT 2018],
Employee [empId=2, empName=name2, joiningDate=Fri Mar 23 13:33:09 EDT 2018],
Employee [empId=3, empName=name3, joiningDate=Mon Mar 26 13:33:09 EDT 2018],
Employee [empId=4, empName=name4, joiningDate=null],
Employee [empId=5, empName=name5, joiningDate=Sat Mar 24 13:33:09 EDT 2018],
Employee [empId=6, empName=name6, joiningDate=Sun Mar 25 13:33:09 EDT 2018]
]
2018-03-26
 maxDatedEmploye : Employee [empId=3, empName=name3, joiningDate=Mon Mar 26 13:33:09 EDT 2018]

 minDatedEmployee : Employee [empId=1, empName=name1, joiningDate=Wed Mar 21 13:33:09 EDT 2018]

Обновление: Что, если сам список пуст?

        Date maxDate = employees.stream().filter(emp -> emp.getJoiningDate() != null).map(Employee::getJoiningDate).max(Date::compareTo).orElse(new Date());
    System.out.println(format.format(maxDate));
    Comparator<Employee> comparator = Comparator.comparing(Employee::getJoiningDate);
    Employee maxDatedEmploye = employees.stream().filter(emp -> emp.getJoiningDate() != null).max(comparator).orElse(null);
    System.out.println(" maxDatedEmploye : " + maxDatedEmploye);

Ответ 4

LocalDate maxDate = dates.stream()
                            .max( Comparator.comparing( LocalDate::toEpochDay ) )
                            .get();

LocalDate minDate = dates.stream()
                            .min( Comparator.comparing( LocalDate::toEpochDay ) )
                            .get();

Ответ 5

class User implements Comparable<User> {
    int UserId;
    String name;
    Date date;
    //all your getter and setters go here 

    public int compareTo(User usr1) {
         return this.getDate().compareTo(usr1.getDate());
    }
}

Затем используйте

Collections.sort(YourUserList);