使用集合对具有空值的多个参数进行排序

Question

I wanted to sort list of records in my using multiple comparators. It contains null values as well for some fields. I used java collections, to sort it, but it works only for one filed which is places at top.

My code:

Collections.sort(userListResult, new Comparator<UserDto>()
{
    @Override
    public int compare(UserDto userDto1, UserDto userDto2)
    {          

        if (userDto1.getLastLoginDate() != null && userDto2.getLastLoginDate() != null && userDto2.getLastLoginDate().compareTo(userDto1.getLastLoginDate()) != 0) {
            return userDto2.getLastLoginDate().compareTo(userDto1.getLastLoginDate());
        }
        if (userDto1.getLastLoginDate() != null && userDto2.getLastLoginDate() == null) {
            return -1;
        }
        if (userDto1.getLastLoginDate() == null && userDto2.getLastLoginDate() != null) {
            return 1;
        }

        if (userDto1.getLastActionDate() != null && userDto2.getLastActionDate() != null && userDto2.getLastActionDate().compareTo(userDto1.getLastActionDate()) != 0) {
            return userDto2.getLastActionDate().compareTo(userDto1.getLastActionDate());
        }
        if (userDto1.getLastActionDate() == null && userDto2.getLastActionDate() == null) {
            return 0;
        }
        if (userDto1.getLastActionDate() == null && userDto2.getLastActionDate() != null) {
            return 1;
        }
        if (userDto1.getLastActionDate() != null && userDto2.getLastActionDate() == null) {
            return -1;
        }

        if (userDto1.getLastName() != null && userDto2.getLastName() != null && userDto1.getLastName().compareTo(userDto2.getLastName()) != 0) {
            return userDto1.getLastName().compareTo(userDto2.getLastName());
        }
        if (userDto1.getLastName() != null && userDto2.getLastName() == null) {
            return -1;
        }
        if (userDto1.getLastName() == null && userDto2.getLastName() != null) {
            return 1;
        }
        if (userDto1.getFirstName().compareTo(userDto2.getFirstName()) != 0) {
            return userDto1.getFirstName().compareTo(userDto2.getFirstName());
        }
        if (userDto1.getFirstName() != null && userDto2.getFirstName() == null) {
            return -1;
        }
        if (userDto1.getFirstName() == null && userDto2.getFirstName() != null) {
            return 1;
        }
        return 0;
    }
});*/

I wanted to sort descending of LastLoginDate, LastActionDate. Either or both fields might be null in some cases. And ascending of names. And to display all other null field values(LastLoginDate, LastActionDate) at last. But when i sort, with this code it takes only the first defined method to sort. Example, if i have 3 records having same, "LastLoginDate" then it should check sort with "LastActionDate" values. But its not sorted with this logic.

So i added another algorithm to sort, since some fields has null values it is not properly sorted.

My second logic:

if (userDto1.getLastLoginDate() != null && userDto2.getLastLoginDate() != null && userDto2.getLastLoginDate().compareTo(userDto1.getLastLoginDate()) != 0) {
        compareLastLoginDate = userDto2.getLastLoginDate().compareTo(userDto1.getLastLoginDate()); 
    }
         if (userDto1.getLastActionDate() != null && userDto2.getLastActionDate() != null && userDto2.getLastActionDate().compareTo(userDto1.getLastActionDate()) != 0) {
                compareLastActionDate = userDto2.getLastActionDate().compareTo(userDto1.getLastActionDate());
            }
         if (userDto1.getLastName() != null && userDto2.getLastName() != null && userDto1.getLastName().compareTo(userDto2.getLastName()) != 0) {
                compareLastName = userDto1.getLastName().compareTo(userDto2.getLastName());
            }
         if (compareLastLoginDate == 0) {
             return ((compareLastActionDate == 0) ? compareLastName : compareLastActionDate);
         }
         else {
             return compareLastLoginDate;
         }

Here i am getting the integer values to compare it "compareLastActionDate, compareLastName, compareLastLoginDate".

Answer 1

I would recommend you to use the java8 comparators, it provides some helpful methods to compare based in multiple attributes with thenComparing() .

If you don't want to / can't use, try two wrap part of that repetitive code in one or multiple custom Comparator or in a helper method.

Here, for simplicity , the following helper method is used ( in java8 you can use method references ):

public static <R extends Comparable<? super R>> int nullSort(R r1, R r2,
                                                             boolean ascending,
                                                             boolean nullsFirst) {
    R rs1 = ascending ? r1 : r2, rs2 = ascending ? r2 : r1;
    if (rs1 != null && rs2 != null) return rs1.compareTo(rs2);
    else if (r1 == null && r2 == null) return 0;
    else if (r1 == null) return (nullsFirst ? -1 : 1);
    else return (nullsFirst ? 1 : -1);
}

In your method, you could use this nullSort() method, to check first for lastLoginDate descending and nulls last (false, false) , then lastActionDate again descending and nulls last.

A difference with your comparator is that you stop evaluating here when both are null. It would be better that when lastLoginDate and lastActionDate are nulls you keep ordering by lastName and firstName . If you don't want this, you can add if (result == 0) return 0; after the second if in the method below.

Then after the dates are compared, you compare lastName and then firstName both ascending and nulls last (true, false) .

@Override
public int compare(UserDto dto1, UserDto dto2) {
    int result;
    if ((result = nullSort(dto1.getLastLoginDate(), dto2.getLastLoginDate(), 
            false, false)) != 0) return result;
    if ((result = nullSort(dto1.getLastActionDate(), dto2.getLastActionDate(), 
            false, false)) != 0) return result;
    if ((result = nullSort(dto1.getLastName(), dto2.getLastName(), 
            true, false)) != 0) return result;
    if ((result = nullSort(dto1.getFirstName(), dto2.getFirstName(), 
            true, false)) != 0) return result;
    return 0;
}

Answer 2

I used simple date format to filter the list. Here is my working code,

public int compare(UserDto userDto1, UserDto userDto2) {
    DateFormat format = new SimpleDateFormat("yyyy-MM-dd");

    if (userDto1.getLastLoginDate() != null && userDto2.getLastLoginDate() != null && userDto2.getLastLoginDate().compareTo(userDto1.getLastLoginDate()) != 0) {
        String Date1 = format.format(userDto1.getLastLoginDate());
        String Date2 = format.format(userDto2.getLastLoginDate());
        if (Date1.equals(Date2)) {
            if (userDto1.getLastActionDate() != null && userDto2.getLastActionDate() != null && userDto2.getLastActionDate().compareTo(userDto1.getLastActionDate()) != 0) {
                String Date3 = format.format(userDto1.getLastActionDate());
                String Date4 = format.format(userDto2.getLastActionDate());
                if (Date3.equals(Date4)) {
                    if (userDto1.getLastName() != null && userDto2.getLastName() != null && userDto1.getLastName().compareTo(userDto2.getLastName()) != 0) {
                        return userDto1.getLastName().compareTo(userDto2.getLastName());
                    }
                } else {
                    return userDto2.getLastActionDate().compareTo(userDto1.getLastActionDate());
                }
            }
        } else {
            return userDto2.getLastLoginDate().compareTo(userDto1.getLastLoginDate());
        }

    }
    if (userDto1.getLastLoginDate() != null && userDto2.getLastLoginDate() == null) {
        return -1;
    }
    if (userDto1.getLastLoginDate() == null && userDto2.getLastLoginDate() != null) {
        return 1;
    }
    if (userDto1.getLastLoginDate() == null && userDto2.getLastLoginDate() == null) {
        return userDto1.getLastName().compareTo(userDto2.getLastName());
    } }