sort by two fields

[Moses Marfo]

Hi
I will like to sort an object having two instance variables(location and event date).

Location can be blank. I will like all objects with blank locations to appear at the top of the list, sorted by event date.

How do I do this?

What I did was implemet Comparator and pass it to "Collections.sort" along with the list. I was able to get records without value for location to appear at the top
by returning 1 from the compare method if either object being compare has a blank location but I'm unable to sort the blank location records by event date.

Below is the compare method

 
public int compare(EventAuditReportVO object1,
				EventAuditReportVO object2) {
if("OfficeLocation".equals(reportArgs.getSortBy())){
				if(StringUtils.isNotBlank(object1.getUserOffice()) && StringUtils.isNotBlank(object2.getUserOffice())){
					compare = object1.getUserOffice().compareToIgnoreCase(object2.getUserOffice());
				}
				if(StringUtils.isBlank(object1.getUserOffice()) || StringUtils.isBlank(object2.getUserOffice())){
					compare = 1;
				}
				if(compare==0){
					compare = object1.getEventDateTime().compareTo(object2.getEventDateTime());
				}
			}  
}
 

[Rob Spoor]

Obviously that's not your entire code; after all, this method won't compile.

How comparing on multiple fields usually works is as follows:

int compare = <first comparison>;
if (compare == 0)
{
    compare = <second comparison>;
}
if (compare == 0)
{
    compare = <third comparison>;
}
// etc
return compare; // returns 0 if all comparisons return 0

Note: you can nest the ifs, but that may cause nesting ifs too deeply

In your case, the first comparison is somewhat like this:

boolean blank1 = StringUtils.isNotBlank(object1.getUserOffice());
boolean blank2 = StringUtils.isNotBlank(object2.getUserOffice());
int compare = (blank1 ? (blank2 ? 0 : -1) : (blank2 ? 1 : object1.getUserOffice().compareToIgnoreCase(object2.getUserOffice())));

You may feel uncomfortable with using nested tertiary operators; you can use simple if statements instead. All that matters is the following matrix:
- blank1 && blank2: both are blank so 0
- blank1 && !blank2: only the first one is blank, so the first one goes before the second one. -1
- !blank1 && blank2: only the second one is blank, so the second one goes before the first one. 1
- !blank1 && !blank2: neither is blank so compare the two

[Jim Hoglund]

Another approach would be to add a key or index attribute (on the way in
to the collection) that combines the other attributes in proper order. Then
a simple one field sort could be used.

Jim...

[Moses Marfo]

thanks Rob for the solution
the list is being sorted properly now

Jim ........can you please expound on the alternative solution

[Jim Hoglund]

Moses: How many location/eventDate pairs are you working with,
do you have a storage structure in mind, and how dynamic is the
list; how frequently are pairs added, changed or removed? It would
be helpful to know a bit more about the context of your questions?

Jim... (and greetings to Rob, our fine moderator.)

[Moses Marfo]

Jim
The program is an event audit report which allows event records retrieved from database to be sorted based on entered creteria.
The Object that I'm using to store the retrieved records consist of more than just location and event date, there is also user name and event description
The List of records consist of two lists obtained from database using two separate queries. The number of records in the list can range from 0 - 1000 or more.
The list is not updated once retrieved from the database, it is just for display purposes. I hope that is sufficient information.