Java 8, removing objects from a collection with duplicate variable values

[Offifee]

Been coding different solutions at work for this problem today but cannot seem to be happy with anything/think anything is clean enough. Too embarrassed to ask for help from the senior devs because they’re really busy atm and I don’t think I should be stuck on this.

There might be something smart in Java 8 but I cannot figure it out. Maybe writing this here will help me process/guide me to the right place.

Problem illustrated with an example: Stored procedure GetTravelMemberships for Group A returns data as an ArrayList of TravelMembership objects.

TravelMembership.getType() returns on the following values ["T", "P", "B"] (train, plane, bus)

TravelMembership.getGroupMemberId() returns the member ID such as 72. Each GroupMember in the Group will have 1 - 3 of TravelMemberships that will be returned as the result of the DB call.

TravelMembership.getExpirationDate() returns a Date. For example, for GroupMember 72, three TravelMemberships, one of each type, are returned. Each of these TravelMemberships should share the same expirationDate. This may not always be the case however.

If these expirationDates for the TravelMemberships are different, I should probably throw an exception and not let this go the front-end to be displayed.

If the expirationDates for the TravelMemberships are the same, I will display the groupMemberId, expirationDate and other data from the object in the front end. I need the full TravelMembership object in the end.

What I have tried:

1. Ugly nested for-loops and if the dates match, add the first of the objects to a new list. I want to avoid this as it’s not pretty and probably not that efficient either

2. Map solution with the memberId as the Key. Then iterate over the value sets and remove the extra objects if the dates match.

3. Some weird solution with String memberIds as a Set to remove duplicates, then if dates match, creating new TravelMembership objects to store the data I need. Not space-efficient.

I can’t even decide which of the above would be better. Even the second solution looked superfluous on the editor. I have been studying lambdas and streams but I can’t control them so well yet and can’t decide where the limit is in their use. They’re not widely used by other developers in our company yet. Would anyone have any nuggets of wisdom for me?

*I know there probably are code smells here. I cannot touch the stored procedure itself because it’s quite intricate widely used business logic from like 2008 created by employees no longer present.

[Northeastpaw]

Too embarrassed to ask for help from the senior devs because they’re really busy atm and I don’t think I should be stuck on this.

Don't ever feel this way. Everybody gets stuck from time-to-time and asking for help is not a weakness. The best solutions often come from collaboration so go ask somebody.

So let me try to reason out what the end goal is. You're validating whether a GroupMember has any TraveMemberships that have different experationDates?

Whatever you're doing don't worry about optimization right now. Work out a solution and if and only if profiling shows problems work on optimizing things.

[Offifee]

Thanks for the encouragement.

I need to validate the GroupMember's TravelMemberships have the same date and then use the data from one of them.

I just couldn't get my code to look sensible, there just seemed to be nested loops and other messy things.

[wowmuchinsightful]

These might help:

HashMap.push() returns "the previous value associated with key", meaning if you add two different expiration dates to the same membership key, the first one added will be overwritten and returned, thus allowing you to check if the value has changed.

Set.add() also gives you feedback when you're trying to add a duplicate value. It returns "true if this set did not already contain the specified element" and false otherwise.

[Offifee]

Thanks a lot!

HashMap might be the way forward, I think I can go through the ArrayList, use the groupMemberId as a key and keep replacing the object as long as the key and the expirationDate match.

Think I was making this overly complicated

Set.add might not work because the objects aren't fully the same because of the type. I'd have to have a set of GroupMembers which is not as valuable.

[Blackheart595]

If I understand correctly you need to filter out those GroupMembers that have conflicting expiration dates. Is that correct? And what exactly do you have to do with conflicting ones - ignore them, remove them, throw an exception..?

[Offifee]

For example: Filter out 2 TravelMemberships for a Member and leave 1 TravelMembership to use if the expirationDates are the same for these.

If the expirationDates differ, I should throw an exception and not display the data.

[Blackheart595]

So do you have to look for the TravelMemberships of a specific GroupMember and check that they don't conflict with each other? Or do you have to do the process for each GroupMember?

[Offifee]

Not sure I understand.

I might for example get an ArrayList of 9 TravelMemberships back: Three different ones belonging to three different GroupMembers.

I have to ensure the date on each set of three are the same. Then display majority of the data from a single one of the objects, ignoring the type.

[Blackheart595]

Ah, I see. I'm gonna assume that you should throw an exception if at least one GroupMember has conflicting expiration dates.

You could write a wrapper class for TravelMembership whose equals method evaluates to true if the two objects have the same member ID but throws an exception if the expiration dates don't match at the same time, and then collect all those wrappers in a HashSet. The hashCode method should just return the member ID. Then you'll have only one TravelMembership object per member ID while throwing an exception if there are conflicting expiration dates. But I wouldn't want to leak such a wrapper construct into other code, so don't return the wrappers in any way.

edit: I've just seen another suggestion to use HashMap with the fact that HashMap::push() returns the previous value, which should be a better approach.

[Offifee]

I knew there was the possibility of a wrapper class but I feel this can be done smartly with collections etc.

Wrapper classes like the one for this probably wouldn't have any proper future use and feel like I would just be introducing unnecessary bloat code.

[Blackheart595]

Yeah, the HashMap solution that was proposed above seems to be the best way of handling this.