Java How To Keep Unique Values In A List of Objects

Question

I currently am retrieving a list of objects List<NprDto> (The NprDto class contains accountId, theDate1, and theDate2) from a query that returns results where the NprDto has duplicate accountIds. I need to have a List<NproDto> of only unique accountIds but keep the object. It only needs to add the first accountId it comes across and ignores the rest.

I'm currently trying this:

private List<NprDto> getUniqueAccountList(List<NprDto> nonUniqueAccountList) throws Exception {

    Map<Long,NprDto> uniqueAccountsMapList = new HashMap<Long,NprDto>();
    List<NprDto> uniqueAccountsList = null;

    if(nonUniqueAccountList != null && !nonUniqueAccountList.isEmpty()) {
        for(NprDto nprDto : nonUniqueAccountList) {
            uniqueAccountsMapList.put(Long.valueOf(nprDto.getAccountId()), nprDto);
        }
    }

    uniqueAccountsList = new ArrayList<NprDto>(uniqueAccountsMapList.values());

    return uniqueAccountsList;

}

But this doesn't seem to be working because when I iterate through the returned uniqueAccountsList later it only picks up the first object.

Any help would be greatly appreciated.

Are you sure you have unique account numbers? I think your design is good (saves a lot of the work you need to do a Set), and your code looks good too. I'm betting your problem is somewhere else. — RalphChapin
– RalphChapin, Commented Nov 21, 2013 at 16:57
I had an output statement in the wrong location. Everything is working fine. Thanks! — Richard
– Richard, Commented Nov 21, 2013 at 16:59

Masudul · Accepted Answer · 2015-06-26 05:05:44Z

10

I need to have a List of only unique accountIds but keep the object.

You should use Set<NprDto>. For that you need to override equals and hasCode at NproDto class.

class NprDto{
   Long accountId;
   .......

 @Override
 public boolean equals(Object obj) {
   NproDto other=(NproDto) obj;
   return this.accountId==other.accountId;
 }

 @Override
 public int hashCode() {
    return accountId.hashCode();
 }
}

Change your getUniqueAccountList as follows:

private Set<NprDto> getUniqueAccountSet(){    
  Map<Long,NprDto> uniqueAccountsMapList = new HashMap<Long,NprDto>();
  Set<NprDto> uniqueAccs = new HashSet<NprDto>(uniqueAccountsMapList.values());    
  return uniqueAccs;
}

edited Jun 26, 2015 at 5:05

answered Nov 21, 2013 at 16:29

Masudul

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Comments

Adam Arold · Accepted Answer · 2013-11-21 16:34:24Z

6

What you need here is a LinkedHashSet. It removes duplicates and keeps insertion order. You do not need TreeSet here because it sorts and changes the order of the original List.

If preserving insertion order is not important use a HashSet.

answered Nov 21, 2013 at 16:34

Adam Arold

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Community · Accepted Answer · 2017-05-23 12:18:07Z

0

Actually you need to implements equals and hascode method, it will be good for you

Remove duplicates from a list

Java Set contains the Unique value but its unsorted collection. List is sorted collection but contains the duplicates objects.

edited May 23, 2017 at 12:18

CommunityBot

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answered Nov 21, 2013 at 16:33

Asif Bhutto

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noone · Accepted Answer · 2013-11-21 16:33:05Z

-1

What you need to do is implement the equals, hashCode and compareTo methods for NprDto to match two objects as equal when their ID is the same. Then you can filter all duplicates as easy as this:

private List<NprDto> getUniqueAccountList(List<NprDto> nonUniqueAccountList) {
    return new ArrayList<NprDto>(new LinkedHashSet<NprDto>(nonUniqueAccountList));
}

answered Nov 21, 2013 at 16:33

noone

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