create a unique list by using loop

Question

I have this master and detail object to store header and detail list:

public class MasterDetail {
    private Master master;
    private List<Detail> details;
}

Now I am trying to create a unique list where master - trx date is distinct.

Example input list:

1 | 22/11/2012 | 10.00
2 | 22/11/2012 | 10.00
3 | 23/11/2012 | 11.00
4 | 23/11/2012 | 12.00

The out I needed:

Master1: 22/11/2012
Detail1: 10.00
         10.00

Master2: 23/11/2012
Detail2: 11.00
         12.00

I tried:

List<MasterDetail > masterDetails = new ArrayList<>();
for(Input input: inputList) {
   // this is where to assign the input master and detail into an object
   master = new Master(input.getTransactionDate);
   details = new Detail(input.getAmount);
   MasterDetail assign = new MasterDetail(master, details);
   if (!masterDetails.isEmpty()) {
       for (int i=0; i < masterDetails.size(); i++) {
                if (masterDetails .get(i).getMaster().getTransDate().equals(input.getTransactionDate())) {
                    masterDetails.set(i, assign); // I think the logic problem here
                } else {
                    masterDetails.add(assign );
                }
            };
   } else {
      masterDetails.add(assign)
   }
}

The above code will give many duplicate records.

You are not using input for construction of you MasterDetail. master and detail do not change value — Scary Wombat
– Scary Wombat, Commented Oct 1, 2021 at 0:43
this approach is so bad... you can create a Map<Date, List<Details>> and it will be much more efficient since you won't have to iterate on all the master-details on each input you want to add. — MaxG
– MaxG, Commented Oct 1, 2021 at 0:49

rkechols · Accepted Answer · 2021-10-01 00:45:59Z

I think a good improvement would be to use a Map<TransDate, MasterDetail> to help you efficiently retrieve the MasterDetail based on the date of the Input in question.

However, I don't know the details of how you're storing dates, and if that'd work easily as the key for a map. If you wanted to stick with a List data structure for whatever reason, here's what I'd do:

List<MasterDetail> masterDetails = new ArrayList<>();
for(Input input: inputList) {
    // find if there's a MasterDetail with the date in question
    MasterDetail target = null;
    for(MasterDetail mDetail : masterDetails){
        if(mDetail.getMaster().getTransDate().equals(input.getTransactionDate()) {
        // we found it
        target = mDetail;
        break;
    }
    if(target == null) {  // never found it, so make a new one
        target = new MasterDetail(input.master, input.detail); // however you create one
        masterDetails.add(target);
    }
    target.details.add(detail);
}

There are still some details I don't know about your code, but at least that logic should make it so there's only one MasterDetail for each date.

score 1 · Accepted Answer · 2021-10-01 04:25:22Z

Try this.

record Input(LocalDate transactionDate, double amount) { }
record Master(LocalDate transDate) { }
record Detail(double amount) { }
record MasterDetail(Master master, List<Detail> details) { }

public static void main(String[] args) {
    List<Input> inputList = List.of(
        new Input(LocalDate.of(2012, 11, 22), 10.0),
        new Input(LocalDate.of(2012, 11, 22), 10.0),
        new Input(LocalDate.of(2012, 11, 23), 11.0),
        new Input(LocalDate.of(2012, 11, 23), 12.0));
   
    Map<LocalDate, List<Detail>> map = inputList.stream()
        .collect(Collectors.groupingBy(Input::transactionDate,
            Collectors.mapping(e -> new Detail(e.amount()), Collectors.toList())));

    List<MasterDetail> masterDetails = map.entrySet().stream()
        .map(e -> new MasterDetail(new Master(e.getKey()), e.getValue()))
        .toList();

    for (MasterDetail e : masterDetails)
        System.out.println(e);
}

output:

MasterDetail[master=Master[transDate=2012-11-22], details=[Detail[amount=10.0], Detail[amount=10.0]]]
MasterDetail[master=Master[transDate=2012-11-23], details=[Detail[amount=11.0], Detail[amount=12.0]]]

Or you can also do it by for loops.

    Map<LocalDate, List<Detail>> map = new HashMap<>();
    for (Input input : inputList)
        map.computeIfAbsent(input.transactionDate(),
            k -> new ArrayList<>()).add(new Detail(input.amount()));

    List<MasterDetail> masterDetails = new ArrayList<>();
    for (Entry<LocalDate, List<Detail>> entry : map.entrySet())
        masterDetails.add(new MasterDetail(new Master(entry.getKey()), entry.getValue()));

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