How to transform to create a new List such that each object in list has only one element in the nested list each using Java?

Question

I have two classes that look like:

public class A {
    String a;
    String b;
    String v;
    List<Pmt> pmtList;
}

public class Pmt {
    String id;
    String b;
    List<Transaction> trList;
}

How to transform to create a new payment list that can replace current payment list ( pmtList ) such that each payment object has only one Transaction each using Java?

Can someone please suggest how we can implement this logic? Each payment in a payment list can have multiple attributes which should not be modified. For example: if we have 5 payments in existing payment list and each payment has 2 Transactions each, then new payment list will have 10 payment objects.

Edit: @Nikolas Charalambidis answer works perfectly fine.

To set it back to xml document. Can someone pls let me know if this approach is fine

q.forEach(p->{
            //q.forEach(p->{
                BigDecimal s = pmtInf.getSum();
                short t = pmtInf.getNum();
                cs.getGH().setSum(s);
                cs.getGH().setNum((short) t);
                cs.getPmt().add(p);
                document.setCstmrCdtTrfInitn(cs);
                 /str.add(getJAXBObjectToXML(document, Doc.class));
                cs.getPmtInf().clear();
            });

Answer 1

The solution is simple. You need to iterate through each payment ( Ptm ) and then through each transaction ( Transaction ) while retaining the reference to the payment ( Ptm ) for creating the identical one with a single-item list of the transaction ( Transaction ). There are three basic ways I can come up with from scratch and they don't differ in principle:

Java 7 and earlier (though this solution is ok to be used in the later versions):

List<Pmt> list = new ArrayList<>();
    pmtList.forEach(p ->
        p.getTrList().forEach(tr ->
            list.add(new Pmt(p.getId(), p.getB(), singletonList(tr)))));

Java 8 - Java 15 (though this solution is ok to be used in the later versions):

List<Pmt> list = pmtList.stream()
    .flatMap(p -> p.getTrList()
        .stream()
        .map(tr -> new Pmt(p.getId(), p.getB(), singletonList(tr))))
    .collect(Collectors.toList());

Java 16 and newer:

List<Pmt> list = pmtList.stream()
    .mapMulti((Pmt p, Consumer<Pmt> c) -> {
        p.getTrList().forEach(tr ->
            c.accept(new Pmt(p.getId(), p.getB(), singletonList(tr))));
    })
    .toList();