multi level filtering by index of customized list in java 11

Question

Instead of adding and updating these extra sequential number fields, I found useful two stage solution when processing well sorted multi-level collections for GUI (where row indexes are calculated on fly):

ParentEntity filterByParentAndChildListsIndexes(List<ParentEntity> parents, Integer prntIdx, Integer chldIdx){
   AtomicInteger idx = new AtomicInteger(1); // may be '0'
   Optional<ParentEntity> parentOp = parents.stream().filter(p -> idx.getAndIncrement() == prntIdx).findFirst();
   if (parentOp.isPresent()){
      return childFilter(parentOp.get(), chldIdx);
   }
   return null;
}

ChildEntity childFilter(ParentEntity parent, Integer chldIdx){
   AtomicInteger idx = new AtomicInteger(1); // may be '0'
   return parent.getChildEntities().stream().filter(c -> idx.getAndIncrement() == chldIdx).findFirst().orElse(null);
}

but it seems redundant and not flexible... so, I would be grateful for suggestions on existing solutions or suggestions for recursion of such multilevel filters.

Answer 1

You can do it like this:

parents.stream()
        .skip(prntIdx)
        .limit(1)
        .map(ParentEntity::getChildEntities)
        .flatMap(Collection::stream)
        .skip(chldIdx)
        .findFirst()
        .orElse(null)

Or just use List.get():

if (prntIdx < parents.size()) {
    List<ChildEntity> children = parents.get(prntIdx).getChildEntities();
    if (chldIdx < children.size()) {
        return children.get(chldIdx);
    }
}
return null;