[java] How can I turn a List of Lists into a List in Java 8?

I just want to explain one more scenario like List<Documents>, this list contains a few more lists of other documents like List<Excel>, List<Word>, List<PowerPoint>. So the structure is

class A {
  List<Documents> documentList;
}

class Documents {
  List<Excel> excels;
  List<Word> words;
  List<PowerPoint> ppt;
}

Now if you want to iterate Excel only from documents then do something like below..

So the code would be

 List<Documents> documentList = new A().getDocumentList();

 //check documentList as not null

 Optional<Excel> excelOptional = documentList.stream()
                         .map(doc -> doc.getExcel())
                         .flatMap(List::stream).findFirst();
 if(excelOptional.isPresent()){
   Excel exl = optionalExcel.get();
   // now get the value what you want.
 }

I hope this can solve someone's issue while coding...

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