How to iterate a List<List<String>> and create a single list?

Question

I am working on a legacy code which has a List<Student> studentList.

Each student object contains two properties:

• String name;
• List<Address> address;

Class Address contains two properties houseNumber and streetNumber

I want to create a single List which contains name, houseNumber, streetNumber.

houseNumber is an enum so I'll do .toString() and an if condition for null values.

final List<String> finalList = new ArrayList<>();
studentList
        .forEach(x -> {
                    x.getAddressList()
                            .forEach(y -> {
                                finalList.add(x.getName);
                                finalList.add(y.getHouseNumber != null ?
                                        y.getHouseNumber.toString() : "");
                                finalList.add(y.getStreetNumber);
                            });
                }
        );
        

I have written this code but I want it to achieve the result by using flatMap.

Answer 1

May you want this:

var rs = lst.stream().flatMap(x -> x.getAddress().stream()
            .map(y -> List.of(x.getName(), 
                y.getHouseNumber() != null ? y.getHouseNumber().toString() : "",
                y.getStreetNumber())
         )).collect(Collectors.toList());

rs is list of list contains name, houseNumber, streetNumber.