使用 lambda 遍历多个嵌套列表

Question

How to traverse through a list while already traversing through another list using lambdas? I have 1 car Object, a list of Something objects which I am traversing and while doing that I need to traverse through every Wheel object. The output is a list of a different object Clean.

    Car car = new Car();
    List<Something> somethings = new ArrayList<Something>();
    //say we have multiple items in this List
    List<Clean> cleanList =
             somethings.stream()
                    .map(something -> {
                        return car.wheels.stream().map(wheel -> something.clean(car, wheel));                        })
                    .collect(Collectors.toList());

For the following Objects,

class Car {

    String carId;

    String carName;

    List<Wheel> wheels;
}

class Wheel {

    String wheelId;

    String wheelType;
}

class Something {

    String somethingId;

    String somethingType;

    public Clean clean(Car car, Wheel wheel) {
        return new Clean();
    }
}

class Clean {

    String cleanId;

    String cleanType;
}

I want to traverse through every Something for all Wheels of the Car. I tried doing this but it doesn't work.

Answer 1

You would need to flatten it, so a flatMap will do

List<Clean> cleanList =
            somethings.stream()
                    .flatMap(something -> car.wheels.stream().map(wheel -> something.clean(car, wheel)))
                    .collect(Collectors.toList());