Java map with lists of objects to list

Question

Here is my code:

public class CarShop {
    private final HashMap<Brand, List<CarPrototype>> catalog;

    public List<CarPrototype> allPrototypes() {
        List<CarPrototype> prototypes = catalog.values().stream().collect(Collectors.toList())
        return prototypes ;
    }

What I want to do in this method is I want to get a list of all different car prototypes given in car shop catalog. I should not use for loop, so stream comes into play. My solution is adds only lists of prototypes, and I struggle finding how to change it so it add specific prototypes themselves.

That is a bizzarde to have requiremet to NOT use most common solutions to known problems since the beginning of programming. — Antoniossss
– Antoniossss, Commented Apr 6, 2020 at 17:06

Minn · Accepted Answer · 2020-04-06 17:14:23Z

3

If you have a Stream<List<Foo>> and want to convert it to List<Foo> you can use flatMap:

Stream<List<Foo>> stream = ...;
Stream<Foo> flatStream = stream.flatMap(List::stream);
List<Foo> list = flatStream.collect(Collectors.toList());

For your specific example:

public List<CarPrototype> allPrototypes() {
    List<CarPrototype> prototypes = catalog.values().stream()
        .flatMap(List::stream)
        .collect(Collectors.toList());
    return prototypes;
}

answered Apr 6, 2020 at 17:14

Minn

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