如何使用Java 8 Lambdas将对象列表转换为Map <Object，Object>

Question

I have List of Objects say Car which Needs to be converted to Map.

    Public Class Car {
        private Integer carId;
        private Integer companyId;
        private Boolean isConvertible;
        private String carName;
        private String color;
        private BigDecimal wheelBase;
        private BigDecimal clearance;
    }

I have another object which I want to treat as key of Map.

    public class Key<L, C, R> {
        private L left;
        private C center;
        private R right; 
   }

I want to create a map from List of Car objects.

List<Car> cars;
Map<Key, Car> -> This map contains Key object created from 3 field of Car object namely carId, companyId, isConvertible.

I am unable to figure out how to do this using Java 8 Lambda

cars.stream.collect(Collectors.toMap(?, (c) -> c);

In above statement, in place of ?, I want to create object of Key class using values present in current car object. How can I achieve this?

Answer 1

You can do:

Function<Car, Key> mapper = car -> new Key(car.getId(), 
                                           car.getCompanyId(), 
                                           car.isConvertible());
cars.stream().collect(Collectors.toMap(mapper, Function.identity());

Answer 2

cars.stream().collect(Collectors.toMap(c->new Key(c.getId(),...), Function.identity());

Answer 3

You can also try this :

    Map<Key, Car> carMapper = new HashMap<>();

    carlist.forEach(car -> carMapper.put(new Key(car.getCarId(), car.getCompanyId(), car.getIsConvertible()), car));