值作为 HashMap 中对象的数组列表

Question

I want to create a HashMap that stores key as an Integer value which is the ID of a restaurant. The value should be a List of the Restaurant objects. But my IDE is not happy with the way I am doing it while adding the restaurant object to the List. Here is my code:

public List getTopPerformers(List<RestaurantInfo> restaurants){

    HashMap <Integer, List<RestaurantInfo>> map = new HashMap<Integer,
                                             List< RestaurantInfo>>();
             // Key is restaurant ID. Value is the Object of Class RestaurantInfo
    List<RestaurantInfo> ll;
    for(RestaurantInfo restaurant: restaurants){

        map.put(restaurant.cityId, ll.add(restaurant));

    }
}

My Restaurant class has properties as cityId, orderCount and restaurantId.

The map.put(restaurant.cityId, ll.add(restaurant)); line gives error as follows and obviously it will never compile.

no suitable method found for put(int,boolean)
method HashMap.put(Integer,List<RestaurantInfo>) is not applicable

(actual argument boolean cannot be converted to List by method invocation conversion)

Answer 1

ll.add(restaurant) returns boolean.

So, when you do:

map.put(restaurant.cityId, ll.add(restaurant));

you are trying to add (int, boolean) to a map of type: (Integer,List)

Also, below code will add all restaurants to every cityid:

List<RestaurantInfo> ll = new List<RestaurantInfo>();
for(RestaurantInfo restaurant: restaurants){
    ll.add(restaurant);
    map.put(restaurant.cityId, ll);
}

I think what you need is:

List<RestaurantInfo> ll;
for (RestaurantInfo restaurant: restaurants) {
  // If restaurant is from the same city which is present in the map then add restaurant to the existing list, else create new list and add.
  if (map.containsKey(restaurant.cityId)) {
    ll = map.get(restaurant.cityId);
  } else {
    ll = new List<RestaurantInfo>();
  }
  ll.add(restaurant);
  map.put(restaurant.cityId, ll);
}

Answer 2

  map.put(restaurant.cityId, ll.add(restaurant));

In this statement,

ll.add(restaurant)

return value for add operation is boolean , which is why you are getting that error.

What you may need to do would be something like:

ll.add(restaurant);
map.put(restaurant.cityId, ll);

Answer 3

add(E) function of a collection returns boolean: true If the data E is added and the collection structure has been changed (Returns false if this collection does not permit duplicates and already contains the specified element).

Hence:

for(RestaurantInfo restaurant: restaurants){
        map.put(restaurant.cityId, ll.add(restaurant));
    }

is essentially equivalent to:

for(RestaurantInfo restaurant: restaurants){
            map.put(restaurant.cityId, boolean);
        }

So, First add the resutaurant instances to list ll one by one and then add ll list instance to the map .

You might want to do something like this:

RestaurantInfo restaurant =  resturants.get(0);
int cityId = restaurant.cityId;

List<RestaurantInfo> ll = new ArrayList<>();

for(RestaurantInfo restaurant: restaurants){
            ll.add(restaurant);
        }

 map.put(cityId, ll);