Java 值作为 HashMap 中对象的数组列表
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Values as an arraylist of Objects in HashMap
提问by noobcoder
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:
我想创建一个 HashMap 将键存储为整数值，即餐厅的 ID。该值应该是餐厅对象的列表。但是我的 IDE 对我在将餐厅对象添加到列表时的做法不满意。这是我的代码：
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.
我的餐厅类具有 cityId、orderCount 和 restaurantId 等属性。
The map.put(restaurant.cityId, ll.add(restaurant));line gives error as follows and obviously it will never compile.
该map.put(restaurant.cityId, ll.add(restaurant));行给出如下错误，显然它永远不会编译。
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)
（不能通过方法调用转换将实参boolean转成List）
采纳答案by Akshay
ll.add(restaurant) returns boolean.
ll.add(restaurant) 返回布尔值。
So, when you do:
所以，当你这样做时：
map.put(restaurant.cityId, ll.add(restaurant));
you are trying to add (int, boolean) to a map of type: (Integer,List)
您正在尝试将 (int, boolean) 添加到类型为：(Integer,List) 的映射中
Also, below code will add all restaurants to every cityid:
此外，以下代码会将所有餐厅添加到每个 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:
我认为你需要的是：
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);
}
回答by kosa
map.put(restaurant.cityId, ll.add(restaurant));
In this statement,
在这份声明中，
ll.add(restaurant)
returnvalue for add operation is boolean, which is why you are getting that error.
return添加操作的值是boolean，这就是您收到该错误的原因。
What you may need to do would be something like:
您可能需要做的是：
ll.add(restaurant);
map.put(restaurant.cityId, ll);
回答by Sage
add(E)function of a collectionreturns boolean: trueIf the data Eis added and the collectionstructure has been changed (Returns falseif this collection does not permit duplicates and already contains the specified element).
add(E)集合的函数返回布尔值：true如果E添加了数据并且集合结构已更改（false如果此集合不允许重复并且已经包含指定的元素，则返回）。
Hence:
因此：
for(RestaurantInfo restaurant: restaurants){
map.put(restaurant.cityId, ll.add(restaurant));
}
is essentially equivalent to:
本质上等同于：
for(RestaurantInfo restaurant: restaurants){
map.put(restaurant.cityId, boolean);
}
So, First add the resutaurantinstances to list llone by one and then add lllist instance to the map.
因此，首先将resutaurant实例添加到列表中ll，然后将ll列表实例添加到map.
You might want to do something like this:
你可能想做这样的事情：
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);
