如何使用 Guava 的 ListMultimap 对键列表中的元素求和

Question

Example:

I want to sum the elements inside every key, which is basically an ArrayList of elements "double numbers".The Guava ArrayListMultimap is declared in this way:

ListMultimap <String, Double> multimap = LinkedListMultimap.create(); 

and displayed like this "3 keys of ArrayList":

HH_0=[0.1, 0.4, 0.2, 0.3, 0.1], 
HH_1=[0.1,0.2], 
HH_2=[0.1] 

The summation should be stored in an array or any simple structure like this output:

array_sum=[1.1,0.3,0,1]

the actual size of multimap is 8 and not 3 because if any key has more than one value in its arraylist, means the key is duplicate and that's reason of using multimap of Guava , it can store duplicated keys with multiple (different or same) values in an ArrayList with the contrary in case of a simple Hashmap.

My essay:
I extracted the keys in array_Str of Strings to compare with. We suppose the length of the wanted array_sum[ ] is already initiated with 3:

double sum = 0.0;
String array_Str = {"HH_0","HH_1","HH_2"};

       for (int i = 0; i < array_sum.length; i++) {
           for (Entry<String, Double> entry : multimap.entries())  {
               
               
           if( entry.getKey().equals(array_sum[i])) {
               
               sum=sum+ entry.getValue();
           }
               
           }
           array_sum[i]=sum;   
       }

Any guidance is very appreciated.

Answer 1

Using Java 8 streams to simplify just a little:

for (int i = 0; i < array_str.length; i++) {
  array_sum[i] = multimap.get(array_str[i]).stream().mapToDouble(d -> d).sum();
}