How to sum values in a Map with a stream?
I want the equivalent of this with a stream:
public static <T extends Number> T getSum(final Map<String, T> data) {
T sum = 0;
for (String key: data.keySet())
sum += data.get(key);
return sum;
}
This code doesn't actually compile because 0 cannot be assigned to type T, but you get the idea.
Solution 1:
You can do this:
int sum = data.values().stream().mapToInt(Integer::parseInt).sum();
Solution 2:
Here's another way to do this:
int sum = data.values().stream().reduce(0, Integer::sum);
(For a sum to just int
, however, Paul's answer does less boxing and unboxing.)
As for doing this generically, I don't think there's a way that's much more convenient.
We could do something like this:
static <T> T sum(Map<?, T> m, BinaryOperator<T> summer) {
return m.values().stream().reduce(summer).get();
}
int sum = MyMath.sum(data, Integer::sum);
But you always end up passing the summer. reduce
is also problematic because it returns Optional
. The above sum
method throws an exception for an empty map, but an empty sum should be 0. Of course, we could pass the 0 too:
static <T> T sum(Map<?, T> m, T identity, BinaryOperator<T> summer) {
return m.values().stream().reduce(identity, summer);
}
int sum = MyMath.sum(data, 0, Integer::sum);
Solution 3:
Integer count = intMap.values().stream().mapToInt(d-> d).sum();
Get the HashMap.values
method to Stream and collect mapToIn
with lambda expression ending with aggregation sum method.