How to Sum a List of Integers with Java Streams

How to sum a list of integers with java streams?

This will work, but the i -> i is doing some automatic unboxing which is why it "feels" strange. mapToInt converts the stream to an IntStream "of primitive int-valued elements". Either of the following will work and better explain what the compiler is doing under the hood with your original syntax:

integers.values().stream().mapToInt(i -> i.intValue()).sum();
integers.values().stream().mapToInt(Integer::intValue).sum();

Java 8 Streams - Why can't I sum a Stream of Integers?

Calling stream() on a List will get you a general-purpose Stream, which can handle any reference type, not just numeric types. It doesn't make sense to include a sum method on Stream, because it doesn't make sense to sum URLs, Classes, Threads, or any other non-numeric reference type.

You could sum the elements by calling reduce:

numbers.stream().reduce(0, (a, b) -> a + b)

But that would involve a lot of unboxing and boxing. It would be best to sum them as you have it, by converting it to an IntStream, which operates on ints, and calling sum() (or summaryStatistics(), which includes count, average, max, and min along with sum).

You could even use IntStream.of and avoid boxing the values even once.

IntStream.of(1, 2, 3).sum()

How to sum every elements in a list of a list using stream?

You can do it like this:

List<List<Integer>> cases = Arrays.asList(Arrays.asList(1, 1, 2), Arrays.asList(3, 3, 2), Arrays.asList(4, 5, 1));

List<Integer> result = cases.stream()
        .map(list -> list.stream().mapToInt(Integer::intValue).sum())
        .collect(Collectors.toList());

System.out.println(result); // print [4, 8, 10]

Sum only first few values of the list using Java8

Use limit() as shown below

int sum = list.stream().sorted().limit(3).mapToInt(Integer::intValue).sum();

How to sum the values in Listint[] using Java 8

You want to flatMap to an IntStream. After that, taking the sum is easy.

int sum = counts.stream()
        .flatMapToInt(IntStream::of)
        .sum();

Calculating List of cumulative sums from List of integers with Java streams

Try this.

int[] a = {4, 5, 8, -11, 9, 5, -7, 4, 6, -6, -8, -11, 80, -32, -56, -15, 5, -49};
Arrays.parallelPrefix(a, (x, y) -> x + y);
System.out.println(Arrays.toString(a));

output:

[4, 9, 17, 6, 15, 20, 13, 17, 23, 17, 9, -2, 78, 46, -10, -25, -20, -69]

Java map / collect for Stream to sum List of Lists

Because you have a List<List<Integer>>, it seems that a vertical sum should produce a List<Integer>, as each column will have its own sum. Assuming every row in the List<List<Integer>> contains the same number of elements, it would be a good idea to first find the transpose of the List<List<Integer>>, sum every row, and collect it back into a List<Integer>. You can do it using the following:

List<List<Integer>> list = Arrays.asList(
    Arrays.asList(1, 2, 3, 4),
    Arrays.asList(5, 6, 7, 8)
);

List<Integer> sumList = IntStream.range(0, list.get(0).size())
                                 .mapToObj(i -> list.stream()
                                                    .mapToInt(l -> l.get(i))
                                                    .sum())
                                 .collect(Collectors.toList());

System.out.println(sumList);

Output:

[6, 8, 10, 12]

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