How to split odd and even numbers and sum of both in a collection using Stream

Question

How can I split odd and even numbers and sum both in a collection using stream methods of Java 8?

public class SplitAndSumOddEven {

    public static void main(String[] args) {

        // Read the input
        try (Scanner scanner = new Scanner(System.in)) {

            // Read the number of inputs needs to read.
            int length = scanner.nextInt();

            // Fillup the list of inputs
            List<Integer> inputList = new ArrayList<>();
            for (int i = 0; i < length; i++) {
                inputList.add(scanner.nextInt());
            }

            // TODO:: operate on inputs and produce output as output map
            Map<Boolean, Integer> oddAndEvenSums = inputList.stream(); // Here I want to split odd & even from that array and sum of both

            // Do not modify below code. Print output from list
            System.out.println(oddAndEvenSums);
        }
    }
}

Cleancut

Please provide some code to show what you have done so far. what have you attempted. where are you getting suck at – Adelladella

pls see updated question now! @MarquisBlount – Cleancut

Answer 1

You can use Collectors.partitioningBy which does exactly what you want:

Map<Boolean, Integer> result = inputList.stream().collect(
       Collectors.partitioningBy(x -> x%2 == 0, Collectors.summingInt(Integer::intValue)));

The resulting map contains sum of even numbers in true key and sum of odd numbers in false key.

Deportment

How can it be changed to return the count of odd vs even instead of the sum? Collectors.counting() does not seem to work. – Brunswick

@NoviceUser counting() returns long, so you need to change to Map<Boolean, Long> – Deportment

Answer 2

It's easiest (and cleanest) to do it in two separate stream operations, like such:

public class OddEvenSum {

  public static void main(String[] args) {

    List<Integer> lst = ...; // Get a list however you want, for example via scanner as you are. 
                             // To test, you can use Arrays.asList(1,2,3,4,5)

    Predicate<Integer> evenFunc = (a) -> a%2 == 0;
    Predicate<Integer> oddFunc = evenFunc.negate();

    int evenSum = lst.stream().filter(evenFunc).mapToInt((a) -> a).sum();
    int oddSum = lst.stream().filter(oddFunc).mapToInt((a) -> a).sum();

    Map<String, Integer> oddsAndEvenSumMap = new HashMap<>();
    oddsAndEvenSumMap.put("EVEN", evenSum);
    oddsAndEvenSumMap.put("ODD", oddSum);

    System.out.println(oddsAndEvenSumMap);
  }
}

One change I did make was making the resultant Map a Map<String,Integer> instead of Map<Boolean,Integer>. It's vey unclear what a key of true in the latter Map would represent, whereas string keys are slightly more effective. It's unclear why you need a map at all, but I'll assume that goes on to a later part of the problem.

Bottom

An enum would be even better than strings. – Darbies

Answer 3

    List<Integer> numberList = Arrays.asList(1,3,4,60,12,22,333456,1239);
    Map<Boolean, List<Integer>> evenOddNumbers = numberList.stream()
            .collect(Collectors.partitioningBy( x -> x%2 == 0));
    System.out.println(evenOddNumbers);

Output:

{false=[1, 3, 1239], true=[4, 60, 12, 22, 333456]}

and if want to separate it out the use below one:

List<Integer> evenNums = partitions.get(true);
List<Integer> oddNums = partitions.get(false);

Rad

Answer 4

Easiest way to find even numbers from list and then performing addition of even numbers using Stream :

int number = l.stream().filter(n->n%2==0).mapToInt(n -> n).sum();
        System.out.println(number);
    
    

Blub

Answer 5

Below is the two use case of separating the ODD and EVEN numbers from an Integer array -

..............................................................................

    int[] arr = {45,23,6,8,9,2};

Map<Boolean, List<Integer>> strm = Arrays.stream(arr).boxed().collect(Collectors.partitioningBy(i -> i%2 == 0));  
System.out.println(strm);  
// Output - {false=[45, 23, 9], true=[6, 8, 2]}

System.out.println(Arrays.stream(arr).mapToObj(ob -> Integer.valueOf(ob)).collect(Collectors.groupingBy(t-> t%2 == 0 ?"ODD" : "EVEN")));
// Output - {EVEN=[6, 8, 2], ODD=[45, 23, 9]}

Platitudinous

Answer 6

You can try the code snippet

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13);

// Sum of even numbers
int evenSum = numbers.stream()
  .filter(num -> num % 2 == 0)
  .mapToInt(Integer::intValue)
  .sum();

System.out.println("Sum of Even numbers: " + evenSum);

// Sum of odd numbers
int oddSum = numbers.stream()
  .filter(num -> num % 2 != 0)
  .mapToInt(Integer::intValue)
  .sum();

 System.out.println("Sum of Odd numbers: " + oddSum);

Rarefied