Using Streams instead of for loop in java 8

Question

int [] numbers = {1,2,3,4,5,6,7,8};
int [] doubleNumbers = new int[numbers.length];
int [] tripleNumbers = new int[numbers.length];


for(int index = 0; index < numbers.length; index++)
{
    doubleNumbers[index] = numbers[index] * 2;  
    tripleNumbers[index] = numbers[index] * 3;
}

System.out.println("Double Numbers");
Arrays.stream(doubleNumbers).forEach(System.out::println);

System.out.println("Triple Numbers");
Arrays.stream(tripleNumbers).forEach(System.out::println);

I have above code where I have used for loop and double and triple the numbers and stored it in different arrays in single loop. Can anybody help me to write the same code using streams with its map and other methods without iterating numbers array twice.

Answer 1

You can do it like this:

IntStream.range(0, numbers.length)
  .forEach(index -> {
    doubleNumbers[index] = numbers[index] * 2;
    tripleNumbers[index] = numbers[index] * 3;
  });

Answer 2

You can apply the doubling and tripling within the stream:

public static void main(String[] args) {
    int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8};

    System.out.println("Double Numbers");
    Arrays.stream(numbers).map(x -> x * 2).forEach(System.out::println);

    System.out.println("Triple Numbers");
    Arrays.stream(numbers).map(x -> x * 3).forEach(System.out::println);
}

though technically that's still iterating over the array twice.

As a trick you could instead collect the results in a Map:

Map<Integer, Integer> m = Arrays.stream(numbers)
        .boxed()
        .collect(Collectors.toMap(
                x -> x * 2,
                x -> x * 3
        ));