Java Stream: Examples

Saurabh Sharma

Streams are convenient way to reduce the line of codes, but an understanding them is equally challenging. I write this blog, after realising a desperate attempt at solving a trivial problem just took overwhelming time. (Could be that I am getting old, but never late to learn things again)

 

public class PlaySteams {
    public static void main(String []args) {
        int[] a = new int[] {1,1,1,2,2,2,3,3,3,4,4,4};
        //Stream Sum of the array
        long sum = IntStream.range(0, a.length).count();

        System.out.printf("Sum: %d\n", sum);
        // Maximum
        System.out.printf("Max: %d\n", IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).distinct().max().getAsInt());
        // Minimum
        System.out.printf("Min: %d\n", IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).distinct().min().getAsInt());

        IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).distinct().forEach(x -> System.out.printf("%d ", x));
        System.out.printf("Average (Distinct): %f\n", IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).distinct().average().getAsDouble());

        System.out.printf(" Number of elements : %d\n", IntStream.of(a).asLongStream().count());

        IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).forEach(x -> System.out.printf("%d ", x));
        System.out.printf("Average (All): %f\n", IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).average().getAsDouble());

        List<Integer> listOfIntegers = IntStream.of(a).boxed().collect(Collectors.toList());
        System.out.println(listOfIntegers.toString());

        System.out.println("Length: ");
        System.out.println(listOfIntegers.size());

        // Creating a map
        // 1 -> 3
        // 2 -> 3 ...
        System.out.println(IntStream.range(0, a.length).map(i -> a[i]).boxed().collect(Collectors.groupingBy(Function.identity(), Collectors.counting())).toString());

        // Using list
        Map<Integer, Long> valueMap = listOfIntegers.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
        System.out.println(valueMap.toString());

        // Map of indexes
        // Indexes and value of the index
        System.out.println(IntStream.range(0, a.length).boxed().collect(Collectors.groupingBy( i -> a[Integer.valueOf(i)], Collectors.toList())));


    }
}

The example set remains the same for all the invocation.

int[] a = new int[] {1,1,1,2,2,2,3,3,3,4,4,4};

Example – 1:

long sum = IntStream.range(0, a.length).count();
System.out.printf("Sum: %d\n", sum);

It simply runs over the length of the array and returns the total number of elements

Sum: 12

Example – 2:

Return the maximum

System.out.printf("Max: %d\n", IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).distinct().max().getAsInt());
Max: 4

Example – 3:

Returns the minimum

System.out.printf("Min: %d\n", IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).distinct().min().getAsInt());
Min: 1

Example – 4:

Run through the list and print distinct items

IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).distinct().forEach(
x -> 
System.out.printf("%d ", x));
1 2 3 4 

Example – 5:

Average of the distinct items

System.out.printf("Average (Distinct): %f\n", IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).distinct().average().getAsDouble());
Average (Distinct): 2.500000

Example – 6:

Different way of getting the count of elements.

System.out.printf(" Number of elements : %d\n", IntStream.of(a).asLongStream().count());
Number of elements : 12

Example – 7:

IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).forEach(x -> System.out.printf("%d ", x));

Get the list of elements

1 1 1 2 2 2 3 3 3 4 4 4 

Example – 8:

System.out.printf("Average (All): %f\n", IntStream.range(0, a.length).map(i -> Integer.valueOf(a[i])).average().getAsDouble());
Average (All): 2.500000

Example – 9:

List<Integer> listOfIntegers = IntStream.of(a).boxed().collect(Collectors.toList());
System.out.println(listOfIntegers.toString());
[1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4]

Example – 10:

System.out.println(IntStream.range(0, a.length).map(i -> a[i]).boxed().collect(Collectors.groupingBy(Function.identity(), Collectors.counting())).toString());

Creates a map of the value and the number of times it repeats

1=3, 2=3, 3=3, 4=3}

Example – 11:

Map<Integer, Long> valueMap = listOfIntegers.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
        System.out.println(valueMap.toString());

Using list that we had in Example - 9 to get the map.

{1=3, 2=3, 3=3, 4=3}

Example – 12:

Returns the list of index for each value

System.out.println(IntStream.range(0, a.length).boxed().collect(Collectors.groupingBy( i -> a[Integer.valueOf(i)], Collectors.toList())));
{1=[0, 1, 2], 2=[3, 4, 5], 3=[6, 7, 8], 4=[9, 10, 11]}

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