Java 8 Stream and operation on arrays

Question

There are new methods added to java.util.Arrays to convert an array into a Java 8 stream which can then be used for summing etc.

int sum =  Arrays.stream(myIntArray).sum();

Multiplying two arrays is a little more difficult because I can't think of a way to get the value AND the index at the same time as a Stream operation. This means you probably have to stream over the indexes of the array.

//in this example a[] and b[] are same length
int[] a = ...
int[] b = ...
 
int[] result = new int[a.length];

IntStream.range(0, a.length).forEach(i -> result[i] = a[i] * b[i]);

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Commenter @Holger points out you can use the map method instead of forEach like this:

int[] result = IntStream.range(0, a.length).map(i -> a[i] * b[i]).toArray();

You can turn an array into a stream by using Arrays.stream():

int[] ns = new int[] {1,2,3,4,5};
Arrays.stream(ns);

Once you've got your stream, you can use any of the methods described in the documentation, like sum() or whatever. You can map or filter like in Python by calling the relevant stream methods with a Lambda function:

Arrays.stream(ns).map(n -> n * 2);
Arrays.stream(ns).filter(n -> n % 4 == 0);

Once you're done modifying your stream, you then call toArray() to convert it back into an array to use elsewhere:

int[] ns = new int[] {1,2,3,4,5};
int[] ms = Arrays.stream(ns).map(n -> n * 2).filter(n -> n % 4 == 0).toArray();

Be careful if you have to deal with large numbers.

int[] arr = new int[]{Integer.MIN_VALUE, Integer.MIN_VALUE};
long sum = Arrays.stream(arr).sum(); // Wrong: sum == 0

The sum above is not 2 * Integer.MIN_VALUE. You need to do this in this case.

long sum = Arrays.stream(arr).mapToLong(Long::valueOf).sum(); // Correct

Please note that Arrays.stream(arr) create a LongStream (or IntStream, ...) instead of Stream so the map function cannot be used to modify the type. This is why .mapToLong, mapToObject, ... functions are provided.

Take a look at why-cant-i-map-integers-to-strings-when-streaming-from-an-array