[java] Java 8 Stream and operation on arrays

I have just discovered the new Java 8 stream capabilities. Coming from Python, I was wondering if there was now a neat way to do operations on arrays like summing, multiplying two arrays in a "one line pythonic" way ?

Thanks

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

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();

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