Java 8 Stream and operation on arrays

Question

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

User · Answer

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

User · Answer

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 - gt  n   2   Arrays stream ns  filter n - gt  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 - gt  n   2  filter n - gt  n   4    0  toArray

User · Answer

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 - gt  result i    a i    b i      EDIT  Commenter  Holger points out you can use the map method instead of forEach like this   int   result   IntStream range 0  a length  map i - gt  a i    b i   toArray

User · Answer

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

[java] Java 8 Stream and operation on arrays

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