Java 8 Stream and operation on arrays
JavaArraysJava 8Java StreamJava Problem Overview
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
Java Solutions
Solution 1 - Java
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]);
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();
Solution 2 - Java
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][1], 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();
[1]: http://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html "Link to Java SE8 API"
Solution 3 - Java
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
Solution 4 - Java
Please note that Arrays.stream(arr) create a LongStream (or IntStream, ...) instead of Stream
Take a look at why-cant-i-map-integers-to-strings-when-streaming-from-an-array