Using Java Streams and Collectors
This short guide covers how to use Java 8 Streams and Collectors to slice and dice lists, including computing sums, averages, and partitioning.
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Join For FreeJava 8 provides Streams, which makes many Collection operations easy. Streaming items from a collector and filtering the data are trivial, as well as are sorting, searching, and computing aggregates. That is, if you are familiar with the many Collectors functions available. We present some of these functions here.
Summing an Integer List
Computing the sum total of numbers in a List? No longer do you need a loop, an iterator, or temporary variables. Assuming numbers
contains a List of integers, the following neatly computes the result.
List<Integer> numbers = ...;
int sum = numbers.stream().reduce(0, (x, y) > x+y);
And the following illustrates a collection operation to collect integers from a streams pipeline into a List of integers.
Random random = new Random();
List<Integer> numbers = random
.ints(1, 100)
.limit(10)
.boxed()
.collect(Collectors.toList());
Here is another way of computing the sum using Stream.collect() instead of Stream.reduce() as above. You can use either alternative as per your preference.
int sum = numbers.stream().collect(Collectors.summingInt(x>x));
// prints:
[90, 93, 61, 84, 26, 95, 61, 19, 51, 44] => sum = 624
Computing Averages
Computing the average of a list of numbers is similarly a piece of cake. The Collectors provide an averagingInt() method for the purpose.
double avg = numbers.stream().collect(Collectors.averagingInt(x>x));
// prints:
[90, 93, 61, 84, 26, 95, 61, 19, 51, 44] => avg = 62.4
Maximum and Minimum
Let us also cover finding the maximum and minimum of a List of numbers while we are at it.
Optional<Integer> max = numbers.stream().collect(Collectors.maxBy(Integer::compare));
Optional<Integer> min = numbers.stream().collect(Collectors.minBy(Integer::compare));
// prints:
[90, 93, 61, 84, 26, 95, 61, 19, 51, 44] => max = 95
[90, 93, 61, 84, 26, 95, 61, 19, 51, 44] => min = 19
Summarizing in One Shot
Or why bother computing sum, average, etc. separately? Just use summarizingInt() as shown.
IntSummaryStatistics r = numbers.stream().collect(Collectors.summarizingInt(x > x));
// prints:
[21, 99, 13, 11, 14, 99, 77, 42, 32, 34] => IntSummaryStatistics{count=10, sum=442, min=11, average=44.200000, max=99}
Partitioning a List
Let us see how to partition a List of numbers into two lists using a criterion (such as values greater than 50):
Map<Boolean,List<Integer>> parts = numbers.stream().collect(Collectors.partitioningBy(x > x > 50));
System.out.println(numbers + " =>\n" +
" true: " + parts.get(true) + "\n" +
" false: " + parts.get(false) + "\n");
// prints;
[77, 52, 52, 15, 81, 59, 38, 70, 55, 61] =>
true: [77, 52, 52, 81, 59, 70, 55, 61]
false: [15, 38]
Summary
The Java 8 Collectors class provides useful implementations, which can be used by Streams' collect() method. Some of these operations presented here include computing sums, averages, maximums, and minimums. Partitioning a List based on a predicate returns a pair of Lists enclosed in a Map.
If you enjoyed this article and want to learn more about Java Streams, check out this collection of tutorials and articles on all things Java Streams.
Published at DZone with permission of Jay Sridhar, DZone MVB. See the original article here.
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