Using Java Collectors

1. Introduction

Java 8 provides the new Streams facility 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.

2. 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

3. 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

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

5. 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}

6. Partitioning a List

Let us see how to partition a List of numbers into two lists using a criterion (such as value 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

Java 8 Collectors class provides useful implementations which can be used by the Streams collect() method. Some of these operations presented here include computing sums, averages, maximum and minimum. Partitioning a List based on a predicate returns a pair of Lists enclosed in a Map.