# Using Java Streams and Collectors

# 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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Java 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.

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