Java Stream collect() Method Examples

Java Stream collect() performs a mutable reduction operation on the elements of the stream. This is a terminal operation.

What is Mutable Reduction Operation?

A mutable reduction operation process the stream elements and then accumulate it into a mutable result container. Once the elements are processed, a combining function merges all the result containers to create the result.

Java Stream collect() Method Signature

There are two variants of Java Stream collect() method.

1. <R> R collect(Supplier<R> supplier, BiConsumer<R, ? super T> accumulator,BiConsumer<R, R> combiner)
2. <R, A> R collect(Collector<? super T, A, R> collector)

The Collector is an interface that provides a wrapper for the supplier, accumulator, and combiner objects. The second method is useful when we are utilizing the Collectors class to provide built-in Collector implementation. The three parameters of the collect() function are:

1. supplier: a function that creates a new mutable result container. For the parallel execution, this function may be called multiple times and it must return a fresh value each time.
2. accumulator is a stateless function that must fold an element into a result container.
3. combiner is a stateless function that accepts two partial result containers and merges them, which must be compatible with the accumulator function.

Stream collect() Method Examples

Let’s look at some examples of Stream.collect() method.

1. Concatenating List of Strings

Let’s say you want to concatenate the list of strings to create a new string. We can use Stream collect() function to perform a mutable reduction operation and concatenate the list elements.

List<String> vowels = List.of("a", "e", "i", "o", "u");

// sequential stream - nothing to combine
StringBuilder result = vowels.stream().collect(StringBuilder::new, (x, y) -> x.append(y),
		(a, b) -> a.append(",").append(b));
System.out.println(result.toString());

// parallel stream - combiner is combining partial results
StringBuilder result1 = vowels.parallelStream().collect(StringBuilder::new, (x, y) -> x.append(y),
		(a, b) -> a.append(",").append(b));
System.out.println(result1.toString());

Output:

aeiou
a,e,i,o,u

• The supplier function is returning a new StringBuilder object in every call.
• The accumulator function is appending the list string element to the StringBuilder instance.
• The combiner function is merging the StringBuilder instances. The instances are merged with each other with a comma between them.
• In the first case, we have a sequential stream of elements. So they are processed one by one and there is only one instance of StringBuilder. There is no use of the combiner function. That’s why the output produced is “aeiou”.
• In the second case, we have a parallel stream of strings. So, the elements are processed parallelly and there are multiple instances of StringBuilder that are being merged by the combiner function. Hence, the output produced is “a,e,i,o,u”.
• If the stream source is ordered such as List, the collect() method maintains the order while processing. If the stream source is unordered such as Set, then the collect() method can produce different results in each invocation.

If you want to concatenate the list of strings, we can use the method references to reduce the code size.

String result2 = vowels.parallelStream()
		.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append)
		.toString();

2. Stream collect() to List using Collectors Class

The Collectors class provides many useful implementations of the Collector interface. Let’s look at an example where we will filter the list of integers to select only even integers. Stream filter() is an intermediate operation and returns a stream. So, we will use the collect() function to create the list from this stream.

List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6);
		
List<Integer> evenNumbers = numbers.stream().filter(x -> x % 2 == 0).collect(Collectors.toList());
System.out.println(evenNumbers);  // [2, 4, 6]

The Collectors.toList() returns a Collector implementation that accumulates the input elements into a new List.

3. Stream collect() to a Set

We can use Collectors.toSet() to collect the stream elements into a new Set.

List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6);

Set<Integer> oddNumbers = numbers.parallelStream().filter(x -> x % 2 != 0).collect(Collectors.toSet());
System.out.println(oddNumbers); // [1, 3, 5]

3. Stream collect() to Map

We can use Collectors.toMap() function to collect the stream elements to a Map. This method accepts two arguments for mapping key and the corresponding value in the Map.

List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6);

Map<Integer, String> mapOddNumbers = numbers.parallelStream().filter(x -> x % 2 != 0)
		.collect(Collectors.toMap(Function.identity(), x -> String.valueOf(x)));
System.out.println(mapOddNumbers); // {1=1, 3=3, 5=5}

4. Collectors joining() Example

We can use Collectors joining() methods to get a Collector that concatenates the input stream CharSequence elements in the encounter order. We can use this to concatenate a stream of strings, StringBuffer, or StringBuilder.

jshell> String value = Stream.of("a", "b", "c").collect(Collectors.joining());
value ==> "abc"

jshell> String valueCSV = Stream.of("a", "b", "c").collect(Collectors.joining(","));
valueCSV ==> "a,b,c"

jshell> String valueCSVLikeArray = Stream.of("a", "b", "c").collect(Collectors.joining(",", "{", "}"));
valueCSVLikeArray ==> "{a,b,c}"

jshell> String valueObject = Stream.of("1", new StringBuffer("2"), new StringBuilder("3")).collect(Collectors.joining());
valueObject ==> "123"

Output:

Conclusion

Java Stream collect() is mostly used to collect the stream elements to a collection. It’s a terminal operation. It takes care of synchronization when used with a parallel stream. The Collectors class provides a lot of Collector implementation to help us out.

References

• Stream collect() API Doc
• Collector Interface
• Collectors API Doc