Streams in Java

A Streams represents a sequence of elements and supports different kind of operations to perform computations upon those elements. With Java 8, Collection interface has two methods to generate a Stream: stream() and
parallelStream(). Stream operations are either intermediate or terminal. Intermediate operations return a Stream so multiple intermediate operations can be chained before the Stream is closed. Terminal operations are either void or return a non-stream result.

Using Streams

A Stream is a sequence of elements upon which sequential and parallel aggregate operations can be performed. Any given Stream can potentially have an unlimited amount of data flowing through it. As a result, data received from a Stream is processed individually as it arrives, as opposed to performing batch processing on the data altogether. When combined with lambda expressions they provide a concise way to perform operations on
sequences of data using a functional approach.


Stream fruitStream = Stream.of("apple", "banana", "pear", "kiwi", "orange");

fruitStream.filter(s -> s.contains("a"))
.map(String::toUpperCase) .sorted().forEach(System.out::println);



The operations performed by the above code can be summarized as follows:

  1. Create a Stream containing a sequenced ordered Stream of fruit String elements using the static factory method Stream.of(values).
  2. The filter() operation retains only elements that match a given predicate (the elements that when tested by the predicate return true). In this case, it retains the elements containing an “a”. The predicate is given as a lambda expression.
  3. The map() operation transforms each element using a given function, called a mapper. In this case, each fruit String is mapped to its uppercase String version using the method-reference String::toUppercase. Note that the map() operation will return a stream with a different generic type if the mapping
    function returns a type different to its input parameter. For example on a Stream calling .map(String::isEmpty) returns a Stream
  4. The sorted() operation sorts the elements of the Stream according to their natural ordering (lexicographically, in the case of String).
  5. Finally, the forEach(action) operation performs an action which acts on each element of the Stream, passing it to a Consumer. In the example, each element is simply being printed to the console. This operation is a terminal operation, thus making it impossible to operate on it again.

Note that operations defined on the Stream are performed because of the terminal operation. Without a terminal operation, the stream is not processed. Streams can not be reused. Once a terminal operation is called, the Stream object becomes unusable.

Operations (as seen above) are chained together to form what can be seen as a query on the data.

Closing Streams

Note :that a Stream generally does not have to be closed. It is only required to close streams that operate on IO channels. Most Stream types don’t operate on resources and therefore don’t require closing.

The Stream interface extends AutoCloseable. Streams can be closed by calling the close method or by using trywith-resource statements.

An example use case where a Stream should be closed is when you create a Stream of lines from a file:

try (Stream lines = Files.lines(Paths.get("somePath"))) {

The Stream interface also declares the Stream.onClose() method which allows you to register Runnable handlers
which will be called when the stream is closed. An example use case is where code which produces a stream needs
to know when it is consumed to perform some cleanup.

public StreamstreamAndDelete(Path path) throws IOException {
    return Files.lines(path).onClose(() ->  someClass.deletePath(path));

The run handler will only execute if the close() method gets called, either explicitly or implicitly by a try-withresources statement.

Processing Order

A Stream object’s processing can be sequential or parallel.

In a sequential mode, the elements are processed in the order of the source of the Stream. If the Stream is ordered (such as a SortedMap implementation or a List) the processing is guaranteed to match the ordering of the source. In other cases, however, care should be taken not to depend on the ordering (see: is the Java HashMap keySet() iteration order consistent?).


List integerList = Arrays.asList(0, 1, 2, 3, 42);

// sequential
long howManyOddNumbers =
.filter(e -> (e % 2) == 1) .count();
System.out.println(howManyOddNumbers); // Output: 2

Parallel mode allows the use of multiple threads on multiple cores but there is no guarantee of the order in which elements are processed.

If multiple methods are called on a sequential Stream, not every method has to be invoked. For example, if a Stream is filtered and the number of elements is reduced to one, a subsequent call to a method such as sort will not occur. This can increase the performance of a sequential Stream — an optimization that is not possible with a parallel Stream.


// parallel
long howManyOddNumbersParallel = integerList.parallelStream()
.filter(e -> (e % 2) == 1)
System.out.println(howManyOddNumbersParallel); // Output: 2

Differences from Containers (or Collections)

While some actions can be performed on both Containers and Streams, they ultimately serve different purposes and support different operations. Containers are more focused on how the elements are stored and how those
elements can be accessed efficiently. A Stream, on the other hand, doesn’t provide direct access and manipulation to its elements; it is more dedicated to the group of objects as a collective entity and performing operations on that entity as a whole. Stream and Collection are separate high-level abstractions for these differing purposes.

Consuming Streams

A Stream will only be traversed when there is a terminal operation, like count(), collect() or forEach(). Otherwise, no operation on the Stream will be performed.

In the following example, no terminal operation is added to the Stream, so the filter() operation will not be invoked and no output will be produced because peek() is NOT a terminal operation.

IntStream.range(1, 10).filter(a -> a % 2 == 0).peek(System.out::println);

This is a Stream sequence with a valid terminal operation, thus an output is produced. You could also use forEach instead of peek:

IntStream.range(1, 10).filter(a -> a % 2 == 0).forEach(System.out::println);



After the terminal operation is performed, the Stream is consumed and cannot be reused.

Although a given stream object cannot be reused, it’s easy to create a reusable Iterable that delegates to a stream pipeline. This can be useful for returning a modified view of a live data set without having to collect results into a temporary structure.

List list = Arrays.asList("FOO", "BAR");
Iterable iterable = () ->;

for (String str : iterable) {
for (String str : iterable) {



This works because Iterable declares a single abstract method Iterator iterator(). That makes it effectively a functional interface, implemented by a lambda that creates a new stream on each call. In general, a Stream operates as shown in the following image:

NOTE: Argument checks are always performed, even without a terminal operation:

try {
IntStream.range(1, 10).filter(null);
} catch (NullPointerException e) {
System.out.println(“We got a NullPointerException as null was passed as an argument to


We got a NullPointerException as null was passed as an argument to filter()

Creating a Frequency Map

The groupingBy(classifier, downstream) collector allows the collection of Stream elements into a Map by classifying each element in a group and performing a downstream operation on the elements classified in the same group.

A classic example of this principle is to use a Map to count the occurrences of elements in a Stream. In this example, the classifier is simply the identity function, which returns the element as-is. The downstream operation counts the number of equal elements, using counting().

Stream.of("apple", "orange", "banana", "apple")
.collect(Collectors.groupingBy(Function.identity(), Collectors.counting())) .entrySet().forEach(System.out::println);

The downstream operation is itself a collector (Collectors.counting()) that operates on elements of type String and produces a result of type Long. The result of the collect method call is a Map.

This would produce the following output:


Infinite Streams

It is possible to generate a Stream that does not end. Calling a terminal method on an infinite Stream causes the Stream to enter an infinite loop. The limit method of a Stream can be used to limit the number of terms of the Stream that Java processes.

This example generates a Stream of all natural numbers, starting with the number 1. Each successive term of the Stream is one higher than the previous. By calling the limit method of this Stream, only the first five terms of the Stream are considered and printed.

// Generate infinite stream - 1, 2, 3, 4, 5, 6, 7, …
IntStream naturalNumbers = IntStream.iterate(1, x -> x + 1);

// Print out only the first 5 terms



Another way of generating an infinite stream is using the Stream.generate method. This method takes a lambda of type Supplier.

// Generate an infinite stream of random numbers
Stream infiniteRandomNumbers = Stream.generate(Math::random);

// Print out only the first 10 random numbers

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