Creating Streams in Java

Learn to create streams of primitives and objects in Java using some most popular ways. We will learn to create finite as well as infinite streams.

1. Creating Finite Streams

1.1. Empty Stream

We can use Stream.empty() method to create an empty stream.

Stream<String> emptyStream = Stream.empty();

1.2. From Values

In Java, the Stream.of() creates a stream of the supplied values as var-args, array or list.

static <T> Stream<T> of(T... values);

Let us see a few examples to create a stream of values.

Stream<Integer> stream = Stream.of(1,2,3,4,5,6,7,8,9);  //from var args

Stream<Integer> stream = Stream.of( new Integer[]{1,2,3,4,5,6,7,8,9} );  //from array

Employee[] arrayOfEmps = {
    new Employee(1, "A", LocalDate.of(1991, 1, 1), 10000d),
    new Employee(2, "B", LocalDate.of(1992, 1, 1), 20000d),
    new Employee(3, "C", LocalDate.of(1993, 1, 1), 30000d)
};

Stream<Employee> employeeStream = Stream.of(arrayOfEmps);

1.3. From Collections

We can also get the stream from Java collection classes such as List, Map and Set.

List<String> list = Arrays.asList("A", "B", "C", "D");
Stream<String> stream = list.stream();

Similarly, get a stream from Map.

Map<String, Integer> map = new HashMap<>();
map.put("A", 1);

Stream<String> keyStream = map.keySet().stream();
Stream<Integer> valStream = map.values().stream();
Stream<Map.Entry<String, Integer>> entryStream = map.entrySet().stream();

We can also get the stream using utility classes such as Arrays and Collections.

String[] arr = { "A", "B", "C", "D" };

Stream<String> stream = Arrays.stream(arr);

1.4. Stream.Builder

The Stream.Builder class follows the builder pattern where we add items to the stream in steps, and finally call the method build() to get the stream.

Stream<String> streamBuilder = Stream.<String>builder()
                                            .add("A")
                                            .add("B")
                                            .build();

2. Creating Infinite Streams

Use the following methods to create infinite streams in Java.

• iterate(seed, function) – accepts two parameters – a seed which is the first term in the stream, and a function  to produce the value of the next item in the stream. We can limit the stream using the limit() method.
• generate(supplier) – accepts a Supplier that provides an infinite series of elements which are placed in the stream. The limit() method can then be called in the stream chain to stop the series after a certain number of elements. This is suitable for generating constant streams, streams of random elements, etc.

2.1. Stream.iterate()

An example is to generate an infinite stream of even numbers starting from 0 using the iterate() function.

Stream<Integer> infiniteEvenNumbers = Stream.iterate(0, n -> n + 2).limit(10);

2.2. Stream.generate()

A similar example creates a stream of 10 random numbers between 0 and 99 using generate() function.

Random rand = new Random();

Stream<Integer> stream =
    Stream.generate(() -> rand.nextInt(100)).limit(20);

3. Conclusion

In this Java 8 stream tutorial, we learned to finite stream elements as well as infinite streams of elements. We saw the usage of limit() function which is used to pick the first N elements from an infinite stream.

Happy Learning !!

Sourcecode on Github