Streams

Stream Creation

A Stream is always created from a data source. Every pipeline starts here.

Ways to Create a Stream

Source	Code
Collection	`list.stream()`
Array	`Arrays.stream(arr)`
Explicit values	`Stream.of(1, 2, 3)`
Builder	`Stream.builder().add(1).add(2).build()`
Iterate (infinite)	`Stream.iterate(seed, fn)`
Generate (infinite)	`Stream.generate(supplier)`
Range of ints	`IntStream.range(1, 5)`, `rangeClosed(1, 5)`

1. From a Collection

Every Collection (List, Set, Queue) has a stream() method.

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

A Map has no stream() method. Stream its keySet(), values(), or entrySet() instead.

Map<Integer, String> map = Map.of(1, "Ravi", 2, "Anita");

map.keySet().stream();
map.values().stream();
map.entrySet().stream();

2. From an Array

int[] numbers = {1, 2, 3, 4, 5};
IntStream s1 = Arrays.stream(numbers);       // primitive array gives IntStream

String[] names = {"Akash", "Rahul", "Amit"};
Stream<String> s2 = Arrays.stream(names);    // object array gives Stream<String>

3. Using Stream.of()

Creates a Stream from explicitly provided values.

Stream<Integer> stream = Stream.of(1, 2, 3, 4, 5);

4. Using Stream.Builder

Stream.Builder<Integer> builder = Stream.builder();

builder.add(1);
builder.add(2);
builder.add(3);

Stream<Integer> stream = builder.build();

5. Using Stream.iterate()

Generates a sequence from a starting value and a function that produces the next value.

// With limit
Stream.iterate(1000, n -> n + 5000)
.limit(5)
.forEach(System.out::println);
// 1000, 6000, 11000, 16000, 21000

// With a stop condition (Java 9+)
Stream.iterate(1, n -> n <= 20, n -> n * 2)
.forEach(System.out::println);
// 1, 2, 4, 8, 16

6. Using Stream.generate()

Produces elements from a Supplier.

Stream.generate(() -> "Hello")
.limit(3)
.forEach(System.out::println);
// Hello, Hello, Hello

Stream.generate(Math::random)
.limit(3)
.forEach(System.out::println);

iterate() and generate() can produce an infinite Stream. Always restrict them with limit() or a stop condition.

7. Ranges of Numbers

IntStream.range(1, 5).forEach(System.out::println);        // 1 2 3 4   (end excluded)
IntStream.rangeClosed(1, 5).forEach(System.out::println);  // 1 2 3 4 5 (end included)

8. Other Sources

Source	Code
String characters	`"hello".chars()`
Empty Stream	`Stream.empty()`
Files (lines)	`Files.lines(path)`
Regex split	`Pattern.compile(",").splitAsStream(s)`

Remember

Collections use .stream(), arrays use Arrays.stream(), values use Stream.of(), and infinite sequences use iterate() or generate() with limit().