Introduction to Java 8 Stream API: Creation, Intermediate and Terminal Operations

In Java, collections and arrays are frequently used, but before Java 8 manipulating them required verbose code. Java 8 added the Stream API, a declarative abstraction that lets developers process data in a way similar to SQL queries.

Streams provide several advantages: they have no storage of their own, are designed for functional programming, execute lazily (operations run only when a result is needed), and can be consumed only once, mirroring the behavior of iterators.

Streams can be created in two main ways. First, any existing collection can produce a stream via the stream() (or parallelStream()) method. Second, a stream can be built directly with Stream.of(...). Example:

List<String> strings = Arrays.asList("Hollis", "HollisChuang", "hollis", "Hello", "HelloWorld", "Hollis");
Stream<String> stream = strings.stream();

Stream<String> stream2 = Stream.of("Hollis", "HollisChuang", "hollis", "Hello", "HelloWorld", "Hollis");

Common intermediate operations include filter , map , limit/skip , sorted and distinct . Each operation returns another stream, allowing them to be chained. Examples:

List<String> strings = Arrays.asList("Hollis", "", "HollisChuang", "H", "hollis");
strings.stream().filter(s -> !s.isEmpty()).forEach(System.out::println);

List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
numbers.stream().map(i -> i * i).forEach(System.out::println);

numbers.stream().limit(4).forEach(System.out::println);

numbers.stream().sorted().forEach(System.out::println);

numbers.stream().distinct().forEach(System.out::println);

A typical pipeline might combine several intermediate steps before a terminal operation:

List<String> strings = Arrays.asList("Hollis", "HollisChuang", "hollis", "Hello", "HelloWorld", "Hollis");
Stream s = strings.stream()
    .filter(str -> str.length() <= 6)
    .map(String::length)
    .sorted()
    .limit(3)
    .distinct();

Terminal operations produce a final result and consume the stream. Frequently used terminals are forEach , count and collect . After a terminal operation the stream cannot be reused.

Random random = new Random();
random.ints().limit(10).forEach(System.out::println);

List<String> strings = Arrays.asList("Hollis","HollisChuang","hollis","Hollis666","Hello","HelloWorld","Hollis");
System.out.println(strings.stream().count());

strings = strings.stream()
    .filter(s -> s.startsWith("Hollis"))
    .collect(Collectors.toList());
System.out.println(strings);

Overall, the Stream API offers a concise, functional style for processing collections, improving readability and productivity. This article covered how to create streams, the most useful intermediate operations, and the essential terminal operations.