Stream API

The Stream API in Java allows you to process collections of objects in a declarative way, leveraging functional programming. A Stream does not store data, but instead carries elements from a source (such as a list or an array) through a pipeline of operations.

A Stream pipeline consists of three parts:

1. A source (e.g., list.stream())
2. Zero or more intermediate operations (which return a new Stream, e.g., filter, map)
3. A terminal operation (which produces a final result or side-effect and closes the Stream, e.g., collect, forEach, reduce)

Common Intermediate Operations

Intermediate operations are lazy: they are not executed until a terminal operation is invoked.

• filter(Predicate): filters elements based on a boolean condition.
• map(Function): transforms each element into another object by applying a function.
• sorted(): sorts the elements of the Stream.

List<String> names = Arrays.asList("Marco", "Anna", "Giovanni");
List<String> filtered = names.stream()
                             .filter(name -> name.startsWith("M"))
                             .map(name -> name.toUpperCase())
                             .collect(Collectors.toList()); // "MARCO"

Common Terminal Operations

• forEach(Consumer): performs an action for each element of the Stream.
• collect(Collector): gathers the Stream results into a container (e.g., a list using Collectors.toList()).
• reduce(identity, BinaryOperator): combines the Stream elements into a single cumulative value.

List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
int sum = numbers.stream()
                 .reduce(0, (a, b) -> a + b); // Sums all numbers: 10

Try it yourself