The Stream API

A Stream is a sequence of elements that supports functional-style, declarative operations — filtering, transforming, and aggregating data without writing manual loops. Streams build directly on lambdas and functional interfaces from Day 22.

Creating a Stream

List names = Arrays.asList("Alice", "Bob", "Charlie", "Dave");
Stream nameStream = names.stream();

Intermediate Operations — filter() and map()

Intermediate operations return a new stream and are lazy — nothing actually executes until a terminal operation is called.

List result = names.stream()
    .filter(name -> name.length() > 3)   // keep names longer than 3 chars
    .map(String::toUpperCase)             // transform each to uppercase
    .collect(Collectors.toList());

System.out.println(result); // [ALICE, CHARLIE, DAVE]

Terminal Operations — Where Execution Actually Happens

The reduce() Operation

List numbers = Arrays.asList(1, 2, 3, 4, 5);
int sum = numbers.stream()
    .reduce(0, (a, b) -> a + b);
System.out.println(sum); // 15

Chaining Multiple Operations

List scores = Arrays.asList(55, 90, 42, 78, 88, 30);
long passCount = scores.stream()
    .filter(s -> s >= 50)
    .count();
System.out.println(passCount); // 4

map() vs flatMap()

map() transforms each element one-to-one. flatMap() is used when each element itself produces a stream (e.g., a list of lists), flattening the nested structure into one stream.

Interview tip: Streams do NOT modify the original collection — they produce a new result. A common mistake is expecting names.stream().filter(...) to change the original names list; it never does, since streams are designed around immutability and functional purity.