Stream basics: interview questions & scenarios

Model answer

A Java stream is a one-shot pipeline over a source. The source can be a List, array, file, generated sequence or custom Spliterator; the stream describes operations to apply as elements are pulled through. A collection stores elements. A stream does not store elements as its main job, and it is not meant to be reused after a terminal operation.

The shape is:

source -> intermediate operations -> terminal operation

filter, map, peek, sorted and limit are intermediate operations because they return another stream. count, sum, toList, findFirst, forEach and collect are terminal operations because they start traversal and produce a result or final side effect.

Model answer

import java.util.List;

public class Main {
    public static void main(String[] args) {
        var pipeline = List.of("ann", "bob", "carla").stream()
                .peek(name -> System.out.println("peek " + name))
                .filter(name -> name.length() > 3)
                .map(name -> {
                    System.out.println("map " + name);
                    return name.toUpperCase();
                });

        System.out.println("built");
        System.out.println(pipeline.toList());
    }
}

It prints:

built
peek ann
peek bob
peek carla
map carla
[CARLA]

The intermediate operations are lazy. Creating pipeline builds stage objects; it does not traverse the list. toList() is the terminal operation, so it starts pulling elements through the chain. For a stateless sequential pipeline, elements move vertically: ann is peeked and filtered out, bob is peeked and filtered out, then carla is peeked, mapped and added to the result.