What Is a Stream in Java?

A Stream is a sequence of elements that supports functional-style operations to process data. It's not a data structure itself, but a pipeline for processing data.

Streams allow you to:

• Transform data (map)

• Filter data (filter)

• Aggregate data (reduce, collect)

• Chain multiple operations in a readable way

Traditional Loop Example

List<String> names = Arrays.asList("Anna", "Bob", "Charlie");
for (String name : names) {
    if (name.startsWith("A")) {
        System.out.println(name.toUpperCase());
    }
}

Stream-Based Equivalent

names.stream()
     .filter(name -> name.startsWith("A"))
     .map(String::toUpperCase)
     .forEach(System.out::println);

Why Use Streams?

• Concise: Less boilerplate code.

• Readable: Clear flow of data.

• Declarative: Focus on what to do, not how.

• Parallelizable: Easily switch to .parallelStream() for multithreading.

How Streams Work: The Pipeline

A stream has three steps:

1. Source: Where the data comes from (List, Set, Array, etc.)

2. Intermediate Operations: Transformations (map, filter, sorted)

3. Terminal Operation: Final action (forEach, collect, count)

Example:

List<String> cities = List.of("Berlin", "London", "Paris", "Barcelona");

cities.stream()                     // 1. Source
      .filter(c -> c.length() > 6)  // 2. Intermediate
      .sorted()
      .forEach(System.out::println); // 3. Terminal

Important Notes

• Streams are lazy: Intermediate operations don’t run until a terminal operation is invoked.

• Streams are one-time use: Once consumed, you must recreate it to reuse.

• Streams don't modify the original collection.

Summary

• A Stream is a powerful tool for processing collections.

• You define a pipeline of operations to filter, transform, and consume data.

• Streams help write cleaner, more functional, and declarative code.

Assignment: Lesson 1 – Getting Started with Streams

Objective:

Get hands-on experience using Java Streams to filter, map, and print data.

Part 1: Conceptual Questions

1. What is a Stream in Java?

2. What are the three parts of a stream pipeline?

3. How are Streams different from traditional loops?

Assignment 1 – Easy

Task:

Write a Java program that:

• Creates a list of names: "Alice", "Bob", "Clara", "Daniel", "Eva"

• Uses a stream to:

  • Filter names that start with "A" or "C"

  • Convert them to uppercase

  • Print them

Expected Output:

ALICE
CLARA

Assignment 2 – Challenge

Task:

Create a method called filterAndTransform() that:

• Accepts a List<String> of names

• Returns a new list of names that:

  • Have length ≥ 5

  • Are reversed ("Charlie" → "eilrahC")

  • Are sorted in reverse alphabetical order

Hint:

Use map(), filter(), sorted(), and collect(Collectors.toList()).

Sample Output (input: "Anna", "Robert", "Clara", "Eva", "Charlie"):

["treboR", "eilrahC", "aralC"]