Java Streams Explained

1️⃣ What is a Stream in Java?

    A Stream in Java (introduced in Java 8) is a sequence of data elements that supports functional-style operations like filtering, mapping, and reducing. It helps in processing collections efficiently.

Key Features of Streams:

✅ Lazy Evaluation – Operations execute only when needed.
✅ Functional Programming – Uses lambda expressions.
✅ Parallel Processing – Supports parallel execution for better performance.
✅ Immutability – Does not modify the original collection.

2️⃣ How to Create a Stream?

📌 Creating Streams from Collections

List<String> names = Arrays.asList("Alice", "Bob", "Charlie");

Stream<String> stream = names.stream();

📌Creating Streams from Arrays

Stream<String> stream = Stream.of("Apple", "Banana", "Cherry");

3️⃣ Common Stream Operations

📌 1. Filtering (Filter Elements Based on Condition)

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

List<Integer> evenNumbers = numbers.stream()

                                   .filter(n -> n % 2 == 0)

                                   .collect(Collectors.toList());

System.out.println(evenNumbers); // Output: [2, 4]

📌 2. Mapping (Transform Each Element)

List<String> names = Arrays.asList("alice", "bob", "charlie");

List<String> uppercaseNames = names.stream()

                                   .map(String::toUpperCase)

                                   .collect(Collectors.toList());

System.out.println(uppercaseNames); // Output: [ALICE, BOB, CHARLIE]

📌 3. Sorting

List<Integer> numbers = Arrays.asList(5, 2, 8, 1, 3);

List<Integer> sortedNumbers = numbers.stream()

                                     .sorted()

                                     .collect(Collectors.toList());

System.out.println(sortedNumbers); // Output: [1, 2, 3, 5, 8]

📌 4. Reducing (Aggregate Elements)

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

int sum = numbers.stream().reduce(0, Integer::sum);

System.out.println(sum); // Output: 15

4️⃣ Parallel Streams (Performance Boost)

If you have a large dataset, you can use parallel streams to improve performance.

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int sum = numbers.parallelStream().reduce(0, Integer::sum);
System.out.println(sum); // Output: 15

5️⃣ Difference Between Stream and Collection

Feature	Collection (List, Set)	Stream
Storage	Stores elements	Does not store elements
Modification	Can modify elements	Cannot modify original collection
Iteration	External iteration (for-loop)	Internal iteration
Processing	Sequential only	Supports parallel execution

6️⃣ Summary

✔ Streams process data efficiently using functional programming
✔ Lazy evaluation and supports parallel execution
✔ Does not modify the original collection