🛡️ What is the Stream API?

Introduced in Java 8, the Stream API allows you to process sequences of elements in a functional and declarative manner. It does not store data; instead, it conveys elements from a source (like a collection) through a pipeline of computational steps.

⏰ When to Use

• Bulk Data Operations: Filtering, sorting, and transforming large collections.
• Parallel Processing: Easily leveraging multi-core processors with parallelStream().
• Declarative Code: Replacing imperative for-loops with more readable "What vs How" logic.

📊 Complexity & Performance

• Lazy Evaluation: Intermediate operations (like filter) are not executed until a terminal operation (like collect) is called. This avoids unnecessary passes over the data.
• Overhead: For very small collections, a simple for-loop might be faster due to the overhead of stream creation.
• Parallelism: Parallel streams have overhead for splitting and merging. Only use them when the task is computationally expensive enough to justify the overhead.

💻 Code Example: Functional Pipeline

java Standard
import java.util.*;
import java.util.stream.*;

public class StreamDemo {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "David", "Edward");

        // Filter names longer than 4 chars, convert to upper case, and collect
        List<String> result = names.stream()
            .filter(n -> n.length() > 4)
            .map(String::toUpperCase)
            .sorted()
            .collect(Collectors.toList());

        System.out.println(result); // [ALICE, CHARLIE, DAVID, EDWARD]
    }
}

⚠️ Interview Pitfalls

1. Reuse Trap: A stream can only be operated on (traversed) once. Attempting to reuse a stream after a terminal operation throws IllegalStateException.
2. Side Effects: Avoid modifying the source collection inside a stream operation. This leads to unpredictable behavior, especially in parallel streams.
3. Ordering: parallelStream() does not guarantee order unless you use forEachOrdered().
4. Intermediate vs Terminal: Know that peek() is an intermediate operation and won't run unless a terminal operation like count() or collect() is triggered.