Stream API

1. What is the Stream API?

The Stream API lets you process collections of data (like lists) in a clean, declarative style — describing what should happen (filter, transform, count) rather than manually writing loops to do it step-by-step.

2. Why is it used?

It significantly reduces the amount of code needed for common data-processing tasks, like filtering a list of students by marks, or converting a list of names to uppercase, all in a few readable lines.

3. Real-Life Example

Think of a factory assembly line where raw material passes through several stages — cleaning, cutting, packaging — each stage doing one specific job before passing the result to the next. A Java Stream processes data through a similar chain of steps.

4. Syntax

java
collection.stream()
    .filter(condition)
    .map(transformation)
    .collect(Collectors.toList());

5. Example Program

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

public class StreamDemo {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);

        List<Integer> evenNumbers = numbers.stream()
            .filter(n -> n % 2 == 0)
            .collect(Collectors.toList());

        System.out.println(evenNumbers);
    }
}

Output:

[2, 4, 6]

6. Key Points to Remember

• A stream doesn't store data itself — it simply processes data from an existing source, like a List.
• Stream operations don't modify the original collection; they produce a new result.
• Common operations include filter(), map(), sorted(), collect(), and forEach().