StreamAPI.md

Stream API in Java

The Stream API in Java is a part of the Java Collections Framework (introduced in Java 8) that allows functional-style operations on collections, making data processing faster, more readable, and parallelizable.

It is not directly part of OOP (Object-Oriented Programming) but falls under Functional Programming principles in Java.

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Key Features of Stream API

1. Functional Programming – Uses lambda expressions and method references.
2. Declarative Approach – Focuses on "what to do" rather than "how to do it."
3. Lazy Evaluation – Operations are performed only when needed.
4. Parallel Processing – Supports parallel execution for improved performance.
5. Chainable Operations – Allows method chaining to process collections efficiently.

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How Stream API Works

A Stream processes a sequence of elements in three stages:

1. Creation → Generate a Stream from a Collection or an array.
2. Processing (Intermediate Operations) → Transform data (e.g., filter(), map(), sorted()).
3. Terminal Operations → Produce a result (e.g., collect(), forEach(), count()).

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Example Usage

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

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

        // Convert to uppercase and filter names starting with 'A'
        List<String> result = names.stream()
                .map(String::toUpperCase)   // Intermediate Operation
                .filter(name -> name.startsWith("A")) // Intermediate
                .collect(Collectors.toList()); // Terminal Operation

        System.out.println(result); // Output: [ALICE]
    }
}

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Common Stream Operations

Type	Method	Purpose
Creation	stream(), of(), generate()	Create a stream
Intermediate	map(), filter(), sorted(), distinct()	Transform data
Terminal	collect(), forEach(), count(), reduce()	Produce a result
Parallel	parallelStream()	Process in parallel