6. Java Date/Time, NIO File I/O & Multithreading

Chapters 21-24: java.time API, Path & Files, try-with-resources, Thread/Runnable, ExecutorService & CompletableFuture

Module 1: Date & Time API (java.time) & NIO File Operations

Using modern immutable date types and try-with-resources file management.

🇬🇧 English Definition

java.time provides modern immutable date/time classes (LocalDate, LocalDateTime, ZonedDateTime). java.nio.file.Path and Files offer modern filesystem utility operations with try-with-resources auto-closing.

🇮🇳 Hinglish Definition

java.time date aur time ke liye modern immutable classes deta hai. try-with-resources AutoCloseable files/streams ko automatically close karta hai.

Core Concepts & Rules

  • LocalDate & Duration: LocalDate.now(); Duration.ofHours(5).toMinutes();
  • try-with-resources: try (var reader = Files.newBufferedReader(path)) { ... }

🔍 Line-by-Line Code Breakdown

  1. Line 2: LocalDate.now() captures current system date.
  2. Line 3: Duration.ofHours(5).toMinutes() calculates duration minutes (300).

🎯 Interview Focus & Questions

  • Q: Advantage of try-with-resources?
    A: Automatically closes resources (files, sockets, DB connections) implementing AutoCloseable preventing memory leaks.
💻 Practical Java Code Example - Date & Time API (java.time) & NIO File Operations 
import java.time.*;

public class DateTimeDemo {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        Duration d = Duration.ofHours(5);
        System.out.println("Date: " + today);
        System.out.println("5 Hours in Minutes: " + d.toMinutes());
    }
}
📤 Compiled Execution Output:
Date: 2026-10-04 5 Hours in Minutes: 300

Module 2: Multithreading, Concurrency & CompletableFuture

Managing concurrent execution paths, synchronization, thread pools, and async futures.

🇬🇧 English Definition

Multithreading executes multiple concurrent threads. ExecutorService manages worker thread pools. CompletableFuture enables asynchronous pipeline composition.

🇮🇳 Hinglish Definition

Multithreading parallel tasks ke liye use hoti hai. ExecutorService thread pool manage karta hai aur CompletableFuture async pipeline compose karta hai.

Core Concepts & Rules

  • Thread vs Runnable: Runnable separates task logic from thread execution.
  • Synchronization: Prevents race conditions on shared mutable state.
  • CompletableFuture: CompletableFuture.supplyAsync(...).thenApply(...).thenAccept(...);

🔍 Line-by-Line Code Breakdown

  1. Line 1: Import java.util.concurrent.CompletableFuture.
  2. Line 4-7: Asynchronous pipeline supplying 'Java', converting to uppercase, and printing.

🎯 Interview Focus & Questions

  • Q: What is ExecutorService?
    A: A high-level concurrency API managing worker thread pools and submitted tasks.
  • Q: What is synchronization?
    A: Controlled access to shared mutable state avoiding race conditions.
💻 Practical Java Code Example - Multithreading, Concurrency & CompletableFuture 
import java.util.concurrent.CompletableFuture;

public class AsyncDemo {
    public static void main(String[] args) {
        CompletableFuture.supplyAsync(() -> "Java Concurrency")
            .thenApply(String::toUpperCase)
            .thenAccept(System.out::println)
            .join();
    }
}
📤 Compiled Execution Output:
JAVA CONCURRENCY