Java Interview Questions & Answers

Java follows four OOP principles:

• Encapsulation
• Inheritance
• Polymorphism
• Abstraction

HashMap stores data using key-value pairs.

Internal Steps

1. Calculate hashCode()
2. Find bucket index
3. Store entry in bucket
4. Handle collisions using LinkedList or Tree

Lambda expressions provide a concise way to represent anonymous functions.

An interface with exactly one abstract method.

@FunctionalInterface

interface Calculator {

  int add(int a, int b);

}

Common Functional Interfaces

• Predicate
• Function
• Consumer
• Supplier

Stream API enables functional-style processing of collections.

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

numbers.stream()

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

  .forEach(System.out::println);

Benefits

• Readable Code
• Functional Programming
• Parallel Processing

Optional helps avoid NullPointerException.

Common Methods

• of()
• ofNullable()
• orElse()
• orElseGet()
• isPresent()

JVM memory is divided into several areas used during program execution.

Employee employee = new Employee();

int age = 30;

employee object is stored in Heap while age is stored in Stack.

Garbage Collection automatically removes objects that are no longer reachable.

Benefits

• Automatic Memory Management
• Reduces Memory Leaks
• Improves Stability

ClassLoader dynamically loads Java classes into memory.

Types

• Bootstrap ClassLoader
• Platform ClassLoader
• Application ClassLoader

Serialization converts an object into a byte stream.

public class Employee

implements Serializable {

  private String name;

}

Uses

• Caching
• Messaging Systems
• File Storage
• Network Transfer

Reflection allows inspection and modification of classes at runtime.

Class<?> clazz = Employee.class;

Method[] methods = clazz.getMethods();

Common Framework Usage

• Spring Framework
• Hibernate
• JUnit

Synchronization prevents multiple threads from accessing critical sections simultaneously.

public synchronized void deposit() {

  balance += 100;

}

ConcurrentHashMap provides thread-safe access without locking the entire map.

Advantages

• Thread Safe
• Better Performance Than Hashtable
• High Concurrency Support

Key Components

• Order Service
• Inventory Service
• Payment Service
• Notification Service
• Message Queue
• Cache Layer

Expected Discussion

• Database Transactions
• Event Driven Architecture
• Scalability
• Failure Handling

Channels

• Email
• SMS
• Push Notifications
• In-App Notifications

Architecture

Client
  ↓
Notification API
  ↓
Message Queue
  ↓
Email / SMS / Push Workers

Services communicate by publishing and consuming events.

Order Created Event

      ↓

Kafka Topic

      ↓

Inventory Service
Payment Service
Notification Service

Benefits

• Loose Coupling
• Scalability
• Resilience
• Independent Deployments

Prevents cascading failures when dependent services are unavailable.

States

• Closed
• Open
• Half Open

Approaches

• Two Phase Commit (2PC)
• Saga Pattern
• Compensating Transactions

Cache Layers

• Application Cache
• Redis
• CDN Cache
• Database Cache

Cache Patterns

• Cache Aside
• Write Through
• Write Behind
• Read Through

Areas To Investigate

• Heap Size
• GC Logs
• Thread Dumps
• Memory Leaks
• CPU Profiling

-Xms2G
-Xmx4G

-XX:+UseG1GC

-XX:MaxGCPauseMillis=200

Core Components

• URL Generator
• Database
• Cache
• Analytics
• Redirect Service

Scalability Considerations

• Read Replicas
• Redis Cache
• Rate Limiting
• Load Balancing

Core Technologies

• Spring Boot
• WebSocket
• Redis Pub/Sub
• Kafka
• MySQL/PostgreSQL

Client

   ↕

WebSocket Server

   ↕

Redis Pub/Sub

   ↕

Chat Service

   ↕

Database

Features

• Presence Detection
• Typing Indicators
• Read Receipts
• Message Persistence
• Horizontal Scaling