• Understand OOP, collections, exceptions, and file handling.

• Create Account
• Deposit / Withdraw
• Transfer money
• Transaction history
• Persist data to file (serialization or file I/O)

• OOP (encapsulation, abstraction)
• List, Map
• Exception handling
• File I/O
• equals/hashCode
• Basic concurrency (optional: simulate concurrent transactions)

• Make it thread-safe
• Add logging

Understand internal working of collections.

• Custom HashMap
• Custom Stack
• Custom Queue
• LRU Cache

• Hashing
• Collision handling
• Linked list
• Generics
• Time complexity

If you cannot implement HashMap, you do not understand Java deeply.

Master concurrency.

• Read large file
• Process lines in parallel
• Aggregate results
• Benchmark single-thread vs multi-thread

• Thread
• ExecutorService
• Callable/Future
• Synchronization
• ConcurrentHashMap
• Race conditions

Use:

• Spring Boot
• MySQL or PostgreSQL
• Spring Data JPA

• Register/Login
• JWT Authentication
• CRUD Users
• Pagination
• Sorting
• Validation
• Global Exception Handling

• Password hashing
• Role-based authorization
• Unit tests

• REST APIs
• DTO pattern
• Repository pattern
• Transactions
• Indexing

Deploy it (mandatory):

• Amazon Web Services
• Render
• Railway

• Products
• Cart
• Orders
• Payment simulation
• Inventory management

• Caching (Redis)
• Optimistic locking
• Order state machine
• Rate limiting
• API documentation

Use:

• Spring Security
• Swagger

• Transactions
• ACID
• Race conditions
• DTO mapping
• Service layer separation

Break E-commerce into:

• User Service
• Product Service
• Order Service
• Payment Service

• API Gateway
• Service-to-service communication
• Circuit breaker
• Logging
• Centralized config

Containerize using:

• Docker

• Distributed systems
• Service discovery
• Fault tolerance
• Horizontal scaling

• Generate short URLs
• Redirection
• Analytics
• Rate limiting

• Distributed ID generation
• Caching layer
• Load balancing simulation

Concepts:

• CAP theorem
• Consistency models
• Database sharding
• Index design

Choose one:

• WebSocket
• Message persistence
• Group chat
• Typing indicators

• Order matching engine
• Priority queue
• Concurrent order processing

• Worker nodes
• Job queue
• Retry mechanism

These projects differentiate you from average candidates.

Apply to one project:

• CI/CD pipeline
• Dockerize app
• Basic Kubernetes deployment
• Logging and monitoring

1. Clean architecture
2. Proper package structure
3. Unit + integration tests
4. Meaningful commit history
5. Performance consideration
6. Deployment experience

Work in this order:

1. Core Java projects
2. Concurrency project
3. Spring Boot CRUD
4. Full business system
5. Microservices
6. System design implementation

• Stage 1–2: 2 months
• Stage 3–4: 3 months
• Stage 5–6: 2 months
• Stage 7–8: 1–2 months

Total: 8–9 months serious effort.