Object-Oriented Programming:

• Primitive datatypes and their ranges, wrapper classes
• Abstract classes vs interfaces - this is frequently asked
• Inheritance, polymorphism, encapsulation
• Method overloading vs overriding
• Access modifiers and their scope

Java 8+ Features:

• Java 8 concepts including lambda expressions, streams, functional interfaces
• Optional class and handling null values
• Date/Time API improvements

Collections Framework:

• HashMap operations - inserting values and printing them
• TreeSet and that it implements Comparable interface
• ArrayList, LinkedList, HashSet differences
• Concurrent collections for multithreading

Multithreading & Concurrency

Essential Concepts:

• Race conditions - commonly asked in interviews
• Threading fundamentals for concurrent programming
• Synchronization mechanisms (synchronized keyword, locks)
• Thread safety and best practices

Data Structures & Algorithms

Must-Know Topics:

• Arrays, Strings, Linked Lists, Recursion, Sorting algorithms, Trees, and Dynamic Programming
• String manipulation and algorithms
• Dynamic Programming problems
• Graph algorithms including DFS/BFS

Common Coding Problems:

• Spiral order traversal of matrix, subarray with 0 sum
• Finding best average score from student data
• Tree traversals and operations
• String pattern matching and encoding/decoding

Database & SQL Knowledge

SQL Fundamentals:

• SQL joins, aggregate operators between tables
• Database design principles and normalization
• Different types of joins (INNER, LEFT, RIGHT, FULL OUTER, SELF, CARTESIAN)
• Query optimization and indexing

System Design & Architecture

Distributed Systems:

• Strong fundamentals in distributed systems, databases
• Microservices architecture patterns
• API design and RESTful web services
• Caching strategies and load balancing

Testing & Development Practices

Testing Concepts:

• Unit testing, integration testing, regression testing
• Test-driven development (TDD)
• Multiple environments for applications
• CI/CD pipeline understanding

Key Focus Areas:

1. Core Java mastery - Since you have strong C# background, focus on Java-specific syntax and features
2. Data structures/algorithms - Practice medium to hard LeetCode problems
3. System design - Your IDE project shows large-scale system thinking
4. Financial domain - Basic understanding of trading systems and market data

Common Interview Questions to Prepare

Technical Java Questions:

• Difference between abstract class and interface
• What is JVM and how does it work?
• Explain method overloading vs overriding
• How do wrapper classes work?
• What are the different access modifiers?

Coding Problems:

• Convert number (0-9 billion) to string literal (e.g., 12345 → "Twelve Thousand Three Hundred Forty Five")
• Merge two ArrayLists without using built-in functions, removing duplicates
• Find middle element of a sequence
• Best average score calculation from student data
• Tree-related problems and debugging

System Design Questions:

• How would you design a trading system?
• Explain RESTful web services
• What is the need for multiple environments?
• How would you add a new feature to an application?

SQL Questions:

• Write queries with joins and aggregate functions
• Foreign key relationships between tables
• Database optimization techniques