Java Interview Preparation Roadmap

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1. Core Java

Topic	Subtopics	Example Questions / Practice
OOP Concepts	Inheritance, Polymorphism (compile/runtime), Abstraction, Encapsulation, Interfaces	Explain runtime vs compile-time polymorphism. Design a shape class hierarchy with method overriding.
Java Basics	main method, packages, access modifiers	- Difference between public, private, protected, default. - Explain static keyword and its use cases.
Data Types & Variables	Primitive vs Wrapper, autoboxing/unboxing	- Difference between int and Integer. - What happens during autoboxing?
Strings	String, StringBuilder, StringBuffer, immutability	- Why String is immutable? - Compare StringBuilder vs StringBuffer.
Collections Framework	List, Set, Map, Queue, implementations, differences	- Difference between ArrayList and LinkedList. - Implement LRU cache using LinkedHashMap.
Generics	Bounded types, wildcards, type erasure	- Difference between ? extends T and ? super T. - Why Java uses type erasure?
Exception Handling	Checked vs unchecked, throw vs throws, custom exceptions	- Write a custom exception. - Explain try-with-resources.
Java Memory Model	Stack vs Heap, GC, memory leaks, references	- Explain strong, weak, soft, phantom references. - What is a memory leak in Java?

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2. Java 8+ Features

Topic	Subtopics	Example Questions / Practice
Functional Interfaces	Predicate, Function, Consumer, Supplier	- Use Predicate to filter a list of strings starting with “A”.
Lambdas	Syntax, capturing variables, method references	- Convert an anonymous class into a lambda. - Explain this reference inside a lambda.
Streams API	Map, filter, reduce, collect, parallel streams	- Count distinct words in a list. - Group employees by department using streams.
Optional	Avoiding NPE, ifPresent, orElse	- How to avoid NullPointerException with Optional?
Date/Time API	LocalDate, LocalTime, LocalDateTime, formatting	- Calculate days between two dates using java.time.

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3. Multithreading & Concurrency

Topic	Subtopics	Example Questions / Practice
Threads	Thread vs Runnable vs Callable, Future	- Implement a multi-threaded task using Callable and Future.
Synchronization	synchronized, Locks, Deadlock	- Write a deadlock example and solve it.
Executor Framework	ExecutorService, ScheduledExecutorService	- Schedule a task to run every 5 seconds.
Concurrent Collections	ConcurrentHashMap, CopyOnWriteArrayList	- Difference between HashMap and ConcurrentHashMap.
Java Memory Visibility	volatile, happens-before	- Explain volatile with an example.
ForkJoinPool	Divide & conquer parallelism	- Implement parallel sum of an integer array.

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4. Advanced Java / JVM

Topic	Subtopics	Example Questions / Practice
JVM Internals	Class loading, bytecode, JIT	- Explain JVM memory areas. - What is classloader hierarchy?
Garbage Collection	Serial, Parallel, CMS, G1	- How does G1 GC work? - Explain memory leak examples.
Annotations & Reflection	Custom annotations, reflection API	- Create a custom annotation and process it using reflection.
Serialization	Serializable, Externalizable, transient	- Explain difference between Serializable and Externalizable.
Java Modules	module-info.java, exports, requires	- Explain module system in Java 9+.

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5. Design Patterns & Best Practices

Topic	Subtopics	Example Questions / Practice
Design Patterns	Singleton, Factory, Builder, Observer, Strategy	- Implement a thread-safe singleton. - Use Builder pattern to create an object.
SOLID Principles	SRP, OCP, LSP, ISP, DIP	- Refactor a class violating SRP.
Effective Java Practices	equals()/hashCode(), immutability	- Implement proper equals() and hashCode(). - Make a class immutable.

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6. Spring & Hibernate (if applicable)

Topic	Subtopics	Example Questions / Practice
Spring Core	IoC, DI, Bean lifecycle	- Explain constructor vs setter injection.
Spring Boot	Auto-configuration, profiles, starters	- How does Spring Boot simplify app setup?
Spring MVC	Controllers, REST API	- Write a REST API to get employee details.
Spring Security	Authentication, JWT, OAuth2	- Implement basic JWT authentication.
JPA & Hibernate	Entity lifecycle, caching, relationships	- Map a OneToMany and ManyToOne relationship. - Difference between lazy vs eager fetch.

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7. Common Coding/Design Questions

• Implement LRU Cache using LinkedHashMap.
• Producer-Consumer problem with threads.
• Deadlock demonstration and resolution.
• Custom comparator for sorting.
• Stream-based grouping/filtering/counting problems.
• Thread-safe singleton & object pool implementation.

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💡 Preparation Tips:

1. Start with Core Java → practice coding + conceptual questions.
2. Move to Java 8 features + Collections + OOP.
3. Dive into Multithreading + Concurrency.
4. Cover JVM internals + Garbage Collection + Serialization.
5. Finally, Spring/Hibernate + Design Patterns + Advanced coding.