Java interview questions cover OOP principles, collections, concurrency, JVM internals, and design patterns. Questions range from language fundamentals to advanced topics like garbage collection, thread safety, and reactive programming with modern Java features.
Object-Oriented Programming (15)
Master the four pillars (encapsulation, inheritance, polymorphism, abstraction) and how Java implements them with classes, interfaces, and access modifiers.
easy What are the 4 pillars of OOP in Java?
What interviewers evaluate: Object-Oriented Programming
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easy What is the difference between abstraction and encapsulation?
What interviewers evaluate: Object-Oriented Programming
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easy What is the difference between method overloading and method overriding? Cite relevant examples.
What interviewers evaluate: Object-Oriented Programming
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easy What is polymorphism in Java? Explain compile-time vs runtime polymorphism.
What interviewers evaluate: Object-Oriented Programming
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easy What is the difference between an abstract class and an interface?
What interviewers evaluate: Object-Oriented Programming
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medium Can an interface have static methods? What about default methods?
What interviewers evaluate: Object-Oriented Programming
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medium What is the diamond problem in Java? How does Java 8 handle it with default methods?
What interviewers evaluate: Object-Oriented Programming
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hard What is the Liskov Substitution Principle?
What interviewers evaluate: Object-Oriented Programming
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medium What is the difference between composition and inheritance? Why is composition preferred?
What interviewers evaluate: Object-Oriented Programming
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medium What is the difference between Composition and Aggregation?
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easy What is the 'IS-A' relationship in OOP Java?
What interviewers evaluate: Object-Oriented Programming
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easy Why is Java not a pure object-oriented language?
What interviewers evaluate: Object-Oriented Programming
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easy Can Java be said to be a complete object-oriented programming language?
What interviewers evaluate: Object-Oriented Programming
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easy Explain data encapsulation with an example.
What interviewers evaluate: Object-Oriented Programming
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easy What are the access modifiers in Java and what is their scope?
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Data Types, Variables & Operators (7)
Cover Java's primitive types, autoboxing, type casting, operator precedence, and the gotchas that trip up junior developers in interviews.
easy What are the default values assigned to variables and instances in Java?
What interviewers evaluate: Data Types, Variables & Operators
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easy What do you understand by an instance variable and a local variable?
What interviewers evaluate: Data Types, Variables & Operators
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easy What is the default value of local variables?
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easy What are wrapper classes in Java?
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easy What is autoboxing and unboxing?
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medium What is the difference between the '>>' and '>>>' operators in Java?
What interviewers evaluate: Data Types, Variables & Operators
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medium Why does `System.out.println('b' + 'i' + 't')` print a number?
What interviewers evaluate: Data Types, Variables & Operators
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Strings (9)
Understand String immutability, the String pool, StringBuilder vs StringBuffer, and the performance implications of string operations in Java.
medium Why are strings immutable in Java? What are the reasons beyond the security aspect?
What interviewers evaluate: Strings
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medium What is the difference between String, StringBuffer, and StringBuilder?
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medium How is the creation of a String using `new()` different from that of a literal?
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medium Why is the character array preferred over String for storing confidential information?
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hard Why is it said that the `length()` method of String class doesn't return accurate results?
What interviewers evaluate: Strings
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easy Which should be preferred when there are lots of updates: String or StringBuffer?
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medium What is the String pool in Java?
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medium Why does `String s = "a"; s += "b";` create extra objects?
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medium Why can `==` be true for some strings and false for others?
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Exceptions (11)
Learn checked vs unchecked exceptions, try-with-resources, exception hierarchy, custom exceptions, and Java's exception handling best practices.
easy What is the difference between checked and unchecked exceptions?
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easy What is the difference between the 'throw' and 'throws' keyword in Java?
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easy Do `final`, `finally`, and `finalize` keywords have the same function?
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easy A single try block and multiple catch blocks can co-exist in a Java program. Explain.
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medium Is it possible that the 'finally' block will not be executed? If yes, list the cases.
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medium Will the finally block be executed if `System.exit(0)` is written at the end of the try block?
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medium What happens if you call return in try and finally also has a return?
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easy Is it mandatory for a catch block to follow a try block?
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medium How does an exception propagate in the code?
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easy How do exceptions affect the program if it doesn't handle them?
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medium Write a Java program to create and throw custom exceptions.
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Core Language Features (25)
Cover static vs instance, final keyword, nested classes, generics, annotations, and the language features every Java developer must know.
easy Explain the use of the `final` keyword in variable, method, and class.
What interviewers evaluate: Core Language Features
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easy Can the main method be overloaded?
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easy Why is the main method static in Java?
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medium Can static methods be overridden? Why or why not?
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easy Explain `public static void main(String args[])` in Java.
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easy What is the difference between `equals()` method and equality operator `==` in Java?
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easy How is Java different from C++?
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easy Why does Java not make use of pointers?
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easy What is the difference between Heap and Stack Memory in Java?
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easy When can you use the `super` keyword?
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easy Explain the concept of constructor overloading.
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easy Define copy constructor in Java.
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medium Can you call a constructor of a class inside another constructor?
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easy What are the differences between constructor and method of a class in Java?
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medium Java works as 'pass by value' or 'pass by reference'?
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medium What is the importance of reflection in Java?
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medium What do you understand by marker interfaces in Java?
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hard Explain the term 'Double Brace Initialization' in Java.
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medium What is a singleton class in Java? How do you implement it?
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medium How to not allow serialization of attributes of a class in Java?
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medium What do you understand by Object Cloning and how do you achieve it?
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medium What are shallow copy and deep copy in Java?
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medium What is a ClassLoader?
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medium Tell us something about JIT compiler.
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easy What do you understand about JDK, JRE, and JVM?
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Collections Framework (36)
Master ArrayList, HashMap, ConcurrentHashMap, TreeMap, LinkedList, and the time complexity, thread safety, and use cases of each collection.
medium How is `HashMap` implemented internally in Java?
What interviewers evaluate: 2. Collections Framework
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easy What is the difference between `ArrayList` and `LinkedList`?
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medium What is the difference between `HashMap` and `HashTable`?
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medium What makes a `HashSet` different from a `TreeSet`?
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easy What is the difference between `HashSet` and `HashMap`?
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easy What is the difference between `ArrayList` and `Vector`?
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easy Differentiate between `List` and `Set` in Java.
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easy Differentiate between `Set` and `Map` in Java.
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medium Differentiate between `Iterator` and `ListIterator` in Java.
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medium What is the difference between `Comparable` and `Comparator`?
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hard When should you use `ConcurrentHashMap` vs `Collections.synchronizedMap()`?
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hard What is fail-fast vs fail-safe iterator?
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easy Explain the hierarchy of the Collection framework in Java.
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easy Explain the various interfaces used in the Collection framework.
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easy Differentiate between `Collection` and `Collections` in Java.
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medium How do `TreeMap` and `HashMap` differ?
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medium Can you add a null element into a `TreeSet` or `HashSet`?
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medium Why does `HashMap` allow one null key, but `HashTable` doesn't?
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medium What is a priority queue in Java?
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medium How can you make an `ArrayList` read-only in Java?
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medium How can you synchronize an `ArrayList` in Java?
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medium Why does the `remove()` method run faster in LinkedList than in an array?
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medium How does the size of `ArrayList` grow dynamically? Explain the internal implementation.
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medium Contiguous memory locations are usually used for storing actual values in an array but not in `ArrayList`. Explain.
What interviewers evaluate: 2. Collections Framework
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easy Why does the Java array index start with 0?
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easy What is the difference between an Array and a Collection in Java?
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easy Differentiate between `Array` and `ArrayList`.
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medium What is the default size of the load factor in hashing-based collections?
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hard What is `BlockingQueue`?
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medium What are some best practices while using Java Collections?
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hard Why doesn't the `Map` interface extend the `Collection` interface?
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hard What is the purpose of the `RandomAccess` interface?
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medium What is the difference between `Iterator` and `Enumeration`?
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medium What is the use of the `Properties` class in Java?
What interviewers evaluate: 2. Collections Framework
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hard Why do we need synchronized `ArrayList` when we have `Vector`?
What interviewers evaluate: 2. Collections Framework
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hard What are the benefits of using `ConcurrentHashMap`?
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Multithreading & Concurrency (52)
Cover threads, ExecutorService, locks, synchronized blocks, volatile, atomic classes, and how to write thread-safe code that scales.
easy What is a thread in Java? What is the difference between a thread and a process?
What interviewers evaluate: 3. Multithreading & Concurrency
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easy What are the two ways of creating a thread in Java?
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medium What are the different states of a thread in Java? Explain the thread lifecycle.
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easy What is the difference between `Runnable` and `Thread`?
What interviewers evaluate: 3. Multithreading & Concurrency
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medium What is the difference between `Runnable` and `Callable`? When should you use `Callable` over `Runnable`?
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easy What is the difference between `start()` and `run()` method?
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medium What is synchronization in Java? Why is it necessary?
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medium How does the `synchronized` keyword work?
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medium What is the difference between synchronized method and synchronized block? Which should be preferred?
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hard What is a deadlock? How do you prevent it?
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medium What is the difference between `wait()` and `sleep()`?
What interviewers evaluate: 3. Multithreading & Concurrency
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medium What is the purpose of `wait()`, `notify()`, and `notifyAll()`?
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hard Why are `wait()`, `notify()`, and `notifyAll()` in the `Object` class, not the `Thread` class?
What interviewers evaluate: 3. Multithreading & Concurrency
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medium What is the difference between `notify()` and `notifyAll()`?
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medium What is the purpose of the `volatile` keyword?
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hard What is the difference between the `volatile` keyword and Atomic classes?
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hard How does `ThreadLocal` work?
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hard What is the difference between `synchronized` and `ReentrantLock`?
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hard What is the lock interface? Why is it preferred over synchronized blocks?
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medium What is the difference between class lock and object lock?
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medium What is the difference between user thread and daemon thread?
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medium How can we create daemon threads?
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medium Can you start a thread twice?
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easy Is it possible to call the `run()` method directly to start a new thread?
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medium What is thread safety and why is it important?
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medium What are the ways to achieve thread safety in Java?
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medium What is an atomic operation?
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hard What are the classes in `java.util.concurrent.atomic` package?
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hard What is the `ExecutorService` in Java? How is it different from using threads directly?
What interviewers evaluate: 3. Multithreading & Concurrency
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hard What is the difference between `execute()` and `submit()` methods in `ExecutorService`?
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hard How do you gracefully shut down an `ExecutorService`?
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hard What is the difference between `shutdown()` and `shutdownNow()`?
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hard What is a `Future` in Java and how is it related to `ExecutorService`?
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medium What is a thread pool and why is it used? Explain the different types.
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hard How does `ThreadPoolExecutor` work internally?
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hard What are the core and maximum pool sizes in `ThreadPoolExecutor`?
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hard What is the keep-alive time in `ThreadPoolExecutor`?
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hard What is `CyclicBarrier` and `CountDownLatch`? How do they differ?
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hard What is a `Semaphore`?
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hard What is `BlockingQueue` and how is it used in producer-consumer pattern?
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hard What is thread starvation?
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hard What is livelock? How is it different from deadlock?
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medium Explain context switching.
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hard What is busy spinning?
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hard What is a `ScheduledExecutorService` and how do you use it?
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hard Explain the producer-consumer problem and how to solve it in Java.
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hard Implement a singleton class in a multithreaded environment.
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medium How do threads communicate with each other?
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medium Explain thread priority in Java. What is the default priority?
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hard What is a shutdown hook?
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medium How do you handle exceptions in threads?
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hard What is a `Condition` in Java concurrency?
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Java 8+ Features (Streams, Lambdas, Optional) (27)
Master functional Java: Stream API, Collectors, lambda expressions, method references, Optional, and the modern features that define current Java code.
medium What is a lambda expression in Java? How does it relate to a functional interface?
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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medium What is a functional interface (SAM interface)? Give examples.
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medium What are the most common pre-defined functional interfaces in Java 8?
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easy What is the basic structure/syntax of a lambda expression?
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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medium What is a method reference in Java 8? What does `String::valueOf` mean?
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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medium What is the difference between `map()` and `flatMap()` in Streams?
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medium What are Java 8 Streams? How are they different from Collections?
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medium What are intermediate and terminal operations in Streams? Give examples.
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medium What are the most commonly used intermediate operations?
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medium What is the difference between `findFirst()` and `findAny()`?
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medium What is an `Optional` class? How should it be used?
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medium What are the advantages of using the `Optional` class?
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medium What is the purpose of default methods in interfaces?
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medium Can a functional interface extend/inherit another interface?
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medium What are static methods in interfaces?
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hard What is a stateful intermediate operation? Give examples.
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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hard What is parallel stream? When should you use it?
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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medium What is the feature of the new Date and Time API in Java 8?
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medium What are the important packages for the new Date and Time API?
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medium Explain `LocalDate`, `LocalTime`, and `LocalDateTime` APIs.
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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easy What are the significant advantages of Java 8?
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hard What is MetaSpace? How does it differ from PermGen?
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medium What is `var` (local type inference) introduced in Java 10?
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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medium What are records in Java 14+?
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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hard What are sealed classes in Java 17?
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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hard What are virtual threads in Java 21? How do they differ from platform threads?
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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hard What is pattern matching for switch in modern Java?
What interviewers evaluate: 4. Java 8+ Features (Streams, Lambdas, Optional)
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JVM Architecture & Memory Model (10)
Understand heap vs stack, classloaders, JIT compilation, the Java memory model, and how the JVM executes your code under the hood.
easy What is the difference between Heap and Stack memory in Java?
What interviewers evaluate: JVM Architecture & Memory Model
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hard Explain the Java memory model.
What interviewers evaluate: JVM Architecture & Memory Model
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hard What is PermGen vs Metaspace?
What interviewers evaluate: JVM Architecture & Memory Model
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medium What is a ClassLoader in Java?
What interviewers evaluate: JVM Architecture & Memory Model
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medium What is the JIT compiler?
What interviewers evaluate: JVM Architecture & Memory Model
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easy What are the differences between JVM, JRE, and JDK?
What interviewers evaluate: JVM Architecture & Memory Model
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easy Where do objects get created in Java?
What interviewers evaluate: JVM Architecture & Memory Model
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easy What part of memory (Stack or Heap) is cleaned in the garbage collection process?
What interviewers evaluate: JVM Architecture & Memory Model
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hard What is Eden and Survivor space in the heap?
What interviewers evaluate: JVM Architecture & Memory Model
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hard What are the different regions the heap is divided into for GC?
What interviewers evaluate: JVM Architecture & Memory Model
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Garbage Collection (17)
Cover G1, ZGC, Shenandoah, generational GC, and how to read GC logs, tune heap parameters, and diagnose memory issues in production.
easy What is garbage collection in Java? What is its main objective?
What interviewers evaluate: Garbage Collection
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medium When does an object become eligible for garbage collection?
What interviewers evaluate: Garbage Collection
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medium Can you force garbage collection in Java?
What interviewers evaluate: Garbage Collection
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medium What is the difference between `System.gc()` and `Runtime.gc()`?
What interviewers evaluate: Garbage Collection
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hard What is the difference between minor and major garbage collection?
What interviewers evaluate: Garbage Collection
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hard How does the G1 Garbage Collector work?
What interviewers evaluate: Garbage Collection
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hard How does G1GC differ from CMS?
What interviewers evaluate: Garbage Collection
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hard What is the difference between Serial and Parallel Garbage Collector?
What interviewers evaluate: Garbage Collection
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hard What is the difference between ParNew and DefNew garbage collectors?
What interviewers evaluate: Garbage Collection
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hard What are the different kinds of garbage collectors available in Java?
What interviewers evaluate: Garbage Collection
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hard How do you monitor garbage collection activities?
What interviewers evaluate: Garbage Collection
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hard How do you enable garbage collection logs?
What interviewers evaluate: Garbage Collection
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hard Have you done any garbage collection tuning? What was your approach?
What interviewers evaluate: Garbage Collection
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medium How do you increase heap memory in Java?
What interviewers evaluate: Garbage Collection
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medium What are the best practices to help the garbage collector?
What interviewers evaluate: Garbage Collection
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hard Does garbage collection occur in the permanent generation space?
What interviewers evaluate: Garbage Collection
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medium What is the `finalize()` method? Can you call it directly?
What interviewers evaluate: Garbage Collection
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Memory Leaks & Troubleshooting (8)
Learn how to find and fix Java memory leaks using heap dumps, profilers, and the common patterns that cause OutOfMemoryError in production.
hard What is a memory leak in Java? Discuss common causes.
What interviewers evaluate: Memory Leaks & Troubleshooting
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hard How can you detect and fix memory leaks?
What interviewers evaluate: Memory Leaks & Troubleshooting
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hard Is exceeding the memory limit possible despite having a garbage collector?
What interviewers evaluate: Memory Leaks & Troubleshooting
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medium What are the possible ways to make an object eligible for GC?
What interviewers evaluate: Memory Leaks & Troubleshooting
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hard What are strong vs weak references?
What interviewers evaluate: Memory Leaks & Troubleshooting
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hard What is `OutOfMemoryError`? What are its 3 most likely causes?
What interviewers evaluate: Memory Leaks & Troubleshooting
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medium How do you find how much memory is used and total memory in Java?
What interviewers evaluate: Memory Leaks & Troubleshooting
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hard A thread has a lock on it; calling `sleep()` on that thread will release the lock?
What interviewers evaluate: Memory Leaks & Troubleshooting
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Core Spring Concepts (12)
Cover Inversion of Control, dependency injection, beans, ApplicationContext, and the core Spring concepts that every Java backend developer must know.
medium What are the advantages of using Spring Boot over Spring?
What interviewers evaluate: Core Spring Concepts
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hard How does Spring Boot auto-configuration work?
What interviewers evaluate: Core Spring Concepts
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medium What does the `@SpringBootApplication` annotation do internally?
What interviewers evaluate: Core Spring Concepts
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medium What is an IOC container?
What interviewers evaluate: Core Spring Concepts
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medium What is dependency injection? What is the best way to inject dependency?
What interviewers evaluate: Core Spring Concepts
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medium What is the difference between `@Component`, `@Service`, and `@Repository`?
What interviewers evaluate: Core Spring Concepts
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hard How does Spring manage bean lifecycle?
What interviewers evaluate: Core Spring Concepts
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medium How do you set the Spring bean scope? What supported scopes does it have?
What interviewers evaluate: Core Spring Concepts
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medium What is the difference between `@Bean` and `@Component`?
What interviewers evaluate: Core Spring Concepts
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medium What is `@Configuration` and what problem does it solve?
What interviewers evaluate: Core Spring Concepts
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hard What is the role of `@ConditionalOnProperty`?
What interviewers evaluate: Core Spring Concepts
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hard How do you handle circular dependencies in Spring?
What interviewers evaluate: Core Spring Concepts
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REST APIs & Web (6)
Build production REST APIs in Java covering @RestController, request mapping, validation, exception handling, and HTTP status codes.
medium What is the difference between `@RestController` and `@Controller`?
What interviewers evaluate: REST APIs & Web
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medium What is the difference between `@RequestMapping` and `@GetMapping`?
What interviewers evaluate: REST APIs & Web
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medium What is the difference between `@RequestParam`, `@PathVariable`, and `@RequestBody`?
What interviewers evaluate: REST APIs & Web
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hard Describe the flow of HTTPS requests through a Spring Boot application.
What interviewers evaluate: REST APIs & Web
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medium How do you design a REST API in Spring Boot (high level)?
What interviewers evaluate: REST APIs & Web
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hard How do you version APIs without breaking clients?
What interviewers evaluate: REST APIs & Web
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Spring Boot Operations (8)
Learn Spring Boot Actuator, Profiles, externalized configuration, embedded servers, and the operational concerns of running Spring Boot in production.
medium What is Spring Actuator? What are its advantages?
What interviewers evaluate: Spring Boot Operations
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medium What are the actuator-provided endpoints for monitoring?
What interviewers evaluate: Spring Boot Operations
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medium What is the use of Profiles in Spring Boot?
What interviewers evaluate: Spring Boot Operations
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easy Where do we define properties in a Spring Boot application?
What interviewers evaluate: Spring Boot Operations
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hard What is the difference between `application.properties` and `bootstrap.properties`?
What interviewers evaluate: Spring Boot Operations
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medium Can we override or replace the embedded Tomcat server?
What interviewers evaluate: Spring Boot Operations
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easy What is the default port of Tomcat in Spring Boot?
What interviewers evaluate: Spring Boot Operations
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easy What are starter dependencies?
What interviewers evaluate: Spring Boot Operations
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Spring Boot Tricky Questions (5)
Tackle the gotchas that catch experienced developers: @Transactional pitfalls, lazy initialization, circular dependencies, and N+1 queries.
hard Why does `@Transactional` sometimes not work in Spring Boot?
What interviewers evaluate: Spring Boot Tricky Questions
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hard What causes `LazyInitializationException` in Spring Boot?
What interviewers evaluate: Spring Boot Tricky Questions
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hard Why is `spring.jpa.hibernate.ddl-auto=update` not recommended in production?
What interviewers evaluate: Spring Boot Tricky Questions
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hard Why does field injection cause testing and immutability issues?
What interviewers evaluate: Spring Boot Tricky Questions
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hard What causes N+1 queries in Hibernate and how do you fix it?
What interviewers evaluate: Spring Boot Tricky Questions
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Security & Microservices (11)
Cover Spring Security, JWT, OAuth2, RBAC, inter-service communication, resilience patterns, and the challenges of distributed Java systems.
hard How do you secure REST endpoints in Spring?
What interviewers evaluate: Security & Microservices
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hard What is the difference between OAuth2 and JWT?
What interviewers evaluate: Security & Microservices
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hard How do you implement RBAC in Spring Security?
What interviewers evaluate: Security & Microservices
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hard How do you implement inter-service communication in microservices?
What interviewers evaluate: Security & Microservices
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hard How do you implement retry, fallback, and circuit breaker?
What interviewers evaluate: Security & Microservices
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hard What is the role of API Gateway in microservice architecture?
What interviewers evaluate: Security & Microservices
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hard What is the Saga pattern and when should you use it?
What interviewers evaluate: Security & Microservices
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hard How do you handle data consistency in distributed transactions?
What interviewers evaluate: Security & Microservices
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hard What is Spring Cloud Config Server?
What interviewers evaluate: Security & Microservices
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hard How do you implement distributed tracing?
What interviewers evaluate: Security & Microservices
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hard How do you achieve zero-downtime deployment?
What interviewers evaluate: Security & Microservices
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Design Patterns in Java (20)
Master Singleton, Factory, Strategy, Observer, Builder, and the Java-specific implementations of GoF patterns interviewers expect you to know.
medium What are the different categories of Java design patterns (creational, structural, behavioral)?
What interviewers evaluate: 7. Design Patterns in Java
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medium What is the Singleton pattern? How do you implement it in Java (including thread-safe versions)?
What interviewers evaluate: 7. Design Patterns in Java
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medium How is the Factory pattern different from Abstract Factory?
What interviewers evaluate: 7. Design Patterns in Java
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medium What is the Strategy pattern? Give a real-world use case.
What interviewers evaluate: 7. Design Patterns in Java
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medium Where is the Builder pattern commonly used? Why?
What interviewers evaluate: 7. Design Patterns in Java
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medium What is the Observer pattern?
What interviewers evaluate: 7. Design Patterns in Java
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hard Explain the Proxy pattern with real-world usage.
What interviewers evaluate: 7. Design Patterns in Java
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hard What is the difference between the Adapter and Decorator pattern?
What interviewers evaluate: 7. Design Patterns in Java
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hard What is the Template Method pattern?
What interviewers evaluate: 7. Design Patterns in Java
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hard When do you use the Command pattern?
What interviewers evaluate: 7. Design Patterns in Java
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hard What is the benefit of Chain of Responsibility?
What interviewers evaluate: 7. Design Patterns in Java
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medium What is Dependency Injection? How does Spring use it?
What interviewers evaluate: 7. Design Patterns in Java
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medium How is MVC implemented in Spring?
What interviewers evaluate: 7. Design Patterns in Java
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hard What is AOP (Aspect-Oriented Programming)? Common use cases?
What interviewers evaluate: 7. Design Patterns in Java
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hard How is Lazy Initialization useful in design patterns?
What interviewers evaluate: 7. Design Patterns in Java
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hard Can patterns introduce unnecessary complexity? How do you balance it?
What interviewers evaluate: 7. Design Patterns in Java
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hard What is the Mediator pattern?
What interviewers evaluate: 7. Design Patterns in Java
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hard Give an example of the Flyweight pattern in Java APIs.
What interviewers evaluate: 7. Design Patterns in Java
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hard What pattern does Java's Enum Singleton use?
What interviewers evaluate: 7. Design Patterns in Java
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hard How do you test code that uses Factory or Strategy pattern?
What interviewers evaluate: 7. Design Patterns in Java
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String & Array Manipulation (6)
Practice the coding problems that test fundamentals: palindromes, anagrams, two-pointer, sliding window, and array manipulation techniques.
easy Check if a given string is a palindrome using recursion.
What interviewers evaluate: String & Array Manipulation
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easy Write a Java program to reverse a string.
What interviewers evaluate: String & Array Manipulation
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easy Write a Java program to check if two strings are anagrams.
What interviewers evaluate: String & Array Manipulation
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medium Given an array of non-duplicating numbers from 1 to n where one number is missing, find the missing number.
What interviewers evaluate: String & Array Manipulation
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medium Write a Java program to rotate arrays 90 degrees clockwise by taking matrices from user input.
What interviewers evaluate: String & Array Manipulation
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medium How do you find the intersection of two arrays?
What interviewers evaluate: String & Array Manipulation
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Recursion & Algorithms (6)
Cover Fibonacci, factorial, binary search, DFS, BFS, divide and conquer, and the recursive patterns that show up in Java coding interviews.
easy Write a Java program to print the Fibonacci series using recursion.
What interviewers evaluate: Recursion & Algorithms
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easy Write a Java program to find the factorial of a given number.
What interviewers evaluate: Recursion & Algorithms
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medium Implement Binary Search in Java using recursion.
What interviewers evaluate: Recursion & Algorithms
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medium Write a Java program for solving the Tower of Hanoi problem.
What interviewers evaluate: Recursion & Algorithms
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medium Write a Java program to check if a number is the sum of 2 prime numbers.
What interviewers evaluate: Recursion & Algorithms
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easy Check if any number is a magic number or not.
What interviewers evaluate: Recursion & Algorithms
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Data Structures (9)
Implement and analyze LinkedList reversal, LRU cache, binary trees, heaps, tries, and the data structures every Java engineer should know.
medium How do you reverse a LinkedList without using extra space?
What interviewers evaluate: Data Structures
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hard Implement an LRU cache in Java.
What interviewers evaluate: Data Structures
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medium What is the time complexity of `HashMap` operations (get, put, remove)?
What interviewers evaluate: Data Structures
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easy Explain binary search algorithm and its time complexity.
What interviewers evaluate: Data Structures
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medium How does QuickSort work? Implement it in Java.
What interviewers evaluate: Data Structures
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medium What is the difference between BFS and DFS?
What interviewers evaluate: Data Structures
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medium When is a heap used in Java?
What interviewers evaluate: Data Structures
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hard What is a Trie? Give a use case.
What interviewers evaluate: Data Structures
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hard How do you detect a cycle in a graph?
What interviewers evaluate: Data Structures
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Java-Specific Coding (4)
Solve coding problems using Java's specific features: custom exceptions, producer-consumer with BlockingQueue, thread-safe Singleton, and memoization.
hard Implement the producer-consumer problem using `BlockingQueue`.
What interviewers evaluate: Java-Specific Coding
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hard Implement a thread-safe Singleton.
What interviewers evaluate: Java-Specific Coding
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hard What is dynamic programming? Solve a DP problem in Java.
What interviewers evaluate: Java-Specific Coding
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hard What is memoization? Implement it in Java.
What interviewers evaluate: Java-Specific Coding
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Scenario-Based & System Design Questions (10)
Debug real production scenarios: slow apps, intermittent timeouts, OOM errors, concurrency bugs, and the systematic approach to troubleshooting Java systems.
hard Your app is slow after running for 2 hours. What do you check first?
What interviewers evaluate: 9. Scenario-Based & System Design Questions
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hard An API is timing out intermittently. How do you debug it?
What interviewers evaluate: 9. Scenario-Based & System Design Questions
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hard You see `OutOfMemoryError`. What are the 3 most likely causes?
What interviewers evaluate: 9. Scenario-Based & System Design Questions
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hard Multiple users update the same record simultaneously. How do you avoid wrong data?
What interviewers evaluate: 9. Scenario-Based & System Design Questions
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hard A background job runs twice. How do you prevent duplicates?
What interviewers evaluate: 9. Scenario-Based & System Design Questions
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hard You must handle 100,000 requests per minute. What changes in code or design?
What interviewers evaluate: 9. Scenario-Based & System Design Questions
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hard A thread is stuck. How do you identify why?
What interviewers evaluate: 9. Scenario-Based & System Design Questions
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medium You need to release resources reliably. What pattern do you use?
What interviewers evaluate: 9. Scenario-Based & System Design Questions
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hard How do you approach debugging a production issue end-to-end?
What interviewers evaluate: 9. Scenario-Based & System Design Questions
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hard What testing mix do you typically aim for in full-stack work (unit, integration, e2e)?
What interviewers evaluate: 9. Scenario-Based & System Design Questions
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Database & SQL (Java Context) (11)
Cover JOINs, normalization, indexing, ACID, isolation levels, JPA optimization, and the SQL knowledge expected of senior Java developers.
easy What is the difference between INNER JOIN and LEFT JOIN?
What interviewers evaluate: 10. Database & SQL (Java Context)
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easy What is database normalization?
What interviewers evaluate: 10. Database & SQL (Java Context)
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medium How do you prevent SQL injection in Java?
What interviewers evaluate: 10. Database & SQL (Java Context)
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medium What is indexing and how does it work?
What interviewers evaluate: 10. Database & SQL (Java Context)
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medium What is ACID in databases?
What interviewers evaluate: 10. Database & SQL (Java Context)
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hard Explain database isolation levels.
What interviewers evaluate: 10. Database & SQL (Java Context)
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medium How are transactions managed in Spring?
What interviewers evaluate: 10. Database & SQL (Java Context)
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hard What is optimistic vs pessimistic locking?
What interviewers evaluate: 10. Database & SQL (Java Context)
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medium When should you use NoSQL over RDBMS?
What interviewers evaluate: 10. Database & SQL (Java Context)
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hard What is the use of `JOIN FETCH` in JPQL?
What interviewers evaluate: 10. Database & SQL (Java Context)
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hard How do you profile slow queries?
What interviewers evaluate: 10. Database & SQL (Java Context)
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Top 15 Recommended Questions (9)
Start here. The 15 highest-impact Java interview questions covering HashMap internals, deadlocks, Streams, GC, Spring, and more.
medium How is HashMap implemented internally in Java?
What interviewers evaluate: Collections
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medium Why are Strings immutable in Java?
What interviewers evaluate: Core Java
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medium Explain the Java thread lifecycle and its states.
What interviewers evaluate: Concurrency
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medium What is the Singleton pattern? Implement a thread-safe version.
What interviewers evaluate: Design Patterns
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hard What is garbage collection? Name the different collectors and when to use each.
What interviewers evaluate: JVM
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easy What is the difference between ArrayList and LinkedList?
What interviewers evaluate: Collections
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medium What is the difference between Runnable and Callable?
What interviewers evaluate: Concurrency
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medium What is a functional interface? Give examples.
What interviewers evaluate: Java 8+
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hard What is a memory leak in Java? How do you detect and fix it?
What interviewers evaluate: JVM
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