Java Interview Questions

Java interview questions with clear answers and runnable code examples, from core Java to concurrency, JVM internals, Java 8–21 features, DSA coding problems and low-level design.

369 questions · 198 Easy · 146 Medium · 25 Hard · every answer is collapsible, so try to answer first, then click to check.

Companion repo for Spring and Spring Boot: SpringBoot_Interview_Questions

Difficulty levels

Folders and the files inside them are numbered from basic to advanced, so read them in order. Every question is tagged by difficulty, and within each file questions run from Easy → Medium → Hard (numbered DSA problems keep their order):

Level	Meaning
[Easy]	Fundamentals every Java developer is expected to answer (freshers, screening rounds)
[Medium]	Follow-ups and practical usage (1–3 years of experience)
[Hard]	Internals, production troubleshooting and design trade-offs (3+ years, senior rounds)

How to use this repo

Pick a topic and read the questions without opening the answers.
Answer out loud (about 1 minute each), then expand the answer and compare.
For 10-dsa-coding, solve on paper or LeetCode first (links included), then check the solution and complexity.
Revisit the questions you got wrong the day before the interview.

Suggested study path

Stage	Focus
1. Foundations	All [Easy] questions in `01` to `04`, plus `10-dsa-coding/01-classic-java-programs`
2. Working knowledge	[Medium] questions in collections, multithreading, Java 8 streams, exceptions; DSA problems 1–20
3. Experienced rounds	[Hard] questions: HashMap/ConcurrentHashMap internals, JMM and volatile, JVM troubleshooting, generics; DSA 21–33 and LLD

Topics

Java Basics

Java platform (5: 4 Easy · 1 Medium)

[Easy] Why is Java so Popular?
[Easy] What is Platform Independence?
[Easy] What is ByteCode?
[Easy] Compare JDK vs JVM VS JRE.
[Medium] What is the role of class loader in java ?

Core Java essentials (11: 6 Easy · 5 Medium)

[Easy] What is the main method signature and why is it public static void?
[Easy] What does the static keyword mean for variables, methods, blocks and classes?
[Easy] What is the difference between this and super?
[Easy] What is the difference between == and equals() for Strings?
[Easy] What are varargs?
[Easy] What is the difference between final, finally and finalize?
[Medium] Is Java pass-by-value or pass-by-reference?
[Medium] How do you create an immutable class in Java?
[Medium] What is a marker interface? Give examples.
[Medium] What is the difference between shallow copy and deep copy? How does clone() work?
[Medium] What are records and sealed classes, and why were they added?

Strings (7: 4 Easy · 3 Medium)

[Easy] Are all String’s immutable?
[Easy] What are differences between String and StringBuffer?
[Easy] What are differences between StringBuilder and StringBuffer?
[Easy] Can you give examples of different utility methods in String class?
[Medium] Where are string values stored in memory?
[Medium] Why should you be careful about String Concatenation(+) operator in Loops?
[Medium] How do you avoid the cost of String concatenation in loops?

Wrapper classes and casting (8: 6 Easy · 2 Medium)

[Easy] What are wrapper classes?
[Easy] Why do we need Wrapper Classes in Java?
[Easy] What is Auto Boxing?
[Easy] What is Casting?
[Easy] What is Implicit Casting?
[Easy] What is Explicit Casting?
[Medium] What are the different ways of creating Wrapper Class Instances?
[Medium] What are the advantages of Auto Boxing?

OOP, SOLID and Design Patterns

OOP basics (32: 26 Easy · 6 Medium)

[Easy] What is a Class?
[Easy] What is an Object?
[Easy] What is state of an Object?
[Easy] What is behavior of an Object?
[Easy] What is the super class of every class in Java?
[Easy] Explain about toString method ?
[Easy] Explain inheritance with Examples?
[Easy] What is Method Overloading?
[Easy] What is Method Overriding?
[Easy] Can super class reference variable can hold an object of sub class?
[Easy] Is Multiple Inheritance allowed in Java?
[Easy] What is an Interface?
[Easy] How do you define an Interface?
[Easy] How do you implement an interface?
[Easy] Can you extend an interface?
[Easy] Can a class implement multiple interfaces?
[Easy] What is an Abstract Class?
[Easy] When do you use an Abstract Class?
[Easy] How do you define an abstract method?
[Easy] What is a Constructor?
[Easy] What is a Default Constructor?
[Easy] Will this code compile? (no-arg and one-arg constructors)
[Easy] How do you call a Super Class Constructor from a Constructor?
[Easy] What is the use of this()?
[Easy] Can a constructor be called directly from a method?
[Easy] Is a super class constructor called even when there is no explicit call from a sub class constructor?
[Medium] What is the use of equals method in Java ?
[Medium] What are the important things to consider when implementing equals method?
[Medium] What is the hashCode method used for in Java?
[Medium] Can you explain a few tricky things about interfaces?
[Medium] Compare Abstract Class vs Interface?
[Medium] Will this code compile? (statement before super())

Access and non-access modifiers (15: 11 Easy · 3 Medium · 1 Hard)

[Easy] What is default class modifier?
[Easy] What is private access modifier?
[Easy] What is default or package access modifier?
[Easy] What is public access modifier?
[Easy] What access types of variables can be accessed from a Class in Same Package?
[Easy] What access types of variables can be accessed from a Class in Different Package?
[Easy] What is the use of a final modifier on a class ?
[Easy] What is the use of a final modifier on a method ?
[Easy] What is a Final Variable ?
[Easy] What is a Final Argument ?
[Easy] What is a Static Variable ?
[Medium] What is protected access modifier?
[Medium] What access types of variables can be accessed from a Sub Class in Same Package?
[Medium] What access types of variables can be accessed from a Sub Class in Different Package?
[Hard] What happens when a variable is marked as volatile?

Advanced OOP (9: 5 Easy · 4 Medium)

[Easy] What is Polymorphism?
[Easy] What is the use of instanceof Operator in Java?
[Easy] What is Coupling?
[Easy] What is Cohesion?
[Easy] What is Encapsulation?
[Medium] What is an Inner Class?
[Medium] What is a Static Inner Class?
[Medium] Can you create an inner class inside a method?
[Medium] What is an Anonymous Class?

SOLID and design patterns (8: 2 Easy · 6 Medium)

[Easy] What is the Factory pattern? Give a real example.
[Easy] What is the Builder pattern?
[Medium] Explain the SOLID principles with examples.
[Medium] Write a thread-safe Singleton. Which approach is best?
[Medium] What is the Strategy pattern?
[Medium] What is the Observer pattern?
[Medium] What are Adapter, Decorator, Proxy and Template Method?
[Medium] Composition vs inheritance: which should you prefer?

Exception Handling

Exception handling (18: 13 Easy · 5 Medium)

[Easy] Why is Exception Handling important?
[Easy] What is the need for finally block?
[Easy] In what scenarios is code in finally not executed?
[Easy] Will finally be executed in the program below?
[Easy] Is try without a catch is allowed?
[Easy] Is try without catch and finally allowed?
[Easy] Can you explain the hierarchy of Exception Handling classes?
[Easy] What is the difference between Error and Exception?
[Easy] What is the difference between Checked Exceptions and Unchecked Exceptions?
[Easy] How do you throw an exception from a method?
[Easy] What happens when you throw a Checked Exception from a method ?
[Easy] How do you create a Custom Exception?
[Easy] How do you handle multiple exception types with same exception handling block?
[Medium] What design pattern is used to implement Exception handling Features in most languages?
[Medium] What are the options you have to eliminate compilation errors when handling checked exceptions?
[Medium] Can you explain about try with resources?
[Medium] How does try with resources work?
[Medium] Can you explain a few Exception Handling Best Practices?

Exceptions (advanced) (8: 1 Easy · 5 Medium · 2 Hard)

[Easy] What is the difference between throw and throws?
[Medium] What happens if both try and finally contain a return statement?
[Medium] What is exception chaining and why does it matter?
[Medium] What are the exception rules for method overriding?
[Medium] When would you create a checked vs an unchecked custom exception?
[Medium] What is the difference between ClassNotFoundException and NoClassDefFoundError?
[Hard] What are suppressed exceptions?
[Hard] What is ExceptionInInitializerError?

Collections

Collections basics (27: 22 Easy · 5 Medium)

[Easy] Why do we need Collections in Java?
[Easy] What are the important interfaces in the Collection Hierarchy?
[Easy] What are the important methods that are declared in the Collection Interface?
[Easy] Can you explain briefly about the List Interface?
[Easy] Explain about ArrayList with an example?
[Easy] Can an ArrayList have Duplicate elements?
[Easy] How do you iterate around an ArrayList using Iterator?
[Easy] How do you sort an ArrayList?
[Easy] How do you sort elements in an ArrayList using Comparable interface?
[Easy] How do you sort elements in an ArrayList using Comparator interface?
[Easy] What is Vector class? How is it different from an ArrayList?
[Easy] Can you briefly explain about the Set Interface?
[Easy] What are the important interfaces related to the Set Interface?
[Easy] What is the difference between Set and SortedSet interfaces?
[Easy] Can you give examples of classes that implement the Set Interface?
[Easy] What is a HashSet?
[Easy] What is a LinkedHashSet? How is different from a HashSet?
[Easy] Explain briefly about Queue Interface?
[Easy] Can you briefly explain about the Map Interface?
[Easy] What is a HashMap?
[Easy] What are the different methods in a Hash Map?
[Easy] Difference between Comparable and Comparator
[Medium] What is LinkedList? How is it different from an ArrayList?
[Medium] What is a TreeSet? How is different from a HashSet?
[Medium] Can you give examples of implementations of NavigableSet?
[Medium] What is difference between Map and SortedMap?
[Medium] What is a TreeMap? How is different from a HashMap?

Choosing collections and complexity (6: 1 Easy · 4 Medium · 1 Hard)

[Easy] What is the difference between Iterator and ListIterator?
[Medium] What is the time complexity of common collection operations?
[Medium] What is the difference between List.of(), Arrays.asList() and Collections.unmodifiableList()?
[Medium] How does HashSet work internally?
[Medium] How do you choose the right collection?
[Hard] What are EnumMap, WeakHashMap and IdentityHashMap used for?

Collections internals and concurrent collections (20: 5 Easy · 13 Medium · 2 Hard)

[Easy] What is the difference between HashMap, LinkedHashMap and TreeMap?
[Easy] What are the important interfaces related to the Queue interface?
[Easy] Explain the Deque interface.
[Easy] What are the important static methods in the Collections class?
[Easy] When does a collection throw UnsupportedOperationException?
[Medium] How does HashMap work internally?
[Medium] What is the initial capacity and load factor of a collection?
[Medium] Why must HashMap keys be immutable?
[Medium] How do you build an LRU cache with LinkedHashMap?
[Medium] What is a PriorityQueue?
[Medium] Explain the BlockingQueue interface and give implementations.
[Medium] Can you give an example of the NavigableMap interface?
[Medium] What is the difference between synchronized and concurrent collections?
[Medium] Explain the copy-on-write approach.
[Medium] What is a Lock? How is it different from synchronized?
[Medium] What is the difference between fail-fast and fail-safe iterators?
[Medium] What are atomic operations and atomic classes in Java?
[Medium] Comparable vs Comparator: how do you sort by multiple fields?
[Hard] How does ConcurrentHashMap work in Java 8+?
[Hard] What is the compare-and-swap (CAS) approach?

Multithreading and Concurrency

Multithreading basics (14: 11 Easy · 3 Medium)

[Easy] What is the need for Threads in Java?
[Easy] How do you create a thread?
[Easy] What are the different states of a Thread?
[Easy] What is priority of a thread? How do you change the priority of a thread?
[Easy] What is ExecutorService?
[Easy] What is Callable? How do you execute a Callable from ExecutionService?
[Easy] What is Synchronization of threads ?
[Easy] Can you give an example of a Synchronization block ?
[Easy] What is the use of join methods in threads ?
[Easy] What is the use of yield method?
[Easy] What is Deadlock ?
[Medium] Explain different ways of creating Executor Services?
[Medium] How do you check whether an ExecutionService task executed successfully?
[Medium] Can a static method be synchronized ?

Threading pitfalls and executors (8: 2 Easy · 5 Medium · 1 Hard)

[Easy] What is a daemon thread?
[Easy] What is the difference between Runnable and Callable?
[Medium] What is a race condition? How do you fix it?
[Medium] What makes a class thread-safe?
[Medium] What are deadlock, livelock and starvation?
[Medium] What is the difference between shutdown() and shutdownNow()?
[Medium] What do invokeAll() and invokeAny() do?
[Hard] What is the Fork/Join framework and work stealing?

Advanced concurrency (14: 2 Easy · 9 Medium · 3 Hard)

[Easy] Describe a few other important methods in threads.
[Easy] What is the difference between wait() and sleep()?
[Medium] What are the important methods for inter-thread communication?
[Medium] Can you write a producer-consumer program with wait and notify?
[Medium] Write a program where two threads print odd and even numbers alternately.
[Medium] What does volatile guarantee, and what doesn't it?
[Medium] How do you prevent deadlocks?
[Medium] What is CompletableFuture? Give an example.
[Medium] What are CountDownLatch, CyclicBarrier and Semaphore?
[Medium] What is ReadWriteLock?
[Medium] What are virtual threads (Java 21)?
[Hard] What is the Java Memory Model's happens-before relationship?
[Hard] What is a ThreadPoolExecutor and how do you size it?
[Hard] What is ThreadLocal and when can it cause a memory leak?

JVM, Memory and GC

GC and serialization (12: 6 Easy · 5 Medium · 1 Hard)

[Easy] What is a Garbage Collection?
[Easy] Can you explain the Garbage Collection with an example?
[Easy] What is Serialization?
[Easy] How do you serialize an object using Serializable interface?
[Easy] How do you de-serialize in Java?
[Easy] What do you do if only parts of the object have to be serialized?
[Medium] When is the Garbage Collection run?
[Medium] What are the best practices for Garbage Collection?
[Medium] What are Initialization Blocks?
[Medium] How do you serialize a hierarchy of objects?
[Medium] Are the values of static variables stored when an object is serialized?
[Hard] Are the constructors in an object invoked when it is de-serialized?

JVM memory and performance (9: 5 Medium · 4 Hard)

[Medium] What are the memory areas of the JVM?
[Medium] How does generational garbage collection work?
[Medium] Can a Java application have a memory leak? Give examples.
[Medium] Explain class loading and the delegation model.
[Medium] What is JIT compilation?
[Hard] Which garbage collectors are available, and which would you choose?
[Hard] How do you troubleshoot an OutOfMemoryError or high memory in production?
[Hard] How do you diagnose high CPU or a hung Java service?
[Hard] What is the difference between strong, soft, weak and phantom references?

Java 8, Streams and Functional Programming

Functional programming (16: 14 Easy · 2 Medium)

[Easy] What is functional programming?
[Easy] Can you give an example of functional programming?
[Easy] What is a Functional Interface?
[Easy] What is a stream?
[Easy] Explain about streams with an example?
[Easy] What are intermediate operations in streams?
[Easy] What are terminal operations in streams?
[Easy] What are lambda expressions?
[Easy] Can you give an example of lambda expression?
[Easy] Can you explain the relationship between lambda expression and functional interfaces?
[Easy] What is a Function Functional Interface?
[Easy] What is a Predicate Functional Interface?
[Easy] What is a Consumer Functional Interface?
[Easy] What is a Supplier Functional Interface?
[Medium] What are method references?
[Medium] Can you give examples of functional interfaces with multiple arguments?

Java 8 (advanced) (8: 3 Easy · 5 Medium)

[Easy] Explain the Java 8 Date and Time API.
[Easy] What is the difference between a Collection and a Stream?
[Easy] What are the main functional interfaces in java.util.function?
[Medium] What are default methods, and how is the diamond problem resolved?
[Medium] Why must variables used in a lambda be effectively final?
[Medium] What happens with Collectors.toMap() when there are duplicate keys?
[Medium] What is the difference between map(), flatMap() and peek() in streams?
[Medium] What are the different forms of reduce()?

Streams coding questions (16: 8 Easy · 7 Medium · 1 Hard)

[Easy] Find the second highest salary.
[Easy] Partition numbers into even and odd.
[Easy] Flatten a list of lists and sort unique words.
[Easy] Join names of employees older than 30, comma-separated.
[Easy] Sum, max and statistics in one pass.
[Easy] Reverse each word of a sentence.
[Easy] Check whether two strings are anagrams using streams.
[Easy] What is the difference between map and flatMap?
[Medium] Group employees by department and count / average salary.
[Medium] Find the highest-paid employee in each department.
[Medium] Count the frequency of each character in a string.
[Medium] Find the first non-repeating character in a string.
[Medium] Find duplicate elements in a list.
[Medium] Are streams lazy? What happens without a terminal operation?
[Medium] What is Optional and how should you use it?
[Hard] When should you avoid parallel streams?

Generics

Generics (10: 4 Easy · 3 Medium · 3 Hard)

[Easy] What are Generics?
[Easy] Why do we need Generics? Can you give an example of how Generics make a program more flexible?
[Easy] How do you declare a generic class?
[Easy] Can you give an example of a generic method?
[Medium] What are the restrictions in using a generic type that is declared in a class declaration?
[Medium] How can we restrict Generics to a subclass of a particular class?
[Medium] How can we restrict Generics to a superclass of a particular class?
[Hard] What is type erasure?
[Hard] What is the difference between List<?>, List and List<? extends Number>?
[Hard] What is a bounded type parameter with multiple bounds?

Miscellaneous and Java Versions

Arrays, enums and misc (9: 7 Easy · 2 Medium)

[Easy] What are the default values in an array?
[Easy] How do you print the content of an array?
[Easy] How do you compare two arrays?
[Easy] What is an enum?
[Easy] Can you use a switch statement around an enum?
[Easy] What is tokenizing?
[Easy] Can you give an example of tokenizing?
[Medium] What are asserts used for?
[Medium] When should asserts be used?

Java version features (5 → 21) (7: 4 Easy · 3 Medium)

[Easy] What are the new features in Java 5?
[Easy] What are the new features in Java 6 and 7?
[Easy] What are the new features in Java 8?
[Easy] What is a record and when would you use it?
[Medium] What changed in Java 9, 10 and 11?
[Medium] What are the key features from Java 12 to 17?
[Medium] What are the key features of Java 21?

Extra knowledge (web, DB, OS) (26: 14 Easy · 11 Medium · 1 Hard)

[Easy] Difference between API and REST API?
[Easy] What is domain vs. endpoint in API?
[Easy] Difference between Http and Https?
[Easy] What is the difference between authentication and authorization in api?
[Easy] Difference between Abstract class vs. interface?
[Easy] Difference between heap and stack memory?
[Easy] What is JSON? How is it utilized?
[Easy] What is the usage of the PATH variable in an operating system? How is it utilized, when would you like to update it?
[Easy] What is the difference between a database view and a table in a database?
[Easy] What all commands in Unix are you aware of? Please explain their usage.
[Easy] What is shell scripting? What purpose is it used for?
[Easy] Explain how you can read fields sent in a request from the browser using a servlet.
[Easy] Difference between hashmap and hashtable?
[Easy] What is constructor chaining?
[Medium] What is reflection API in Java?
[Medium] Where is the Java Reflection API used in the Spring Framework?
[Medium] What is the stored procedure in JDBC?
[Medium] What is a connection pool? How is it used to improve performance in database applications?
[Medium] What is the difference between a ClassCastException and a ClassNotFoundException in Java?
[Medium] What configurations do you need to make in web.xml to deploy a servlet in a web application?
[Medium] Explain how the session is utilized for security implementation. Please explain who creates the session and how is session information exchanged between the browser and server.
[Medium] What is meant by an entity-relationship (E-R) model? Explain the terms Entity, Entity Type, and Entity Set in a DBMS.
[Medium] What is a database trigger? How is it used to enforce business rules?
[Medium] How does your computer know what path to take to reach the external server
[Medium] What objects are implicitly available in JSP? Explain their usage
[Hard] Handling and fixing 'Out of Memory' exceptions?

DSA, Coding Programs and LLD

Classic Java programs (9: 9 Easy)

[Easy] Reverse a string without using reverse()
[Easy] Check whether a number is prime
[Easy] Fibonacci series (iterative and memoised)
[Easy] Check whether two strings are anagrams
[Easy] Find the missing number in 1..n
[Easy] Move all zeros to the end, keeping order
[Easy] Remove duplicates from a sorted array in place
[Easy] Print a pattern (Floyd's triangle)
[Easy] FizzBuzz

Strings (6: 2 Easy · 4 Medium)

[Medium] 1. Check whether a pattern exists in a text (substring search)
[Easy] 2. Valid palindrome (ignore case and non-alphanumerics)
[Medium] 3. Longest palindromic substring
[Medium] 4. Longest substring without repeating characters
[Medium] 5. Group anagrams
[Easy] 6. String compression

Arrays and hashing (6: 2 Easy · 4 Medium)

[Easy] 7. Two sum
[Easy] 8. Best time to buy and sell stock (I and II)
[Medium] 9. Maximum subarray sum (Kadane's algorithm)
[Medium] 10. Merge overlapping intervals
[Medium] 11. Product of array except self (no division)
[Medium] 12. Subarray sum equals K

Linked list, matrix, stack (5: 3 Easy · 2 Medium)

[Easy] 13. Reverse a linked list
[Medium] 14. Detect a cycle and return where it starts
[Easy] 15. Merge two sorted lists
[Medium] 16. Word search in a matrix
[Easy] 17. Valid parentheses

Trees and binary search (4: 1 Easy · 3 Medium)

[Medium] 18. Level order traversal (BFS)
[Medium] 19. Lowest common ancestor of a binary tree
[Medium] 20. Search in a rotated sorted array
[Easy] Before you say "done" in an interview

Medium and hard problems (13: 9 Medium · 4 Hard)

[Medium] 21. Top K frequent elements
[Medium] 22. Kth largest element in an array
[Medium] 23. Longest consecutive sequence
[Medium] 24. Three sum
[Medium] 25. Container with most water
[Medium] 26. Number of islands
[Medium] 27. Course schedule (cycle detection / topological sort)
[Medium] 28. Coin change (dynamic programming)
[Medium] 29. Validate a binary search tree
[Hard] 30. Trapping rain water
[Hard] 31. Merge k sorted lists
[Hard] 32. Sliding window maximum
[Hard] 33. Minimum window substring

Low-level design (LLD) (3: 2 Medium · 1 Hard)

[Medium] Design an LRU cache with O(1) get and put
[Medium] Design a parking lot
[Hard] Design a rate limiter (token bucket)

Contributing

Found a mistake or want to add a question? Open an issue or a pull request. Keep the existing format:

<details><summary>

## [Medium] Your question?
</summary>

Short answer in your own words.

**Example:**

```java
// minimal code that compiles
```
</details>

⭐ Star the repo if it helped you prepare.

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Java Interview Questions

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Topics

Java Basics

OOP, SOLID and Design Patterns

Exception Handling

Collections

Multithreading and Concurrency

JVM, Memory and GC

Java 8, Streams and Functional Programming

Generics

Miscellaneous and Java Versions

DSA, Coding Programs and LLD

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