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Object Oriented Programming0/49Java Classes & Objects Deep Dive0/27Java Memory Model & Internals0/21Strings0/10Collections Framework0/41Java 8+ Features0/37Exception Handling0/15Multithreading & Concurrency0/31Java Generics0/9Java Newer Features (Java 9–21)0/16Design Principles (Tested in Java Context)0/10Miscellaneous (Frequently Tricky)0/19

Object Oriented Programming

49 questions

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1. What is encapsulation and why does it matter in real-world code?

2. How is encapsulation different from just making fields private?

3. Can a class be encapsulated but still expose mutable state? How would you fix that?

4. What is the difference between encapsulation and data hiding?

5. How do getter/setter methods sometimes break encapsulation rather than enforce it?

6. What is abstraction and how is it different from encapsulation?

7. How do abstract classes and interfaces both achieve abstraction differently?

8. When would you choose an abstract class over an interface in Java?

9. Can you instantiate an abstract class? What happens if you try?

10. What is the purpose of having an abstract method with no body?

11. What is inheritance and what problem does it solve?

12. What is the difference between IS-A and HAS-A relationships?

13. Why does Java not support multiple inheritance with classes?

14. What is the Diamond Problem and how does Java handle it with interfaces?

15. What does super do and when would you use super() vs super.methodName()?

16. What is the difference between method overriding and method hiding?

17. Can constructors be inherited in Java?

18. What is constructor chaining and how does it work with inheritance?

19. What happens when a parent class constructor calls a method that is overridden in the child class?

20. What is a covariant return type?

21. What is polymorphism and what are its two types in Java?

22. What is the difference between compile-time and runtime polymorphism?

23. How does method overloading achieve compile-time polymorphism?

24. How does method overriding achieve runtime polymorphism?

25. What is dynamic method dispatch?

26. Can you override a static method in Java? What actually happens?

27. Can you override a private method? Why or why not?

28. What is the role of the @Override annotation? Is it mandatory?

29. What happens if you call an overridden method from a constructor?

30. How do abstraction and encapsulation complement each other? Give a real example.

31. How does polymorphism depend on inheritance to work?

32. Can you achieve polymorphism without inheritance in Java?

33. Is method overloading really polymorphism? Defend your answer.

34. How would you explain the difference between all four pillars to a junior developer using a single real-world analogy?

35. If inheritance creates tight coupling, when should you prefer composition over inheritance?

36. How does the Liskov Substitution Principle relate to inheritance and polymorphism?

37. Can encapsulation be violated through inheritance? How?

38. How do interfaces help achieve both abstraction and polymorphism simultaneously?

39. What are all the differences between an interface and an abstract class in Java?

40. Since Java 8, interfaces can have default and static methods — does that blur the line with abstract classes?

41. Can an interface have a constructor? Why or why not?

42. Can an interface extend another interface? Can it extend multiple interfaces?

43. What is a functional interface? Give examples beyond Runnable.

44. What is the difference between a marker interface and a functional interface?

45. What is the purpose of default methods in interfaces and what problem did they solve in Java 8?

46. If a class implements two interfaces that both have the same default method, what happens?

47. Can you have instance variables in an interface?

48. What are the implicit modifiers on interface methods and variables?

49. What is a sealed interface and what problem does it solve? (Java 17+)

Java Classes & Objects Deep Dive

27 questions

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50. How many ways can you create an object in Java?

51. What is the difference between shallow copy and deep copy?

52. What does the clone() method do and why is it considered problematic?

53. What is a copy constructor and how is it different from clone()?

54. What is the default constructor and when does the compiler generate one?

55. If a parent class has no default constructor and a child class constructor doesn't call super() explicitly, what happens?

56. Can a constructor be private? When would you use that?

57. What is a static factory method and why is it sometimes preferred over constructors?

58. What does static mean in Java and what can be marked static?

59. What is the difference between static and instance variables in terms of memory?

60. Can a static method access instance variables? Why or why not?

61. Can a static method be overridden? What is method hiding?

62. What is a static initializer block and when does it run?

63. What is the order of execution: static blocks, instance blocks, and constructors?

64. Can you use this inside a static method?

65. What is a static nested class vs an inner class?

66. What are all the things final can be applied to in Java?

67. What does it mean for a variable to be final? Does it guarantee immutability?

68. What is a blank final variable?

69. What is effectively final in Java 8+ and why does it matter for lambdas?

70. What does a final class mean? Give examples from JDK.

71. What does a final method mean and when would you use it?

72. Can you have a final abstract class? Why or why not?

73. What are all the uses of the this keyword in Java?

74. What is this() constructor call and what are its constraints?

75. Can this be used in a static context?

76. What is the difference between this.variable and just variable inside a method?

Java Memory Model & Internals

21 questions

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77. What are the different memory areas in JVM?

78. What is stored in the heap vs the stack?

79. What is the method area / metaspace?

80. What is the difference between stack memory and heap memory in terms of thread safety?

81. Where are static variables stored?

82. Where are String literals stored and why?

83. What is the String Pool and how does it work?

84. What is the difference between new String("hello") and "hello"?

85. What is garbage collection and how does the JVM decide what to collect?

86. What are the different generations in the JVM heap (Young, Old, Metaspace)?

87. What is the difference between Minor GC and Major GC?

88. What are strong, weak, soft, and phantom references?

89. What is a memory leak in Java? Can GC prevent all memory leaks?

90. What common coding patterns cause memory leaks?

91. What is the difference between G1GC and ZGC at a high level?

92. What does System.gc() do? Should you ever call it?

93. What is finalization and why is it deprecated?

94. Is Java pass-by-value or pass-by-reference?

95. If objects are passed by reference, why do we say Java is pass-by-value?

96. Show with an example how reassigning an object reference inside a method doesn't affect the original.

97. How does this behavior differ when you mutate the object vs reassign the reference?

Strings

10 questions

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98. Why is String immutable in Java?

99. What are the advantages of String immutability?

100. What is the difference between String, StringBuilder, and StringBuffer?

101. When would you use StringBuilder over String concatenation?

102. What does String.intern() do?

103. How does == vs .equals() behave with Strings?

104. What is the output of: new String("abc") == new String("abc")? Why?

105. What happens to strings during concatenation in a loop — what does the compiler do?

106. What are some useful String methods introduced in Java 11, 12, and later?

107. What is String.format() vs String.formatted() vs text blocks (Java 15+)?

Collections Framework

41 questions

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108. What is the Collections framework hierarchy in Java?

109. What is the difference between Collection and Collections?

110. What is the difference between Iterable, Collection, List, Set, and Map?

111. What is RandomAccess interface and which collections implement it?

112. What is the difference between ArrayList and LinkedList?

113. When would you use LinkedList over ArrayList?

114. What is the internal structure of ArrayList? How does it resize?

115. What is the default capacity of ArrayList and how does growth work?

116. What is the difference between size() and capacity() in ArrayList?

117. What is CopyOnWriteArrayList and when would you use it?

118. What is Vector and how is it different from ArrayList?

119. What are the differences between HashSet, LinkedHashSet, and TreeSet?

120. How does HashSet internally work? What does it use under the hood?

121. How does TreeSet maintain order? What is required for objects stored in it?

122. What happens if you add a duplicate to a HashSet?

123. What is the contract between equals() and hashCode()?

124. What is the internal structure of HashMap?

125. What happens during a HashMap collision?

126. What is the load factor and threshold in HashMap?

127. What changed in HashMap in Java 8 regarding collision handling?

128. What is the difference between HashMap, LinkedHashMap, and TreeMap?

129. What is ConcurrentHashMap and how is it different from HashMap and Hashtable?

130. How does ConcurrentHashMap achieve thread safety without locking the whole map?

131. What is computeIfAbsent, merge, and getOrDefault in Map?

132. What happens if you use a mutable object as a HashMap key?

133. What is WeakHashMap and when would you use it?

134. What is EnumMap and why is it more efficient than HashMap for enum keys?

135. What is the difference between Queue and Deque?

136. What is the difference between offer(), add(), poll(), remove(), peek(), and element()?

137. What is PriorityQueue and how does it order elements?

138. What is ArrayDeque and when would you prefer it over LinkedList as a queue?

139. What is BlockingQueue and where is it used?

140. What is the difference between Comparable and Comparator?

141. When would you use Comparator over Comparable?

142. What is Comparator.comparing() and how do you chain comparators?

143. What is natural ordering?

144. How does TreeMap or TreeSet use Comparable/Comparator?

145. What is a fail-fast iterator? Give an example.

146. What is ConcurrentModificationException and when does it occur?

147. What is a fail-safe iterator? Which collections use it?

148. Can you modify a collection while iterating using Iterator.remove()?

Java 8+ Features

37 questions

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149. What is a lambda expression and what problem does it solve?

150. What are the built-in functional interfaces in java.util.function?

151. What is the difference between Function, BiFunction, Consumer, Supplier, Predicate?

152. What is a method reference and what are its four types?

153. What is the difference between a lambda and an anonymous inner class?

154. Can a lambda expression throw a checked exception?

155. What is the difference between Runnable and Callable?

156. What is the Stream API and how is it different from a for loop?

157. What is the difference between intermediate and terminal operations?

158. What does it mean for streams to be lazy?

159. What is the difference between map() and flatMap()?

160. What is the difference between findFirst() and findAny()?

161. What is the difference between forEach() and forEachOrdered()?

162. What is the difference between reduce() and collect()?

163. What are Collectors.toList(), groupingBy(), partitioningBy(), joining()?

164. What is a parallel stream and when should you use it?

165. What are the dangers of parallel streams?

166. Can you reuse a stream after a terminal operation?

167. What is Stream.of() vs Arrays.stream() vs Collection.stream()?

168. How do you handle checked exceptions inside a stream pipeline?

169. What is peek() used for and what are its gotchas?

170. What is the difference between stream().filter().findFirst() vs a for-loop with a break?

171. What is Optional and what problem does it solve?

172. What is the difference between Optional.of(), Optional.ofNullable(), and Optional.empty()?

173. What is the difference between get(), orElse(), orElseGet(), and orElseThrow()?

174. What is the performance difference between orElse() and orElseGet()?

175. Is it good practice to use Optional as a method parameter or field? Why or why not?

176. How do you chain Optional operations using map() and flatMap()?

177. Why were default methods introduced in Java 8?

178. What problem do default methods solve in the Collections framework specifically?

179. How is conflict resolved when two interfaces provide the same default method?

180. What was wrong with java.util.Date and Calendar?

181. What is the difference between LocalDate, LocalTime, LocalDateTime, and ZonedDateTime?

182. What is Instant and when would you use it over LocalDateTime?

183. What is Duration vs Period?

184. How do you parse and format dates using DateTimeFormatter?

185. Why is the new Date/Time API considered thread-safe?

Exception Handling

15 questions

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186. What is the exception hierarchy in Java?

187. What is the difference between Error and Exception?

188. What is the difference between checked and unchecked exceptions?

189. When should you use a checked vs unchecked exception?

190. What is the difference between throw and throws?

191. What happens in a finally block if the try block has a return statement?

192. Can a finally block itself throw an exception?

193. What is exception chaining and why is it useful?

194. What is try-with-resources and what interface must a resource implement?

195. What is a multi-catch block?

196. What is the difference between getMessage(), getCause(), and printStackTrace()?

197. Should you ever catch Exception or Throwable directly?

198. What are some common anti-patterns in exception handling?

199. What is exception swallowing and why is it dangerous?

200. How do you design a custom exception hierarchy for an application?

Multithreading & Concurrency

31 questions

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201. What is a thread and how is it different from a process?

202. What are the different ways to create a thread in Java?

203. What is the difference between Thread.start() and Thread.run()?

204. What are the different states in a thread lifecycle?

205. What is the difference between sleep(), wait(), yield(), and join()?

206. What is a daemon thread?

207. What is thread priority and does it guarantee execution order?

208. What is a race condition? Give a concrete example.

209. What does the synchronized keyword do?

210. What is the difference between a synchronized method and a synchronized block?

211. What is intrinsic lock / monitor lock?

212. What is volatile and when is it not enough?

213. What is the difference between volatile and synchronized?

214. What is a deadlock? How do you detect and prevent it?

215. What is a livelock vs a deadlock?

216. What is thread starvation?

217. What is ReentrantLock and how is it different from synchronized?

218. What is ReadWriteLock and when would you use it?

219. What is CountDownLatch vs CyclicBarrier vs Semaphore?

220. What is an ExecutorService and why is it preferred over creating threads manually?

221. What are the different types of thread pools in Executors?

222. What is Callable vs Runnable and what does Future represent?

223. What is CompletableFuture and how is it different from Future?

224. What are thenApply(), thenCompose(), thenCombine(), and exceptionally() in CompletableFuture?

225. What is ForkJoinPool and what is work-stealing?

226. What is the Java Memory Model?

227. What is the happens-before relationship?

228. What is visibility problem in multithreading?

229. How does volatile ensure visibility but not atomicity?

230. What are atomic classes (AtomicInteger, AtomicReference) and how do they work (CAS)?

231. What is Compare-And-Swap (CAS)?

Java Generics

9 questions

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232. What are generics and what problem do they solve?

233. What is type erasure?

234. What is the difference between List<Object> and List<?>?

235. What is an upper bounded wildcard (? extends T) vs lower bounded (? super T)?

236. What is the PECS principle (Producer Extends, Consumer Super)?

237. Can you create a generic array in Java? Why or why not?

238. What is a generic method vs a generic class?

239. What is a raw type and why should you avoid it?

240. Can you use primitives as generic type parameters?

Java Newer Features (Java 9–21)

16 questions

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241. What is a Record in Java and when would you use it?

242. What does a record automatically generate?

243. Can a record extend a class or implement interfaces?

244. Can you add custom methods to a record?

245. Can you make a record mutable?

246. What is a sealed class and what problem does it solve?

247. What are permits, final, non-sealed, and sealed subclasses?

248. How does a sealed class work with pattern matching and switch expressions?

249. What is pattern matching for instanceof?

250. What is pattern matching in switch (Java 21)?

251. What are guarded patterns?

252. What are text blocks in Java 15+ and when would you use them?

253. What is var (local variable type inference) and what are its limitations?

254. What are switch expressions (Java 14+) vs switch statements?

255. What is the difference between String.isBlank() and String.isEmpty()?

256. What are the improvements to NullPointerException messages in Java 14+?

Design Principles (Tested in Java Context)

10 questions

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257. What are the SOLID principles? Explain each briefly.

258. What is the Single Responsibility Principle and how do you identify a violation?

259. What is the Open/Closed Principle? How do interfaces help achieve it?

260. What is the Liskov Substitution Principle and what does it mean practically?

261. What is the Interface Segregation Principle? Give a bad and good example.

262. What is the Dependency Inversion Principle and how does Spring DI apply it?

263. What is DRY, KISS, and YAGNI?

264. What is cohesion and coupling? What is high cohesion and low coupling?

265. What is the difference between composition and aggregation?

266. What is defensive programming?

Miscellaneous (Frequently Tricky)

19 questions

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267. What is the contract for equals() in Java?

268. What is the contract for hashCode() in Java?

269. Why must you override hashCode() when you override equals()?

270. What happens if two objects are equal but have different hash codes?

271. What happens if two objects have the same hash code but are not equal?

272. What makes a class truly immutable in Java?

273. How do you create an immutable class with a mutable field inside it?

274. Why is immutability important in concurrent programming?

275. What is the difference between Collections.unmodifiableList() and List.of()?

276. What is the difference between widening and narrowing type conversion?

277. What is autoboxing and unboxing? What are the performance pitfalls?

278. What is the Integer cache in Java (-128 to 127)?

279. What is ClassCastException and when does it occur?

280. What is the difference between instanceof and getClass() equality check?

281. What is serialization in Java?

282. What is serialVersionUID and why is it important?

283. What is the transient keyword?

284. What are the problems with Java's default serialization?

285. What are alternatives to Java serialization?