Top 100+ Most Asked Java Interview Questions

Comprehensive collection of the most frequently asked Java interview questions covering fundamentals, OOP concepts, collections, multithreading, exception handling, JVM internals, and advanced topics. Each answer is concise, detailed, and interview-ready.

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1.What is Java and how is it different from other programming languages?

2.Explain the concept of 'Write Once, Run Anywhere' (WORA)

3.What is the Java Virtual Machine (JVM) and its role?

4.Differentiate between JDK, JRE, and JVM

5.What are the primitive data types in Java?

6.What are wrapper classes and why are they used?

7.What are the four main principles of Object-Oriented Programming?

8.What is a class in Java and how do you define one?

9.What is an object in Java?

10.What is a constructor in Java?

11.Explain the 'public static void main(String[] args)' method

12.What is the difference between instance variables and local variables?

13.What are static variables and methods?

14.What is method overloading?

15.What is method overriding?

16.What is inheritance in Java?

17.What is the difference between an interface and an abstract class?

18.What is encapsulation and how is it achieved?

19.What is polymorphism in Java?

20.What is abstraction in Java?

21.What is the difference between == and equals() method?

22.What is the String class and is it mutable or immutable?

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

24.What is garbage collection in Java?

25.Explain the try-catch-finally block

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

27.What is the purpose of the final keyword?

28.What is the this keyword in Java?

29.What is the super keyword in Java?

30.What is autoboxing and unboxing?

31.What is the Java Collections Framework?

32.What is the difference between ArrayList and LinkedList?

33.What is the difference between HashMap and Hashtable?

34.What is the difference between Set and List?

35.What is an Iterator and how is it used?

36.What is the difference between fail-fast and fail-safe iterators?

37.What is multithreading in Java?

38.What is the difference between Thread and Runnable?

39.What is thread synchronization and why is it important?

40.Explain the synchronized keyword

41.What is a deadlock and how can it be prevented?

42.What is the volatile keyword?

43.What are wait(), notify(), and notifyAll() methods?

44.What is the Java Memory Model?

45.How does the JVM organize memory?

46.What is the difference between heap and stack memory?

47.What are generics in Java?

48.What is serialization and deserialization?

49.What is the transient keyword?

50.What is reflection in Java?

51.What are lambda expressions in Java 8?

52.What are functional interfaces?

53.What is the Stream API in Java 8?

54.What is the Optional class in Java 8?

55.What are default methods in interfaces?

56.What is method reference in Java 8?

57.What is the difference between Comparable and Comparator?

58.What is the difference between Runnable and Callable?

59.What is Future in Java?

60.What is ExecutorService?

61.What is the Singleton design pattern?

62.What is the Factory design pattern?

63.What is dependency injection?

64.What is immutability and its benefits?

65.What is the difference between throw and throws?

66.What is a custom exception?

67.What is the difference between error and exception?

68.What is JDBC and its components?

69.What is the difference between Statement and PreparedStatement?

70.What is a connection pool?

71.What is the difference between abstract class and interface in Java 8+?

72.What is composition over inheritance?

73.What are SOLID principles?

74.What is the difference between process and thread?

75.What is the CompletableFuture?

76.What is the fork/join framework?

77.What is CountDownLatch?

78.What is CyclicBarrier?

79.What is Semaphore in Java?

80.What is ConcurrentHashMap and how does it work?

81.What is ThreadLocal?

82.What are atomic variables?

83.What is the difference between ArrayList and Vector?

84.What is the difference between HashSet and TreeSet?

85.What is the difference between HashMap and TreeMap?

86.What is the equals() and hashCode() contract?

87.What is the purpose of the finalize() method?

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

89.What is covariant return type?

90.What is varargs in Java?

91.What are enums in Java?

92.What is annotation in Java?

93.What is the difference between Array and ArrayList?

94.What is the difference between String literal and String object?

95.What is the classpath in Java?

96.What is ClassLoader in Java?

97.What is the difference between Path and Classpath?

98.What is Just-In-Time (JIT) compiler?

99.What are different garbage collection algorithms?

100.What is the difference between heap and metaspace?

101.What is pass-by-value in Java?

102.What is the instanceof operator?

103.What is static import?

104.What is the diamond problem and how does Java handle it?

105.What is the difference between Scanner and BufferedReader?

106.What is NIO in Java?

107.What is the Service Provider Interface (SPI)?

108.What is the difference between JIT and AOT compilation?

109.What is the Module System in Java 9?

110.What are records in Java?