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Java Question Bank

1. What is Java?​

Java is a high-level, object-oriented programming language that is platform-independent due to the Java Virtual Machine (JVM).

2. What are the features of Java?​

Platform-independent, Object-oriented, Simple, Secure, Robust, Multithreaded, Distributed, Dynamic, Portable, and High-performance.

3. What is JVM, JDK, and JRE?​

  • JVM (Java Virtual Machine): Runs Java bytecode.
  • JDK (Java Development Kit): Tools required to develop Java applications.
  • JRE (Java Runtime Environment): Provides libraries and JVM to run Java applications.

4. What is the difference between JDK and JRE?​

  • JDK is used to develop Java applications (includes JRE).
  • JRE is used to run Java applications.

5. What is a Class in Java?​

A class is a blueprint for creating objects, defining properties and methods.

6. What is an Object in Java?​

An instance of a class.

7. What is Inheritance?​

Inheritance allows one class to inherit properties and methods from another.

8. What is Polymorphism in Java?​

Polymorphism allows methods to behave differently based on the object calling them (method overloading and overriding).

9. What is Abstraction?​

Abstraction hides implementation details and shows only functionality.

10. What is Encapsulation?​

Encapsulation wraps data and methods in a single unit (class) and restricts access to them via access modifiers.

11. What are access modifiers?​

Access modifiers define the visibility of classes, methods, and variables (private, public, protected, default).

12. What is the difference between private, protected, and public?​

  • Private: Accessible only within the same class.
  • Protected: Accessible within the same package and subclasses.
  • Public: Accessible from anywhere.

13. What is a Constructor in Java?​

A constructor is a special method used to initialize objects. It has the same name as the class.

14. What is method overloading?​

Method overloading occurs when multiple methods with the same name but different parameters are defined in a class.

15. What is method overriding?​

Method overriding occurs when a subclass provides a specific implementation of a method already defined in its parent class.

16. What is the super keyword in Java?​

super is used to refer to the immediate parent class object and call parent class methods or constructors.

17. What is this keyword in Java?​

this refers to the current object instance.

18. What is a static method?​

A static method belongs to the class rather than any instance and can be called without creating an object.

19. Can we override static methods?​

No, static methods cannot be overridden because they belong to the class, not instances.

20. What is the difference between static and non-static methods?​

  • Static methods are class-level methods.
  • Non-static methods are object-level methods.

21. What is final keyword?​

The final keyword can be used with variables, methods, and classes to prevent modification:

  • Final variable: Cannot be changed.
  • Final method: Cannot be overridden.
  • Final class: Cannot be extended.

22. Can a constructor be made final?​

No, constructors cannot be declared as final.

23. What is an interface?​

An interface is a reference type in Java, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types.

24. Can a class implement multiple interfaces?​

Yes, a class can implement multiple interfaces, providing multiple inheritance.

25. What is the difference between abstract class and interface?​

  • Abstract class: Can have both abstract and concrete methods, can have state.
  • Interface: Can only have abstract methods (Java 7) and static/default methods (Java 8+).

26. What is an abstract class?​

An abstract class cannot be instantiated and may contain abstract methods that must be implemented by subclasses.

27. Can a class extend multiple classes in Java?​

No, Java does not support multiple inheritance of classes.

28. What is the purpose of the main() method?​

The main() method serves as the entry point for Java applications.

29. Why is the main method static?​

The main() method is static so it can be called without creating an instance of the class.

30. Can we overload the main method?​

Yes, the main method can be overloaded, but the JVM calls only the signature public static void main(String[] args).

31. What is the difference between Array and ArrayList?​

  • Array: Fixed in size, supports primitive types.
  • ArrayList: Resizable, supports only objects.

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

  • String: Immutable.
  • StringBuilder: Mutable, not thread-safe.
  • StringBuffer: Mutable, thread-safe.

33. What is a Wrapper class in Java?​

Wrapper classes provide a way to use primitive data types (int, char, etc.) as objects (Integer, Character, etc.).

34. What is Autoboxing and Unboxing?​

  • Autoboxing: Converting a primitive type to its corresponding wrapper class.
  • Unboxing: Converting a wrapper class to its corresponding primitive type.

35. What is Exception Handling in Java?​

Exception handling is managing runtime errors using try, catch, finally, throw, and throws.

36. What are checked and unchecked exceptions?​

  • Checked exceptions: Checked at compile-time (e.g., IOException).
  • Unchecked exceptions: Checked at runtime (e.g., NullPointerException).

37. What is the difference between throw and throws?​

  • throw: Used to explicitly throw an exception.
  • throws: Used in method declaration to specify the exceptions a method can throw.

38. What is a try-catch block?​

A try-catch block is used to handle exceptions by wrapping risky code in try and handling exceptions in catch.

39. What is the finally block?​

The finally block is executed after try-catch, regardless of an exception being thrown.

40. What is a NullPointerException?​

A NullPointerException occurs when trying to use an object reference that has a null value.

41. What is the use of synchronized keyword?​

synchronized is used to ensure that a method or block is accessed by only one thread at a time.

42. What is multithreading?​

Multithreading is the concurrent execution of multiple threads.

43. What is the difference between wait() and sleep()?​

  • wait(): Pauses the thread and releases the lock.
  • sleep(): Pauses the thread but does not release the lock.

44. What is the volatile keyword?​

volatile ensures visibility of changes to variables across threads.

45. What is the difference between Runnable and Thread in Java?​

  • Runnable: Interface that allows the creation of a thread by implementing the run() method.
  • Thread: Class that directly extends thread capabilities.

46. What is a Deadlock?​

A deadlock occurs when two or more threads are blocked forever, waiting for each other.

47. What is garbage collection in Java?​

Garbage collection is the process of reclaiming unused memory automatically in Java.

48. What are memory areas allocated by JVM?​

  • Method Area
  • Heap Area
  • Stack Area
  • PC Registers
  • Native Method Stacks

49. What is the difference between equals() and ==?​

  • equals() checks the equality of objects based on content.
  • == checks reference equality.

50. What is a singleton class?​

A singleton class allows only one instance to be created throughout the application.

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51. What is the hashCode() method?​

The hashCode() method provides a unique integer value for objects, which is used in hashing data structures like HashMap.

52. What is a Map in Java?​

A Map is a collection that stores key-value pairs, where each key is unique.

53. What is the difference between HashMap and Hashtable?​

  • HashMap: Non-synchronized, allows null keys and values.
  • Hashtable: Synchronized, does not allow null keys or values.

54. What is fail-fast in Java?​

Fail-fast behavior occurs when a collection's iterator immediately throws an exception if the collection is structurally modified.

55. What is a fail-safe in Java?​

Fail-safe behavior allows the collection to be modified while iterating over it without throwing an exception (e.g., CopyOnWriteArrayList).

56. What is the Iterator interface?​

The Iterator interface is used to iterate over elements of a collection sequentially.

57. What is the Comparable interface?​

Comparable is used to define the natural ordering of objects by implementing the compareTo() method.

58. What is the Comparator interface?​

Comparator is used to define custom ordering of objects by implementing the compare() method.

59. What is the difference between Comparable and Comparator?​

  • Comparable: Defines natural ordering of objects (implemented in the object itself).
  • Comparator: Defines custom ordering (implemented externally).

60. What is the Collections class?​

Collections is a utility class in Java that provides static methods to manipulate and operate on collections (like sorting, searching).

61. What is an immutable object?​

An immutable object is an object whose state cannot be changed after it is created (e.g., String).

62. How can you make an object immutable?​

To make an object immutable:

  • Declare all fields as final.
  • Don't provide setter methods.
  • Make fields private and ensure no references to mutable objects are returned.

63. What is the difference between StringBuilder and StringBuffer?​

  • StringBuilder: Faster, non-thread-safe.
  • StringBuffer: Slower, thread-safe.

64. What is ConcurrentHashMap?​

ConcurrentHashMap is a thread-safe implementation of HashMap, allowing concurrent access without locking the entire map.

65. What is the difference between Set and List?​

  • Set: Stores unique elements, unordered.
  • List: Allows duplicate elements, ordered.

66. What is a TreeMap in Java?​

TreeMap is a Map implementation that stores its entries in a sorted order based on the natural ordering of keys or a custom Comparator.

67. What is the transient keyword?​

The transient keyword prevents variables from being serialized.

68. What is serialization?​

Serialization is the process of converting an object into a byte stream to be stored or transmitted.

69. What is the Serializable interface?​

Serializable is a marker interface used to indicate that a class's objects can be serialized.

70. What is externalization?​

Externalization is an interface that provides more control over the serialization process by defining custom read and write methods.

71. What is reflection in Java?​

Reflection allows inspection and modification of classes, methods, and fields at runtime.

72. What is the purpose of the Class class in Java?​

The Class class is used to obtain information about a class, such as methods, fields, constructors, and more, at runtime.

73. What are annotations in Java?​

Annotations provide metadata about code but do not affect the program's execution.

74. What are some built-in annotations in Java?​

  • @Override
  • @Deprecated
  • @SuppressWarnings

75. What is the enum type in Java?​

An enum is a special data type that defines a collection of constants.

76. What is a marker interface?​

A marker interface is an interface without any methods or fields, used to indicate special behavior (e.g., Serializable).

77. What is dependency injection?​

Dependency injection is a design pattern where an object's dependencies are injected by an external entity (e.g., frameworks like Spring).

78. What is a lambda expression in Java?​

A lambda expression provides a way to write anonymous methods, enabling functional programming in Java.

79. What is the Optional class?​

Optional is a container object used to avoid null values and handle optional data safely.

80. What is the difference between flatMap() and map() in Java Streams?​

  • map(): Applies a function to each element and returns a stream of transformed values.
  • flatMap(): Flattens nested structures, returning a single-level stream.

81. What are Java Streams?​

Streams represent a sequence of elements supporting sequential and parallel operations on data (introduced in Java 8).

82. What is the difference between ParallelStream and Stream?​

  • Stream: Sequential stream processing.
  • ParallelStream: Divides data into multiple chunks and processes them in parallel.

83. What is method reference in Java?​

Method reference is a shorthand syntax to refer to a method directly using :: (e.g., Class::methodName).

84. What are functional interfaces?​

A functional interface is an interface with exactly one abstract method (e.g., Runnable, Callable).

85. What is a default method in an interface?​

A default method is a method defined in an interface with a default implementation, introduced in Java 8.

86. What is the purpose of Predicate in Java?​

Predicate is a functional interface that represents a condition or filter, typically used in lambda expressions.

87. What is the Supplier functional interface?​

Supplier is a functional interface that takes no arguments and provides a result.

88. What is the Consumer functional interface?​

Consumer represents an operation that takes a single argument and returns no result.

89. What is the Function functional interface?​

Function takes one argument and returns a result, used in mapping operations.

90. What is a stream pipeline?​

A stream pipeline is a sequence of stream operations (intermediate and terminal operations) that process data.

91. What are intermediate and terminal operations in streams?​

  • Intermediate operations: Transform a stream (e.g., filter(), map()).
  • Terminal operations: Produce a result or side-effect (e.g., forEach(), collect()).

92. What is the Collectors class?​

Collectors is a utility class that provides implementations for various reduction operations like toList(), joining(), etc.

93. What is the Optional class used for?​

Optional is used to handle nullable objects gracefully, avoiding NullPointerExceptions.

94. What is method overriding with covariant return types?​

In method overriding, the return type can be a subclass of the original return type.

95. What is the purpose of synchronized blocks?​

A synchronized block ensures that only one thread can execute a block of code at a time.

96. What is the difference between notify() and notifyAll()?​

  • notify(): Wakes up one thread waiting on an object's monitor.
  • notifyAll(): Wakes up all threads waiting on an object's monitor.

97. What is ExecutorService in Java?​

ExecutorService is an interface that provides a way to manage and control thread execution, offering higher-level thread management.

98. What is the difference between submit() and execute() in ExecutorService?​

  • submit(): Submits a task for execution and returns a Future object.
  • execute(): Executes a task without returning any result.

99. What is Future in Java?​

Future represents the result of an asynchronous computation and provides methods to check the completion or cancel the execution of the task.

100. What is Callable in Java?​

Callable is similar to Runnable, but it returns a result and can throw an exception.