Interview Prep Questions

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.