MCS-206 — Java Quick Revision: 75 Questions with Answers

UNIT-1: Basics of Java (Q1–15)

1. What is Java?

Java is a high-level, object-oriented, platform-independent programming language.

2. Why is Java platform independent?

Because Java programs run on JVM using bytecode, not machine code.

3. What is JVM?

Java Virtual Machine executes bytecode and provides platform independence.

4. What is JRE?

Java Runtime Environment contains JVM + runtime libraries required to run Java programs.

5. What is JDK?

Java Development Kit contains compiler (javac), libraries, tools, JRE.

6. Difference between JDK, JRE, and JVM?

7. What is Bytecode?

Intermediate representation executed by JVM.

8. What is a Class?

Blueprint for creating objects.

9. What is an Object?

Instance of a class having state and behavior.

10. What is main() method?

Entry point of Java program:

public static void main(String[] args)

11. What is a variable?

Memory location to store data.

12. Types of variables

Local, Instance, Static.

13. What are data types in Java?

• Primitive: int, byte, short, long, char, float, double, boolean

• Non-primitive: arrays, classes, strings.

14. What is Type Casting?

Converting data type:

• Implicit: small → large

• Explicit: large → small

15. What is a Package?

Collection of classes and interfaces.

UNIT-2: OOP Concepts (Q16–30)

16. What is OOP?

Programming approach based on objects and classes.

17. 4 Pillars of OOP

Encapsulation, Abstraction, Inheritance, Polymorphism.

18. What is Encapsulation?

Wrapping data and methods into a single unit (class).

19. Abstraction

Hiding implementation details and showing only essential features.

20. Inheritance

One class acquires properties of another class using extends.

21. Polymorphism?

Ability to take multiple forms.

Types:

• Compile-time (method overloading)

• Runtime (method overriding)

22. Method Overloading

Same method name but different parameters.

23. Method Overriding

Sub-class provides its own implementation of superclass method.

24. What is Constructor?

Special method used to initialize objects.

25. Types of Constructors

Default, Parameterized, Copy constructor (not built-in but user-defined).

26. What is this keyword?

Refers to current class object.

27. What is super keyword?

Refers to parent class.

28. What is final keyword?

Used to prevent modification:

• final variable → constant

• final method → cannot override

• final class → cannot inherit

29. Difference between Abstract Class and Interface

30. Multiple Inheritance in Java?

Not supported using classes; supported using interfaces.

UNIT-3: Exception Handling & Multithreading (Q31–50)

31. What is an Exception?

Runtime error that disrupts program execution.

32. Exception Handling Keywords

try, catch, finally, throw, throws.

33. Example of exception handling

try {  
   int x = 10 / 0;  
} catch(Exception e) {  
   System.out.println(e);
}

34. Checked vs Unchecked Exception

35. What is finally block?

Executes always, even if exception occurs.

36. What is throw?

Used to manually throw exception.

37. What is throws?

Used in method signature to declare exceptions.

38. What is Multithreading?

Executing multiple tasks simultaneously.

39. Ways to create thread

1. Extend Thread class

2. Implement Runnable interface

40. Thread example

class A extends Thread {
  public void run(){ System.out.println("Hello"); }
}

41. Thread lifecycle

New → Runnable → Running → Blocked → Terminated

42. sleep() method

Pauses execution temporarily.

43. Thread Priority Range

1 (lowest) to 10 (highest)

44. Synchronization

Mechanism to control multiple thread access to shared resources.

45. Deadlock

Two threads waiting for each other forever.

46. yield()

Gives CPU to another waiting thread.

47. join()

Waits for a thread to finish.

48. isAlive()

Checks if thread is still running.

49. Daemon Thread

Background service thread (e.g., garbage collector).

50. Garbage Collection

Automatic memory management in Java.

UNIT-4: I/O, Collections & GUI (Q51–75)

51. What is Stream?

Sequence of data from source to destination.

52. Types of Streams

• Byte streams

• Character streams

53. File Handling Example

File f = new File("test.txt");

54. BufferedReader

Efficient character input stream.

55. What is Serialization?

Saving object state to file using Serializable.

56. What is Collection Framework?

Prebuilt classes for data structures in java.util.

57. List Interface

Ordered, allows duplicates (ArrayList, LinkedList).

58. Set Interface

No duplicates (HashSet, LinkedHashSet, TreeSet).

59. Map Interface

Key-value pairs (HashMap, TreeMap).

60. ArrayList

Resizable array implementation.

61. LinkedList

Doubly linked list structure.

62. Iterator

Used to traverse collections.

63. Comparable vs Comparator

Comparable → natural sorting
Comparator → custom sorting

64. AWT

Abstract Window Toolkit for GUI development.

65. Event Handling

Responding to user actions (mouse, keyboard).

66. Swing

Advanced GUI toolkit (better than AWT).

67. JFrame

Top-level Swing window.

68. Layout Managers

Flow, Border, Grid, Box layouts.

69. JButton Example

JButton b = new JButton("Click");

70. Applet

Java program running in browser (now obsolete).

71. JDBC

API to connect Java with databases.

72. JDBC Steps

Driver load → Connection → Statement → Query → Close connection.

73. Example of JDBC Connection

Connection con = DriverManager.getConnection(url,"user","pass");

74. PreparedStatement

Precompiled SQL statement.

75. Advantages of Java

Secure, portable, distributed, multithreaded, robust, platform independent.