1. What is Java?
Java is a high-level, object-oriented programming language developed by Sun Microsystems. It is platform-independent because of the “write once, run anywhere” principle.
2. What are the main features of Java?
Java offers features like object-oriented design, platform independence, robustness, and security. It also supports multithreading and automatic memory management.
3. What is JVM?
Java Virtual Machine is a virtual machine that runs Java bytecode. It enables platform independence by converting bytecode into machine-specific instructions.
4. What is JDK?
JDK (Java Development Kit) is a software package used to develop Java applications. It includes tools like compiler, debugger, and the JRE.
5. What is JRE?
JRE (Java Runtime Environment) provides libraries and JVM to run Java programs. It does not contain development tools like compilers.
6. What is platform independence?
Platform independence means Java code can run on any system without modification. This is achieved through bytecode executed by JVM.
7. What are OOP concepts in Java?
Java follows four main OOP principles: encapsulation, inheritance, polymorphism, and abstraction. These concepts help in building modular and reusable code.
8. What is a class?
A class is a blueprint or template used to create objects. It contains variables and methods defining object behavior.
9. What is an object?
An object is an instance of a class. It represents a real-world entity with state and behavior.
10. What is inheritance?
Inheritance allows one class to acquire properties of another class. It promotes code reuse and hierarchical relationships.
11. What is polymorphism?
Polymorphism means “many forms” and allows methods to behave differently. It is achieved through method overloading and overriding.
12. What is abstraction?
Abstraction hides implementation details and shows only functionality. It is achieved using abstract classes and interfaces.
13. What is encapsulation?
Encapsulation wraps data and methods into a single unit (class). It protects data using access modifiers.
14. What is method overloading?
Method overloading allows multiple methods with the same name but different parameters. It improves code readability and flexibility.
15. What is method overriding?
Method overriding occurs when a subclass provides a specific implementation of a method. It supports runtime polymorphism.
16. What is a constructor?
A constructor is a special method used to initialize objects. It has the same name as the class and no return type.
17. What is a default constructor?
A default constructor is automatically provided by Java if no constructor is defined. It initializes objects with default values.
18. What is the ‘this’ keyword?
The ‘this’ keyword refers to the current object. It is used to differentiate instance variables from parameters.
19. What is the ‘super’ keyword?
The ‘super’ keyword refers to the parent class object. It is used to access parent class methods and constructors.
20. What are access modifiers?
Access modifiers define visibility of classes, methods, and variables. Examples include public, private, protected, and default.
21. What is static keyword?
The static keyword is used for memory management. Static members belong to the class rather than objects.
22. What is final keyword?
The final keyword is used to restrict modification. It can be applied to variables, methods, and classes.
23. What is String in Java?
String is a class used to represent sequences of characters. Strings are immutable in Java.
24. What is StringBuilder?
StringBuilder is used to create mutable strings. It is faster than String when modifying content frequently.
25. Difference between String and StringBuilder?
String is immutable, while StringBuilder is mutable. StringBuilder performs better for frequent modifications.
26. What is exception handling?
Exception handling is a mechanism to handle runtime errors. It ensures normal program execution using try-catch blocks.
27. What is try-catch block?
Try block contains code that may throw an exception. Catch block handles the exception.
28. What is finally block?
Finally block always executes regardless of exception. It is used for cleanup operations like closing resources.
29. What is throw keyword?
Throw keyword is used to explicitly throw an exception. It helps in custom exception handling.
30. What is throws keyword?
Throws keyword declares exceptions in method signature. It informs the caller about possible exceptions.
31. What is checked exception?
Checked exceptions are checked at compile time. Example: IOException.
32. What is unchecked exception?
Unchecked exceptions occur at runtime. Example: NullPointerException.
33. What is multithreading?
Multithreading allows multiple threads to run simultaneously. It improves performance and CPU utilization.
34. What is thread?
A thread is a lightweight subprocess. It is used to perform tasks concurrently.
35. What is synchronization?
Synchronization controls access to shared resources. It prevents data inconsistency in multithreading.
36. What is deadlock?
Deadlock occurs when two threads wait for each other indefinitely. It stops program execution.
37. What is collection framework?
Java Collection Framework provides classes and interfaces to store data. It includes List, Set, and Map.
38. What is List?
List is an ordered collection that allows duplicates. Example: ArrayList.
39. What is Set?
Set is an unordered collection that does not allow duplicates.
Example: HashSet.
40. What is Map?
Map stores key-value pairs. It does not allow duplicate keys.
41. What is ArrayList?
ArrayList is a resizable array implementation of List. It allows fast retrieval but slower insertion.
42. What is LinkedList?
LinkedList uses nodes to store data. It is efficient for insertion and deletion.
43. Difference between ArrayList and LinkedList?
ArrayList uses dynamic arrays, LinkedList uses nodes. ArrayList is faster for access, LinkedList for modifications.
44. What is HashMap?
HashMap stores key-value pairs using hashing. It allows one null key and multiple null values.
45. What is HashSet?
HashSet stores unique elements using hashing. It does not allow duplicates.
46. What is Iterator?
Iterator is used to traverse collections. It provides methods like hasNext() and next().
47. What is Comparable interface?
Comparable is used to define natural ordering of objects. It contains compareTo() method.
48. What is Comparator interface?
Comparator is used to define custom sorting logic. It provides compare() method.
49. What is serialization?
Serialization converts an object into byte stream. It is used for saving objects or transferring data.
50. What is deserialization?
Deserialization converts byte stream back into an object. It restores the original object state.
Conclusion
Core Java remains the foundation for cracking technical interviews in top MNCs like TCS, Infosys, and Amazon. A strong understanding of concepts such as OOP, collections, multithreading, and exception handling is essential not just for interviews, but for real-world application development.
By consistently practicing these top 50 questions and understanding the logic behind each answer, candidates can confidently tackle both theoretical and coding rounds. Focus on clarity, real-time examples, and problem-solving approaches this is exactly what recruiters in MNCs look for in a Java developer.
