Core Java Quiz ( Intermediate ) - All Questions

This intermediate Core Java quiz is designed for developers who understand Java basics and want to strengthen applied concepts. It covers OOP principles, memory management, strings, collections, exception handling, multithreading basics, and commonly asked Java interview questions.

Question 1: What is the main difference between JDK and JRE?

JDK runs programs, JRE compiles them
JDK contains development tools, JRE only runs programs
JRE is larger than JDK
They are the same

Explanation: JDK includes compiler and tools, while JRE only runs Java programs.

Question 2: What is the purpose of the JVM?

Write Java code
Compile Java code
Execute bytecode on any platform
Manage databases

Explanation: JVM executes bytecode and enables platform independence.

Question 3: Why is Java called platform-independent?

It runs only on Windows
It uses C language internally
Bytecode runs on JVM across platforms
It does not need OS

Explanation: Java bytecode runs on JVM regardless of OS.

Question 4: What is constructor used for?

Destroy objects
Initialize objects
Call methods
Free memory

Explanation: Constructors initialize objects.

Question 5: Which rule applies to constructors?

Must have return type
Can be static
Name must match class name
Can be inherited

Explanation: Constructor name must match class name.

Question 6: What is constructor overloading?

Multiple constructors with same parameters
Multiple constructors with different parameters
Calling parent constructor
Overriding constructors

Explanation: Overloading uses different parameter lists.

Question 7: What happens if no constructor is defined?

Compilation error
Runtime error
Default constructor is created
Object cannot be created

Explanation: Java provides a default constructor.

Question 8: What is the default value of instance variables?

Garbage value
Null or zero values
Must be initialized
Depends on OS

Explanation: Instance variables get default values.

Question 9: What is method overloading resolved at?

Runtime
Compile time
JVM startup
Execution time

Explanation: Method overloading is compile-time polymorphism.

Question 10: What is method overriding resolved at?

Compile time
Runtime
Link time
Load time

Explanation: Method overriding is runtime polymorphism.

Question 11: Which keyword prevents method overriding?

static
private
final
abstract

Explanation: final methods cannot be overridden.

Question 12: Why can't static methods be overridden?

They belong to object
They belong to class
They are abstract
They are final

Explanation: Static methods belong to the class.

Question 13: What is upcasting?

Parent to child reference
Child to parent reference
Object destruction
Type conversion

Explanation: Upcasting assigns child object to parent reference.

Question 14: What is downcasting?

Parent to child reference
Child to parent reference
Implicit casting
Compile-time casting

Explanation: Downcasting converts parent reference to child.

Question 15: Which keyword checks object type before casting?

this
super
instanceof
typeof

Explanation: instanceof checks object type.

Question 16: What is abstraction?

Hiding data
Showing only essential features
Code reuse
Multiple inheritance

Explanation: Abstraction shows essential behavior.

Question 17: Which supports multiple inheritance in Java?

Classes
Abstract classes
Interfaces
Objects

Explanation: Interfaces support multiple inheritance.

Question 18: What is encapsulation?

Code reuse
Binding data and methods
Method hiding
Inheritance

Explanation: Encapsulation binds data with methods.

Question 19: Why are variables made private?

Performance
Security and control
Compilation speed
Memory usage

Explanation: Private variables protect data.

Question 20: Which keyword is used to access private data?

this
super
get/set methods
static

Explanation: Getters and setters access private data.

Question 21: What is String immutability?

Strings cannot be modified
Strings are thread-safe
Strings are faster
Strings are stored in heap only

Explanation: String values cannot be changed once created.

Question 22: Why are Strings immutable?

Security and memory optimization
Performance only
Garbage collection
Compilation speed

Explanation: Immutability improves security and memory reuse.

Question 23: What is String Pool?

Heap memory
Special memory for strings
Stack memory
Cache memory

Explanation: String Pool stores string literals.

Question 24: Which class allows mutable strings?

String
StringBuffer
StringBuilder
Both B and C

Explanation: StringBuffer and StringBuilder are mutable.

Question 25: Difference between StringBuffer and StringBuilder?

Performance
Thread safety
Memory
Syntax

Explanation: StringBuffer is thread-safe, StringBuilder is not.

Question 26: What is the root interface of Collections framework?

List
Set
Collection
Iterable

Explanation: Collection is the root interface.

Question 27: Which collection allows duplicate values?

Set
Map
List
None

Explanation: List allows duplicate elements.

Question 28: Which collection maintains insertion order?

HashSet
TreeSet
LinkedHashSet
Set

Explanation: LinkedHashSet maintains insertion order.

Question 29: Which collection is synchronized?

ArrayList
LinkedList
Vector
HashSet

Explanation: Vector is synchronized.

Question 30: What is Iterator used for?

Sorting
Traversing collections
Indexing
Searching

Explanation: Iterator is used to traverse collections.

Question 31: Which exception occurs at runtime?

IOException
SQLException
RuntimeException
ClassNotFoundException

Explanation: RuntimeException occurs during execution.

Question 32: Which exceptions must be handled explicitly?

Runtime
Unchecked
Checked
All

Explanation: Checked exceptions must be handled.

Question 33: What is finally block used for?

Exception creation
Cleanup code
Handling errors
Stopping execution

Explanation: finally executes cleanup code.

Question 34: What is multithreading?

Multiple programs
Multiple processes
Multiple threads executing concurrently
Parallel JVMs

Explanation: Multithreading runs multiple threads.

Question 35: Which interface is used to create a thread?

Thread
Runnable
Callable
Executor

Explanation: Runnable is commonly used to create threads.

Question 36: Which method starts a thread?

run()
execute()
start()
init()

Explanation: start() begins thread execution.

Question 37: What happens if run() is called directly?

New thread starts
Thread stops
Method executes in current thread
Compilation error

Explanation: run() executes like a normal method.

Question 38: Which keyword ensures thread safety?

volatile
static
final
synchronized

Explanation: synchronized controls concurrent access.

Question 39: What is deadlock?

Thread termination
Threads waiting indefinitely for resources
Fast execution
Memory leak

Explanation: Deadlock is circular waiting.

Question 40: Which memory area stores objects?

Stack
Heap
Method area
Register

Explanation: Objects are stored in heap memory.

Question 41: Which memory area stores local variables?

Heap
Stack
Method area
Class loader

Explanation: Local variables are stored in stack.

Question 42: What is garbage collection?

Manual memory cleanup
Automatic memory management
Thread management
File cleanup

Explanation: GC automatically frees unused memory.

Question 43: Which method suggests garbage collection?

finalize()
gc()
System.gc()
collect()

Explanation: System.gc() requests garbage collection.

Question 44: Which keyword is used to create immutable objects?

static
final
private
const

Explanation: final helps create immutability.

Question 45: Which Java feature improves code reusability?

Encapsulation
Inheritance
Abstraction
Compilation

Explanation: Inheritance enables reuse.

Question 46: What is the main purpose of Core Java?

Framework development
Strong foundation for Java applications
UI creation
Database management

Explanation: Core Java builds a strong programming base.

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Core Java Quiz ( Intermediate )

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Core Java Quiz ( Intermediate )