Java Quiz ( Intermediate ) - All Questions

This intermediate-level Java quiz is designed for developers who understand Java basics and want to strengthen real-world concepts. It focuses on OOP principles, strings, collections, exception handling, memory management, and commonly asked Java interview questions.

Question 1: What is the difference between == and equals() in Java?

Both compare objects
== compares references, equals() compares content
equals() compares references
No difference

Explanation: == checks reference equality, equals() checks logical equality.

Question 2: Why is String immutable in Java?

For faster execution
For security and memory optimization
For garbage collection
For simplicity

Explanation: Immutability improves security and enables string pooling.

Question 3: What is the default value of instance variables?

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

Explanation: Instance variables receive default values.

Question 4: What happens if no constructor is defined in a class?

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

Explanation: Java provides a default constructor.

Question 5: Which type of polymorphism is method overloading?

Runtime
Dynamic
Compile-time
Hybrid

Explanation: Overloading is resolved at compile time.

Question 6: Which type of polymorphism is method overriding?

Compile-time
Static
Runtime
Hybrid

Explanation: Overriding is resolved at runtime.

Question 7: Why can static methods not be overridden?

They are final
They belong to the class, not object
They are private
They are abstract

Explanation: Static methods are class-level methods.

Question 8: What is upcasting in Java?

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

Explanation: Upcasting assigns child object to parent reference.

Question 9: What is downcasting?

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

Explanation: Downcasting converts parent reference to child.

Question 10: Which keyword checks object type before casting?

this
super
instanceof
typeof

Explanation: instanceof checks object type.

Question 11: Which interface allows multiple inheritance in Java?

Class
Abstract class
Interface
Object

Explanation: Interfaces support multiple inheritance.

Question 12: What is encapsulation?

Code reuse
Binding data and methods
Method hiding
Inheritance

Explanation: Encapsulation binds data with methods.

Question 13: Why are variables declared private?

Performance
Security and control
Memory usage
Compilation speed

Explanation: Private variables protect internal state.

Question 14: What is the String Pool?

Heap memory
Special memory for string literals
Stack memory
Cache only

Explanation: String Pool stores string literals.

Question 15: Which class allows mutable strings?

String
StringBuilder
StringBuffer
Both B and C

Explanation: StringBuilder and StringBuffer are mutable.

Question 16: Difference between StringBuilder and StringBuffer?

Syntax
Thread safety
Memory usage
Immutability

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

Question 17: Which collection allows duplicate values?

Set
Map
List
None

Explanation: List allows duplicate elements.

Question 18: Which collection maintains insertion order?

HashSet
TreeSet
LinkedHashSet
Set

Explanation: LinkedHashSet maintains insertion order.

Question 19: Which collection is synchronized by default?

ArrayList
LinkedList
Vector
HashSet

Explanation: Vector is synchronized.

Question 20: What is the purpose of Iterator?

Sorting
Traversing collections
Indexing
Searching

Explanation: Iterator is used to traverse collections.

Question 21: Which exception occurs at runtime?

IOException
SQLException
RuntimeException
ClassNotFoundException

Explanation: RuntimeException occurs during execution.

Question 22: Which exceptions must be handled explicitly?

Unchecked
Runtime
Checked
All

Explanation: Checked exceptions must be handled.

Question 23: What is the purpose of finally block?

Throw exception
Cleanup code
Stop execution
Handle error

Explanation: finally executes cleanup code.

Question 24: What is multithreading?

Multiple programs
Multiple threads running concurrently
Multiple JVMs
Multiple processes

Explanation: Multithreading runs multiple threads simultaneously.

Question 25: Which interface is used to create threads?

Thread
Runnable
Callable
Executor

Explanation: Runnable is commonly used to create threads.

Question 26: Which method starts a thread?

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

Explanation: start() begins thread execution.

Question 27: What happens if run() is called directly?

New thread starts
Compilation error
Runs in current thread
Thread stops

Explanation: run() executes like a normal method.

Question 28: Which keyword ensures thread safety?

volatile
static
final
synchronized

Explanation: synchronized controls concurrent access.

Question 29: What is deadlock?

Thread termination
Circular waiting of threads
Fast execution
Memory leak

Explanation: Deadlock is circular waiting for resources.

Question 30: Which memory area stores objects?

Stack
Heap
Method area
Register

Explanation: Objects are stored in heap memory.

Question 31: Which memory area stores local variables?

Heap
Stack
Method area
Class loader

Explanation: Local variables are stored in stack.

Question 32: What is garbage collection?

Manual memory cleanup
Automatic memory management
Thread management
File cleanup

Explanation: GC frees unused memory automatically.

Question 33: Which method requests garbage collection?

finalize()
gc()
System.gc()
clean()

Explanation: System.gc() requests GC.

Question 34: Which keyword is used to create constants?

static
final
const
immutable

Explanation: final creates constants.

Question 35: Which feature improves code reusability?

Encapsulation
Inheritance
Abstraction
Compilation

Explanation: Inheritance improves reuse.

Question 36: What is abstraction?

Hiding data
Showing essential features only
Code reuse
Method overriding

Explanation: Abstraction shows essential behavior.

Question 37: Which keyword is used to define an abstract method?

virtual
abstract
static
final

Explanation: abstract defines abstract methods.

Question 38: Which package is imported by default?

java.util
java.io
java.lang
java.net

Explanation: java.lang is auto-imported.

Question 39: Which class is the root of Java hierarchy?

Class
Base
Main
Object

Explanation: Object is the root class.

Question 40: Which Java feature enables platform independence?

Compiler
JVM
JDK
JRE

Explanation: JVM enables platform independence.

Question 41: Which collection does not allow duplicates?

List
ArrayList
Set
Queue

Explanation: Set does not allow duplicates.

Question 42: Which access modifier allows visibility within package only?

public
private
protected
default

Explanation: Default access is package-private.

Question 43: Which keyword is used to inherit an interface?

implements
extends
inherits
interface

Explanation: Interfaces extend other interfaces.

Question 44: Which concept binds method call to object at runtime?

Overloading
Overriding
Dynamic binding
Encapsulation

Explanation: Dynamic binding occurs at runtime.

Question 45: Which Java feature supports runtime polymorphism?

Method overloading
Method overriding
Static binding
Final methods

Explanation: Overriding enables runtime polymorphism.

Question 46: What is the main purpose of Java?

UI design
Build secure and scalable applications
Database administration
System programming

Explanation: Java is used to build scalable applications.

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Java Quiz ( Intermediate )

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Java Quiz ( Intermediate )