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Quizzes > Programming & Software

Java OOP Quiz: Check Your Object-Oriented Skills in 15 Questions

Quick, free Java OOP test-15 questions, instant results and brief explanations.

Editorial: Review CompletedCreated By: Gabibo SuperUpdated Aug 26, 2025
Difficulty: Moderate
Questions: 20
Learning OutcomesStudy Material
Colorful paper art depicting a quiz on Java Object-Oriented Programming knowledge test.

This Java object-oriented programming quiz helps you check your grasp of classes, objects, inheritance, encapsulation, and polymorphism in 15 questions. Get instant results with brief explanations so you know what to review for exams or interviews. Want broader practice? Try the computer programming quiz, explore the .NET quiz, or check out a computer science quiz.

Which OOP principle best describes hiding internal state by exposing only necessary methods in Java?
Inheritance
Polymorphism
Abstraction
Encapsulation - correct because it restricts access to internal data via accessors/mutators
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In Java, a subclass providing a specific implementation for a method defined in its superclass is an example of what?
Overloading
Hiding
Shadowing
Overriding - correct because it replaces a superclass method implementation in a subclass
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A Java class marked as final can be subclassed.
True
False
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Which access modifier allows visibility within the same package and to subclasses in other packages?
private
public
protected - correct because protected members are visible to package peers and external subclasses
default (package-private)
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Interfaces in Java can declare constants implicitly using which modifier set?
protected final
public static final - correct because interface fields are implicitly public, static, and final
default
private static
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Which method signature correctly overrides equals from java.lang.Object?
public boolean equals()
public boolean equals(MyType o)
protected boolean equals(Object o)
public boolean equals(Object o) - correct because Object parameter matches the contract
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What is the main benefit of using interfaces over abstract classes for API design in Java?
They execute faster than abstract classes
They are always simpler to implement
They can hold mutable instance fields
They allow multiple inheritance of type - correct because classes can implement many interfaces
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The super keyword in a subclass constructor must be the first statement if used.
False
True
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Which scenario demonstrates covariant return types in Java?
A subclass override returns the same primitive type
A subclass override returns a subtype of the superclass method's return - correct for covariance
A subclass overloads a method with different parameters
A method returns different types based on conditions
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Which access level is the most restrictive in Java?
public
private - correct because it limits access to within the defining class
default (package-private)
protected
undefined
Which statement about Java records relates to OOP design?
Records are immutable data carriers that implicitly define state and canonical constructor - correct
Records always extend Enum
Records cannot implement interfaces
Records are mutable classes with public fields
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Which is a correct reason to prefer composition over inheritance in Java?
Composition reduces coupling to base class internals - correct by delegating behavior
Composition disables polymorphism
Inheritance always breaks encapsulation
Composition increases the number of public fields
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In Java, what does the @Override annotation ensure at compile time?
That a method cannot be overloaded
That a method is final
That a method actually overrides a superclass or interface method - correct because mismatch triggers error
That a method is synchronized
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If class B extends A, upcasting a B reference to A loses the actual object type.
True
False
undefined
Which statement about equals and hashCode is correct in Java OOP?
Objects with equal hash codes must be equal
hashCode should include random values
equals must be synchronized
Equal objects must have equal hash codes - correct per the general contract
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Which statement about cloning in Java is correct?
clone is public in Object by default
Cloneable guarantees deep cloning
clone never throws checked exceptions
Implementing Cloneable and overriding clone is needed to enable object cloning - correct
undefined
What is true about type erasure in Java generics from an OOP perspective?
Generic types are preserved for reflection
Generic type information is removed at runtime - correct because generics are compile-time only
Generics change the runtime object layout
Generics require primitive type parameters
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A class can implement two interfaces that define the same default method without any additional work.
True
False
undefined
Which describes bounded type parameters and bounded wildcards in Java generics?
Using extends or super to constrain type arguments - correct because bounds restrict acceptable types (correct)
Using wildcards only in return types
Limiting methods to static contexts
Specifying arrays as type parameters
undefined
Which is a correct statement about wildcard capture in Java?
Helper methods can capture ? wildcards with a fresh type variable - correct to enable safe assignments
Wildcards can be assigned to any concrete type directly
Capture removes generic bounds
Capture only works for primitives
undefined
0

Learning Outcomes

  1. Identify core OOP principles such as inheritance and polymorphism
  2. Apply encapsulation techniques in Java class design
  3. Analyse relationships between objects and class hierarchies
  4. Demonstrate proficiency in abstract classes and interfaces
  5. Evaluate code snippets for correct OOP implementation
  6. Master best practices for object creation and management

Cheat Sheet

  1. Master the Four Pillars of OOP - Grab your cape and dive into Encapsulation, Inheritance, Abstraction, and Polymorphism - the building blocks of superhero-level Java apps. Understanding these four pillars helps you design code that's robust, flexible, and ready to save the day!
  2. Implement Encapsulation Effectively - Become the guardian of your class's secrets by using private fields and public getters/setters. This keeps your data safe from villainous corruption and ensures each class behaves like a well-oiled machine.
  3. Differentiate Between Abstract Classes and Interfaces - Learn when to share code in an abstract class and when to define a contract with an interface. Mastering this distinction gives you the power to mix and match behaviors across unrelated classes - like assembling your own Java Justice League!
  4. Understand the "IS-A" and "HAS-A" Relationships - Channel your inner detective to spot inheritance ("IS-A") versus composition ("HAS-A") in your code. Modeling real-world relationships correctly will keep your codebase organized and as flexible as a ninja in training.
  5. Practice Method Overriding and Overloading - Polymorphism is your ticket to dynamic, adaptable code - override methods for customized behavior and overload them for multiple input styles. It's like having the ability to wear different hats in the same scene!
  6. Design with Abstract Classes and Interfaces - Craft blueprints with abstract classes for shared logic, and define capabilities with interfaces for ultimate flexibility. This dynamic duo helps your code scale gracefully, no matter how big your project grows.
  7. Handle Multiple Inheritance with Interfaces - Java may banish multiple class inheritance, but interfaces let you inherit multiple behaviors freely. It's like having multiple superpowers without risking a tangled hierarchy!
  8. Utilize Default and Static Methods in Interfaces - Since Java 8, interfaces can have default and static methods - perfect for evolving APIs without breaking old code. Think of it as upgrading your toolkit while keeping all the classic features intact!
  9. Apply Best Practices for Object Creation - Use design patterns like Factory and Singleton to control object creation like a pro. These patterns help you avoid chaos and ensure every instance is made with purpose.
  10. Analyze Code for Proper OOP Implementation - Regularly review and refactor to keep your OOP principles shining bright. Cleaner, more maintainable code is just a refactor away from becoming your new superpower!
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