Java Abstract Class Interview Questions & Answers – With Real Examples, Diagrams & Use Cases
When I started interviewing candidates as a Java engineer years ago, I noticed something surprising:
👉 Most developers can define abstract classes… but very few know when and why to use them.
In 2025, interviewers still love this topic because abstract classes reveal how well you understand OOP design — not just syntax.
So here’s a practical, personal, no-nonsense guide to mastering abstract classes with clarity.
Java Abstract classes are like that strict-but-cool teacher who says, “You must follow the rules… but hey, I’ll give you some freedom too.” They’re super handy when your code has complicated family relationships — like when classes need to behave in a certain way without you micromanaging every single detail. Basically, if a class has the abstract keyword slapped on it, congratulations, it’s now an abstract class! And yes, it can contain both “abstract” mystery methods and fully working normal methods — like a menu with both cooked dishes and “DIY” items.
In simple terms, abstraction in Java hides all the messy, behind-the-scenes code drama and shows you only what you actually need. It’s like ordering food without watching the chaos in the kitchen. 😄
Java abstract class interview questions
1. What is Abstraction in Java?
An abstract class is like a blueprint with a mix of:
Methods with logic (concrete methods)
Methods without logic (abstract methods)
You can extend it, but you can’t create objects of it.
👨💻 My Real-Life Explanation to Candidates:
Think of an abstract class as a “half-built house.”
You know the structure, you know the rooms, but some rooms are still empty—
and the subclass must finish them.
2. Why Can’t We Instantiate an Abstract Class?
Because it’s incomplete.
Creating an object of an abstract class would be like trying to move into a house where the kitchen doesn’t exist yet.
Real-World Use Case
📦 Example: Building a Payment System
You have a base structure:
abstract class Payment {
abstract void initiatePayment();
void validateUser() {
System.out.println("Validating user...");
}
}
Different payment types fill in the missing logic:
class UpiPayment extends Payment {
@Override
void initiatePayment() {
System.out.println("UPI payment initiated");
}
}
class CardPayment extends Payment {
@Override
void initiatePayment() {
System.out.println("Card payment initiated");
}
}
Why this design works:
Every payment must initiate payment → common rule
But how they do it depends on type → custom logic per subclass
3. What is the difference between abstract class and concrete class?
There are basically two big differences between an abstract class and a concrete class… think of it like comparing a “concept” to a “real thing.”
a) You cannot create an object of an abstract class. It’s like that friend who gives great advice but never actually shows up. Only its non-abstract (concrete) subclasses can become “real objects.”
b) An abstract class can have zero or more abstract methods — basically “ideas for methods” with no implementation. A concrete class? Nope. It doesn’t allow any such half-baked ideas. Everything must be fully cooked there. 🍳
And speaking of fully cooked… Java versions!
Every version of Java has its own pros and cons, like different flavors of Maggi. But personally, I’ve always had a soft spot for Java 7. I just love how it plays nicely with non-Java languages on the JVM — like the cool kid who is friends with everyone. Plus, thanks to its sandbox security model, the Java SE platform runs pretty safely too.
Sure, Java 7 is my nostalgic favourite, but I still use newer versions… because who doesn’t like shiny new features to show off in projects?
4. What is Abstract method in Java?
A method in Java that’s declared with the abstract keyword and has zero implementation (yup, no body at all!) is called an abstract method. Think of it like a to-do list item that says, “We’ll figure this out later.” It only has a method signature followed by a semicolon — that’s it. No curly braces, no drama, nothing.
An abstract class may or may not contain abstract methods, which sounds weird, but hey… Java likes to keep us guessing. Just remember: don’t try calling abstract methods inside an abstract class. Why? Because those methods are basically empty promises — they have no body! Calling them too early can create all sorts of chaos due to initialization order.
In short: abstract methods = “idea only,” concrete methods = “actual work,” and you = “trying to keep everything from catching fire.”
Syntax:
abstract type MethodName(arguments); // No body
For example:
abstract void msg(); // No body.
5.Can you make an instance of an abstract class?
Nope! You absolutely cannot create an object of an abstract class.
Trying to do that is like trying to book a cab from a driver who doesn’t exist yet. An abstract class must be subclassed first. If you want to use a method that is already implemented inside the abstract class, you still need to create a subclass, make an object of that subclass, and then call the method. Java basically says, “No shortcuts here, buddy.”
Abstract classes are a mix of both:
Abstract methods → “Ideas only, no body.”
Concrete methods → “Fully working methods.”
And just to be clear, Java won’t let you create an object of an abstract class directly — not with new, not with Class.forName().newInstance(), not even with emotional blackmail. You’ll only get a java.lang.InstantiationException. 😄
But if you create an object of a subclass, the compiler happily initializes both — the subclass and the abstract parent class. It even calls the abstract class constructor behind the scenes, like a ninja.
Any class containing an abstract method must be declared abstract, and those abstract methods finally get their real bodies inside the child classes. That’s where the magic of polymorphism kicks in — different subclasses overriding the same abstract method in different ways.
So basically, an abstract class is like a template or a half-finished blueprint. It provides common behavior, but leaves the specific details for the subclasses to fill in. A class can even be abstract without having any abstract methods — just to make sure nobody tries to create objects from it.
In short:
Abstract class = “Some work done + some work pending”
Subclass = “Don’t worry, boss, I’ll finish the rest.”
protected String name;
public String getName() {
return name;
}
public abstract void function();
}
6. In which scenarios do you prefer using an Abstract Class instead of an interface?
Here are a few situations where using an abstract class just makes life easier:
When your base class wants to act like a “chill boss.”
It provides some default methods but still tells the subclasses, “Hey, you handle the special stuff.”When multiple related classes need to share common data.
Abstract classes can have instance variables, so they’re like a shared fridge for your code — everyone gets access.When you want to enforce a clean architecture.
Abstract classes help maintain a tidy inheritance structure instead of letting your code turn into “Jugaad Engineering.”When the language doesn’t support multiple inheritance.
Java says “nope” to multiple inheritance, so abstract classes become your plan-B for reusing code without creating a Frankenstein monster of classes.When you want to prevent direct object creation.
Basically, you’re saying: “This class is VIP only. Subclasses can enter. Others, please stand outside.”
Java has tons of awesome features, but my absolute favourite is its massive global community. People all over the world build cool projects, write blogs, share tips, and help each other grow. As a Java user, I get to learn from experts, exchange ideas, and even share my own experiments with the community — it feels like being part of one huge, geeky family.
7. Can an Abstract Class have both abstract and non-abstract (concrete) methods? Explain your answer with an example.
Yes, an abstract class can totally have both abstract and non-abstract methods. Think of it as a class that says, “I’ll do some of the work, but you (the subclasses) need to finish the rest.” It sets a common structure while still leaving room for customization.
Take this example: imagine a Vehicle abstract class.
It has two methods — startEngine() and honk().
startEngine() is abstract because every vehicle starts differently. A car might purr, a bike might roar, and an old scooter might need prayers.
honk(), on the other hand, can be a concrete method — because no matter the vehicle, the goal is the same: “Peeep peeep, move please!”
So abstract classes let you define what all subclasses must do, while still helping them with whatever functionality can be shared.
public abstract class Vehicle {
public abstract void startEngine();
public void honk() {
System.out.println("Honking!!!");
}
}
public class Car extends Vehicle {
@Override
public void startEngine() {
System.out.println("Starting car engine!!!");
}
}
public class Motorcycle extends Vehicle {
@Override
public void startEngine() {
System.out.println("Starting motorcycle engine!!!");
}
}
In this above example, the Vehicle class has one abstract method (startEngine) and one concrete method (honk), demonstrating that an abstract class can contain both types of methods.
8. Why abstract class has constructor even though you cannot create object?
You can’t create an object of an abstract class — Java simply won’t let you. But you can create an object of its subclass. And when you do that, something interesting happens behind the scenes.
The moment you create an object of the subclass, its constructor runs… and guess what’s the first thing it does?
It uses super (like a polite kid calling their parent) to invoke the constructor of the abstract class.
So even though you can’t make objects directly from an abstract class, its constructor still gets called — indirectly — through its subclass. It’s like the abstract class saying, “I don’t do direct bookings, talk to my children.”
And here’s the kicker:
If your abstract class doesn’t have a constructor, the subclass won’t even compile. Java is strict like that. Even abstract parents must have a proper constructor so their subclasses don’t freak out during compilation.
9. Why should we create reference to superclass (abstract class reference)?
You can create a reference of the superclass and point it to a subclass object — but remember, that reference can only see the features that already exist in the superclass. It’s like your superclass says, “I’ll allow my kids to have their own personality… but I’ll only recognize the traits I gave them.”
So, if the subclass adds its own fancy new method — something completely original — the superclass reference is clueless. It’s like talking to your dad about your new gaming PC configuration… and he just nods without understanding a word.
This basically means a programmer cannot forcefully access brand-new subclass features unless they use a proper subclass reference. Superclass references only respect what’s already defined at their level. The rest is “invisible mode.”
10. What differentiates the interface from the abstract class?
When interviewers ask this question, they basically want to check whether you truly understand the difference between interfaces and abstract classes in OO ABAP — not just memorized definitions. So keep your answer crisp and focused on how they’re built and what they’re meant to do.
Think of it this way:
An interface is like a strict rulebook — it only lists what must be done, but gives zero clues on how to do it. No implementation, no shortcuts, nothing. Classes can use interfaces to boost their abilities, kind of like adding extra superpowers.
On the other hand, an abstract class is a half-ready blueprint. It can’t be instantiated (you can’t make an object of it), but its subclasses can come to life — as long as they aren’t abstract too. Abstract classes are all about inheritance and shared structure. They let you mix implemented methods with “to-be-implemented-later” abstract ones.
So in short:
Interface: “Here’s what you must do. No instructions included.”
Abstract class: “Here’s what I’ve already done. You finish the rest, champ.”
11. When comparing an abstract class with an interface class, which type offers multiple inheritance support and single-instance support and also allows only public members?
You typically use interfaces in Java when you expect totally unrelated classes to follow the same rules. Interfaces let you define what a class should do, without caring who does it or how they do it. Plus, interfaces give you a sweet bonus — multiple inheritance of type. So you get to say, “Here’s the behavior I want,” while happily ignoring the messy implementation details.
Abstract classes, on the other hand, play a different game. They support single inheritance, but they let you mix and match — you can define some functionality right away and leave the tricky parts for subclasses to finish later. It’s like saying, “I’ll start the work… you complete it.”
With abstract classes, you can declare static and final fields, and you can define public, protected, and even private concrete methods.
But with interfaces? Everything is strict and formal:
All fields are public, static, and final (whether you like it or not).
All methods are public, and until Java 8, they couldn’t even have bodies.
So in short:
Interfaces: “Rules only. No implementation. And yes, everything is public.”
Abstract classes: “Here’s some code, some freedom, and yes… I can keep secrets using private methods too.”
12. Can an Abstract Class Have the Main Method?
Surprisingly… YES!
abstract class Demo {
public static void main(String[] args) {
System.out.println("It works!");
}
}
13. Can an Abstract Class Be Final?
NO.
final means “no one can extend me.”abstract means “someone MUST extend me.”
They contradict each other.
14. Can abstract methods be private?
No, because subclasses need access to implement them.
15. Can an abstract class extend another abstract class?
Yes, and it’s common.
Abstract Class vs Interface – My Interviewer’s Cheat-Sheet
Feature | Abstract Class | Interface |
|---|---|---|
Methods | Concrete + abstract | Mostly abstract (default + static allowed) |
Variables | final & non-final | Always public static final |
Inheritance | Single | Multiple |
Use When… | You need shared logic | You need strict contracts |
💬 My personal note:
I’ve seen candidates get confused here.
The golden rule I tell them:
👉 Use an abstract class when you want to share code.
Use an interface when you want to enforce rules.
Best Practices (From My Experience)
✔ Keep abstract classes lean — don’t overload them
✔ Use meaningful method abstractions
✔ Don’t mix too many responsibilities
✔ Combine abstract classes with interfaces for clean architecture
✔ Prefer interfaces unless shared code is necessary
FAQs – Quick Interview Prep
Q: Is it okay to have zero abstract methods?
Yes. Still useful to prevent instantiation.
Q: Can we have multiple constructors?
Yes.
Q: What happens if subclass doesn’t implement abstract methods?
Subclass must also be abstract.
Q: Performance impact?
Minimal — OOP design, not runtime overhead.
Final Thoughts
Abstract classes are one of the most misunderstood topics…
but once they click, they become your best friend in designing clean, extendable systems.
If I had to summarize everything in one line:
👉 Abstract classes let you define rules + reuse code = cleaner architecture.
Want to prepare for more Java interview concepts?
👉 Check out my guides on Java Lambda Expressions, Generics, and Pattern Matching.
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