Most Common Java OOPs Interview Questions (With Answers)
It should be emphasised that Object Oriented Programming is the heart of Java Development and a key subject for almost every Java job interview. Employers test candidates' knowledge of technicalities, but importantly assess how well he/she understand and apply OOP principles to real-life situations.
In this blog, we summarise the most frequently asked top interview questions in java with short and clear answers to help you prepare almost effortlessly, whether you are a beginner going through one or are a professional brushing up before a change of job.
1. What is Object Oriented Programming (OOP)?
OOP is a programming model that focuses on real-world objects. It organises code in classes and objects to achieve better modularity, code reusability, and scalability. OOP separates programs into reusable components, with each one representing an entity with its state (data) and behaviour (methods).
2. What are the Four Main Pillars of OOP?
The four key principles of OOP are:
Encapsulation-bundling the data with the methods that operate on that data into a single unit and restricting access to some components.
Inheritance-allowing the class to inherit properties and behaviours from another class.
Polymorphism-an ability whereby objects can interact in multiple forms via method overloading or overriding.
Abstraction-hiding the internal functioning of an object and exposing only the necessary features.
3. What is a Class and What is an Object?
A class is the blueprint for creating objects. It defines how the object's structure and behaviour will be. An object is the instance of a class-an entity that holds the state and behavior defined by class. A class is considered a design, while an object is an actual product manufactured from the design.
4. What is Encapsulation?
Encapsulation means keeping the internal state of an object hidden and allowing controlled access through methods. This helps safeguard the data and reduces the complexity of the system by hiding unnecessary details. It enhances the maintainability of code and makes it easier to debug.
5. What is Inheritance?
It implies that it is possible for a class to inherit the fields and methods of another class. The inheriting class is known as the "subclass" or "child class," and the class from which the inheritance is made is the "superclass" or "parent class." Inheritance increases code reusability and encapsulates the hierarchical classification concept.
6. What is Polymorphism?
It means in Latin, "many forms." In Java, it allows one interface to be used for a general class of actions; most commonly forms of polymorphism are both:
Compile-time polymorphism - it is accomplished with method overloading (the same name through many parameters).
Runtime polymorphism is achieved through method overriding (subclass specifies a particular definition of a method already defined in its superclass).
7. Abstraction
Abstraction is hiding the working and internal details in showing only the essential information. It helps in reducing the complexity of programming, thereby increasing its efficiency. In Java, abstraction is provided by abstract classes and interfaces, thus giving developers an opportunity to think more about an object in terms of what it does rather than how it does it.
8. What Is the Difference Between an Abstract Class and an Interface?
An abstract class is one that is only partially implemented. It may contain a mix of method declarations (without bodies) and concrete methods (with bodies). Thus, it embraces inheritance and can own constructors and member variables.
According to an interface's design, a class defines a contract for a method to be implemented. Its role is to provide for complete abstraction and multiple inheritance. An interface in modern Java (Java 8 and above) can also contain default and static methods.
9. The difference between overloading and overriding
Overloading, as it can be defined, occurs when two or more methods within one class have the same name but different parameter lists; resolving this occurs at compile time.
It happens in overriding when a subclass implements a particular behavior to the method which has already been defined in its superclass, and it gets resolved at runtime.
Thus, both represent polymorphism, although the issue of time and scope is different.
This being said, what's the use of 'this'?
In Java, "this" refers to the current instance of a class. Often it is used to differentiate instance variables from parameters which have the same name and invoke one constructor or method from another within the same class. This improves clarity in the code, preventing confusion at object construction or when calling methods.
11. What Are The Uses Of Super Keywords?
Super keyword is used within the child class to refer to the immediate parent class. It can be used to invoke the methods, variables, or constructors of the parent class. This becomes practical when you've overridden a method in the child class but still want to call this method within the parent.
12. Can You Instantiate An Abstract Class?
No, you can't create a direct object of an abstract class. Abstract classes are incomplete and meant for subclasses. To execute an abstract class, you need to subclass it and implement the abstract methods within the subclass, at which point you can create an object of that subclass.
13. What Is Constructor Overloading?
Constructor overloading is an ability within the class to have more than one constructor with different sets of parameters. This will allow flexible initialization of an object depending on different input conditions, thus adding to the usability of the code.
14. What is Object Cloning?
Object cloning is an operation to generate a copy of an object precisely as it exists. It is useful when objects have to be copied without using the class constructor again. In Java, the Cloneable interface along with the clone() method provide a way to implement it, along with certain caveats on deep and shallow copies.
15. What is the Role of the final Keyword in OOP?
The final keyword in Java is for three main occasions:
Final variables: its value cannot change after being assigned.
Final methods: cannot be overridden by subclasses.
Final classes: cannot be inherited by any class.
This keyword helps to maintain the finality of objects and prevents unintended behavior due to inheritance or reassignment.
16. What Difference Exists Between Composition and Inheritance?
Inheritance is a type of relationship in which one class is-another class (eg. Dog is-an Animal). It is hierarchical and encourages reuse through class extension.
Composition is a has-a relationship in which one class contains another class as a part (e.g. Car has an Engine). It tends to be more favorable in design because it provides greater flexibility and reduces tight coupling.
17. Explain How Java Provides Support for Multiple Inheritance.?
Java does not support multiple inheritance using classes to avoid complexity or ambiguity; instead, it supports it through interfaces. A class may implement multiple interfaces, thereby achieving some kind of multiple inheritance with clarity.
18. What Is a Static Method and How Is It Different from Instance Methods?
Static methods are those which are bound to the class, rather than to any instance thereof. Hence they can be called without instantiating the object. Static methods cannot access non-static (instance) variables or methods directly since they do not work with instances.
Instance methods require an object to work with and can access both static and non-static members.
Conclusion
Understanding OOP concepts in Java is essential not just for interviews but also for becoming a perfect software engineer. These questions lay the foundation for advanced topics, being discussed in design patterns, system design, and frameworks like Spring.
To prepare for these topics well:
Study the principles of OOP from the basic to advanced level.
Understand their practical utility.
Be quick to describe concepts and relate them to real-life instances.
While actually preparing for interviews, try to back these questions with scenario-based practice and mock interviews. This way, you won't simply answer the questions; you will impress with your great depth of understanding.
Also, you can attend the master class on the interviews that are held every weekend at Softronix! Join the league today!
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