Java OOPs Interview Questions

Advanced Java OOPs Interview Questions

21. What is the definition of encapsulation?

Answer: Encapsulation is an OOPS concept for creating and defining an object's permissions and restrictions, as well as the variables and methods that make up the object. Making a class's member variables private and offering public getter and setter methods is a straightforward way to demonstrate the concept. Access level modifiers in Java are divided into four categories: public, protected, no modifier, and private.

22. What is polymorphism?

Answer: The appearance of something in multiple forms is known as polymorphism. Polymorphic reference variables, polymorphic methods, polymorphic return types, and polymorphic argument types are all supported by Java.

23. What is method overriding, and how does it work?

Answer: Method overriding is an OOPs feature that allows a child class or subclass to rewrite methods in the base or parent class. The overridden method has the same name as the original, as well as the same signature, which refers to the arguments supplied and the return type.

24. What does it mean when an operator is overloaded?

Answer: Operator overloading refers to the usage of user-defined types to implement operators based on the arguments provided to them. This feature allows you to extend the functionality of operators beyond their built-in uses, enhancing the expressiveness and flexibility of your code while adhering to the principles of object-oriented programming.

25. What is the definition of an abstract class?

Answer: A class with abstract methods is known as an abstract class. Although these methods are declared, they are not defined. If these methods are to be utilized in a subclass, they must be specified in that subclass solely.

26. What are virtual functions?

Answer: Virtual functions are present in the parent class and are overridden by the subclass. These functions are used to achieve runtime polymorphism. They allow a base class to define a method as virtual, indicating that derived classes may override it. When an object of a derived class is accessed through a pointer or reference of the base class, the appropriate overridden function in the derived class is invoked based on the actual object type.

27. What is a constructor?

Answer: Constructor is a type of method that is used to initialize objects of a class and has the same name as the class. They play a crucial role in ensuring the proper instantiation of objects. They can enforce certain constraints or actions during object creation, contributing to the integrity and consistency of the class's instances within a program.

28. Name the different types of constructors.

Answer: The different types of constructors are:-

• Default constructor
• Parameterized constructor
• Copy constructor
• Static constructor
• Private constructor

29. How much memory does a class occupy?

Answer: No memory is used by classes. Class are nothing more than a blueprint from which objects are produced. When objects are created now, the class members and methods are really initialized, which consumes memory.

30. What is an exception?

Answer: An exception is a type of notice that causes a program's usual execution to be interrupted. Exceptions give an error a pattern and pass it on to the exception handler to be resolved. The program's state is saved as soon as an exception is raised.

31. What is the definition of exception handling?

Answer: Exception handling is a crucial notion in Object-Oriented Programming for dealing with errors. Errors can be thrown and caught, and an exception handler implements a centralized mechanism to resolve them.

32. How does Java support multiple inheritance without using various inheritance?

Java achieves multiple inheritance through interfaces. Allowing a class to implement various interfaces can inherit diverse sets of abstract methods and contracts, avoiding the complexities of traditional multiple inheritance. This approach promotes clear separation between interface and class inheritance, enhancing maintainability and adaptability in the codebase.

33. What is the purpose of the final keyword in Java, and how does it impact OOP principles?

The final keyword restricts modifications on classes, methods, and variables. Applied to a class, it prevents inheritance, ensuring the class cannot be extended. When applied to a method, it prevents overriding. While final contributes to achieving immutability and safeguarding against undesired changes, excessive use may curtail the extensibility and flexibility intrinsic to OOP's objectives.

34. Explain the concept of encapsulation and provide an example in Java.

Encapsulation embodies an OOP principle that consolidates data (attributes) and methods (behavior) relevant to the data within a single entity (class). Controlled access to data is achieved through methods facilitating data protection and abstraction. For example, using private instance variables and public getter and setter methods in a Person class ensures disciplined access and manipulation of the person's attributes.

35. Explain the concept of composition in Java OOPs.

Composition entails the creation of complex objects by assembling simpler components or objects. This is achieved by incorporating instances of other classes as member variables within a class. This methodology fosters code reusability and modular design, allowing components to be substituted or added without disrupting the overall structure. For instance, a Car class could comprise instances of Engine, Wheels, and Seat classes, each handling distinct functionalities.

36. What is the role of the super keyword in Java, and how does it relate to inheritance?

The super keyword references the superclass of a derived class. It is frequently employed to invoke overridden superclass methods, facilitating an extension of behavior inherited from the superclass. This mechanism supports code reuse, permitting the utilization of existing functionalities while introducing or modifying behaviors specific to the subclass.

37. What is a singleton design pattern, and how does it ensure the creation of only one instance?

The singleton design pattern ensures the existence of only one instance of a class while offering a universal access point to that instance. This is accomplished by making the class constructor private and presenting a static method to access the single instance. The instance is created upon the first request, and subsequent requests retrieve the existing instance. This pattern often manages singular resources such as configurations or database connections.

38. How does Java implement runtime polymorphism, and why is it essential in OOP?

Java employs runtime polymorphism through method overriding and dynamic method dispatch. By overriding a method from a superclass in a subclass, the actual method called at runtime is based on the object's type. This dynamic method dispatch facilitates adaptable and extensible code, enabling diverse subclasses to offer specialized implementations while adhering to a shared interface.

39. How does Java support method overloading and method overriding, and what is the difference?

Java supports method overloading by enabling a class to define multiple methods sharing the same name but distinct parameter lists. The compiler distinguishes these methods based on parameter number or type. Method overriding, on the other hand, transpires when a subclass supplies a distinct implementation for a method already defined in its superclass. Overriding, marked by the @Override annotation, empowers dynamic method dispatch, a cornerstone of achieving runtime polymorphism.

40. Explain the concept of abstract classes and interfaces in Java, and when would you use each?

Abstract classes are inimitable and intended to be extended by subclasses. They encompass both abstract (unimplemented) and concrete (implemented) methods. Interfaces, conversely, outline contractual obligations for classes to fulfill by offering an assortment of abstract methods to be overridden. Abstract classes suit scenarios necessitating a shared foundation with default implementations, whereas interfaces establish a uniform contract for multiple classes, fostering structured code organization and enabling various inheritances.

Java OOPs Coding Problem Questions

41. Write a Java program to demonstrate encapsulation by creating a Person class with private variables and public getter/setter methods.

• Java

Java

class Person {
    private String name;
    private int age;

    // Getter and Setter methods
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

public class Main {
    public static void main(String[] args) {
        Person person = new Person();
        person.setName("Rohit");
        person.setAge(30);

        System.out.println("Name: " + person.getName());
        System.out.println("Age: " + person.getAge());
    }
}

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Output:

Name: Rohit
Age: 30

42. Write a Java program to demonstrate inheritance by creating a Vehicle superclass and a Car subclass that inherits from Vehicle.

• Java

Java

class Vehicle {
    public void startEngine() {
        System.out.println("Engine started");
    }
}

class Car extends Vehicle {
    public void drive() {
        System.out.println("Car is driving");
    }
}

public class Main {
    public static void main(String[] args) {
        Car car = new Car();
        car.startEngine();  // Inherited method
        car.drive();
    }
}

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Output:

Engine started
Car is driving

43. Write a Java program to demonstrate polymorphism by creating a Shape class and overriding its method in subclasses Circle and Rectangle.

• Java

Java

class Shape {
    public void draw() {
        System.out.println("Drawing a shape");
    }
}

class Circle extends Shape {
    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }
}

class Rectangle extends Shape {
    @Override
    public void draw() {
        System.out.println("Drawing a rectangle");
    }
}

public class Main {
    public static void main(String[] args) {
        Shape shape1 = new Circle();
        Shape shape2 = new Rectangle();
        
        shape1.draw();  // Calls Circle's draw method
        shape2.draw();  // Calls Rectangle's draw method
    }
}

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Output:

Drawing a circle
Drawing a rectangle

44. Write a Java program to demonstrate abstraction by creating an abstract class Animal and implementing its abstract methods in Dog and Cat classes.

• Java

Java

abstract class Animal {
    public abstract void sound();
}

class Dog extends Animal {
    public void sound() {
        System.out.println("Dog barks");
    }
}

class Cat extends Animal {
    public void sound() {
        System.out.println("Cat meows");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal dog = new Dog();
        Animal cat = new Cat();
        
        dog.sound();
        cat.sound();
    }
}

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Output:

Dog barks
Cat meows

45. Write a Java program to demonstrate method overloading by creating a Calculator class with multiple add methods.

• Java

Java

class Calculator {
    // Method to add two integers
    public int add(int a, int b) {
        return a + b;
    }

    // Method to add three integers
    public int add(int a, int b, int c) {
        return a + b + c;
    }

    // Method to add two doubles
    public double add(double a, double b) {
        return a + b;
    }
}

public class Main {
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        
        System.out.println("Sum of 2 integers: " + calc.add(10, 20));
        System.out.println("Sum of 3 integers: " + calc.add(10, 20, 30));
        System.out.println("Sum of 2 doubles: " + calc.add(10.5, 20.5));
    }
}

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Output:

Sum of 2 integers: 30
Sum of 3 integers: 60
Sum of 2 doubles: 31.0

Frequently Asked Questions

Why is Java known as an object-oriented programming language?

Due to the lack of global scope, Java is exclusively an object-oriented language. Everything is an object in java, and all program functions and data are contained within classes and objects. It has a large number of classes organized into packages, as well as a Java object model that is easy to extend.

What are the 4 types of OOP Java?

Java supports four types of OOP(Object-Oriented Programming): Abstraction, Encapsulation, Inheritance, and Polymorphism. These principles guide the creation of flexible code by focusing on data protection, code reuse, and dynamic behavior.

How do I prepare for an OOP interview?  

To prepare for an OOP interview, learn fundamental concepts like classes, objects, inheritance, polymorphism, and encapsulation. Understand design patterns, practice coding exercises, and be ready to explain real-world examples demonstrating your OOP comprehension.

What are OOPs types in Java? 

In Java, OOPs are of four types- Abstraction, Polymorphism, Encapsulation, and Inheritance. These principles contribute to structured and flexible code development by giving importance to data representation, ensuring security, promoting reusability, and enabling dynamic behavior.

Conclusion

In this article, we have discussed Java OOPs Interview Questions. Understanding Object-Oriented Programming (OOP) principles are essential for every Java developer, as they form the foundation of efficient and maintainable code. Mastering concepts like encapsulation, inheritance, polymorphism, and abstraction will not only help you perform well in interviews but also enhance your coding skills for real-world applications.

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