Java OOPS Concepts Interview Questions

Q1.What are the main principles of Object Oriented Programming?

The Main principles of Object Oriented Programming (OOPS) are:

1. Abstraction

2. Encapsulation

3. Inheritance

4. Polymorphism

Q2.What is the difference between Object Oriented Programming language and Object-based Programming language?

Object Oriented Programming languages like Java and C++ follow concepts of OOPS like- Encapsulation, Abstraction, Polymorphism, Inheritance, etc. Object Based Programming languages follow some features of OOPS but they do not provide support for Polymorphism and Inheritance. Egg. JavaScript, VBScript, etc. Object-based Programming languages provide support for Objects and you can build objects from the constructor. These languages also support Encapsulation. These are also known as Prototype-oriented languages

Q3. Java what is the default value of an object reference defined as an instance variable in an Object?

All the instance variable object references in Java are null.

Q4.Why do we need a constructor in Java?

Java is an object-oriented language, in which we create and use objects. A constructor is a piece of code similar to a method. It is used to create an object and set the initial state of the object. A constructor is a special function that has the same name as class name. Without a constructor, there is no other way to create an object. By default, Java provides a default constructor for every object. If we overload a constructor then we have to implement a default constructor.

Q5.Why do we need a default constructor in Java classes?

A Default constructor is a no-argument constructor that is automatically generated by Java if no other constructor is defined. Java specification says that it will provide a default constructor if there is no overloaded constructor in a class. But it does not say anything about the scenario in which we write an overloaded constructor in a class. We need at least one constructor to create an object, that’s why Java provides a default constructor. When we have an overloaded constructor, Java assumes that we want some custom treatment in our code. Due to which it does not provide a default constructor. But it needs a default constructor as per the specification. So it gives an error.

Q6.What is the value returned by the Constructor in Java?

When we call a constructor in Java, it returns the object created by it. That is how we create new objects in Java.

Q7.Can we inherit a Constructor?

No, Java does not support the inheritance of constructor.

Q8. Why constructors cannot be final, static, or abstract in Java?

If we set a method as final it means we do not want any class to override it. However, the constructor (as per Java Language Specification) cannot be overridden. So there is no use of marking it final. If we set a method as abstract it means that it has no body and it should be implemented in a child class. But the constructor is called implicitly when the new keyword is used. Therefore it needs a body. If we set a method as static it means that it belongs to the class, but not a particular object. The constructor is always called to initialize an object. Therefore, there is no use of marking the constructor static.

Q9.What is the purpose of ‘this’ keyword in java?

In Java, ‘this’ keyword refers to the current instance of the object. It is useful for differentiating between instance variables and local variables. It can be used to call constructors. Or it can be used to refer to the instance. In the case of method overriding, this is used for falling the method of the current class.

Q10. Explain the concept of Inheritance.

Inheritance is an important concept in Object Oriented Programming. Some objects share certain characteristics and behaviors. By using Inheritance, we can put the common behavior and characteristics in a base class which is also known as super class. Then all the objects with common behavior inherit from this base class. It is also represented by IS-A relationship. Inheritance promotes code reuse, method overriding and polymorphism.

Q11. Which class in Java is superclass of every other class?

Java is an object-oriented programming language. In Java, the Object class is the superclass of every other class.

Q12. Why Java does not support multiple inheritance?

Multiple Inheritance means that a class can inherit behavior from two or more parent classes. The issue with Multiple Inheritance is that both the parent classes may have different implementations for the same method. So they have different ways of doing the same thing. Now which implementation should the child class choose? This leads to ambiguity in Multiple Inheritance. This is the main reason for Java does not support Multiple Inheritance in implementation.

Let's say you have a class TV and another class AtomBomb. Both have method switchOn() but only TV has switchOff() method. If your class inherits from both these classes then you have an issue that you can switchOn() both parents, but switchOff will only switchOff() TV. But you can implement multiple interfaces in Java.

Q13. In OOPS, what is meant by composition?

Composition is also known as “has-a” relationship. In composition, “has-a” relation relates two classes. E.g. Class Car has a steering wheel. If a class holds the instance of another class, then it is called composition.

Q14. How aggregation and composition are different concepts?

In OOPS, Aggregation and Composition are the types of association relations. A composition is a strong relationship. If the composite object is destroyed, then all its parts are destroyed. E.g. A Car has a Steering Wheel. If the Car object is destroyed, then there is no meaning of the Steering Wheel. In Aggregation, the relationship is weaker than Composition. E.g. A Library has students. If a Library is destroyed, Students still exist. So Library and Student are related by Aggregation. A Library has Books. If the Library is destroyed, the Books are also destroyed. Books of a Library cannot exist without the Library. So Book and Library are related by Composition.

Q15. Why there are no pointers in Java?

In Java there are references instead of pointers. These references point to objects in memory. But there is no direct access to these memory locations. JVM is free to move the objects within VM memory. The absence of pointers helps Java in managing memory and garbage collection effectively. Also it provides developers with convenience of not getting worried about memory allocation and deallocation.

Q16. If there are no pointers in Java, then why do we get NullPointerException?

In Java, the pointer equivalent is Object reference. When we use a . it points to object reference. So JVM uses pointers but programmers only see object references. In case an object reference points to null object, and we try to access a method or member variable on it, then we get NullPointerException.

Q17. What is the purpose of ‘super’ keyword in java?

‘super’ keyword is used in the methods or constructor of a child class. It refers to immediate parent class of an object. By using ‘super’ we can call a method of parent class from the method of a child class. We can also call the constructor of a parent class from the constructor of a child class by using ‘super’ keyword.

Q18. Is it possible to use this() and super() both in same constructor?

No, Java does not allow using both super() and this() in the same constructor. As per Java specification, super() or this() must be the first statement in a constructor.

Q19.What is the meaning of object cloning in Java?

Object.clone() method is used for creating an exact copy of the object in Java. It acts like a copy constructor. It creates and returns a copy of the object, with the same class and with all the fields having the same values as of the original object. One disadvantage of cloning is that the return type is an Object. It has to be explicitly cast to the actual type.