Q1. What is the difference between an Inner Class and a Sub-Class?

Ans: An Inner class is a class which is nested within another class. An Inner class has access rights for the class which is nesting it and it can access all variables and methods defined in the outer class.

A sub-class is a class which inherits from another class called super class. Sub-class can access all public and protected methods and fields of its super class.

Q2. What are the various access specifiers for Java classes?

Ans: In Java, access specifiers are the keywords used before a class name which defines the access scope. The types of access specifiers for classes are:
1. Public : Class,Method,Field is accessible from anywhere.
2. Protected:Method,Field can be accessed from the same class to which they belong or from the sub-classes,and from the class of same package,but not from outside.
3. Default: Method,Field,class can be accessed only from the same package and not from outside of it’s native package.
4. Private: Method,Field can be accessed from the same class to which they belong.

Q3. What’s the purpose of Static methods and static variables?

Ans: When there is a requirement to share a method or a variable between multiple objects of a class instead of creating separate copies for each object, we use static keyword to make a method or variable shared for all objects.

Q4. What is data encapsulation and what’s its significance?

Ans: Encapsulation is a concept in Object Oriented Programming for combining properties and methods in a single unit.
Encapsulation helps programmers to follow a modular approach for software development as each object has its own set of methods and variables and serves its functions independent of other objects. Encapsulation also serves data hiding purpose.

Q5. What is a singleton class? Give a practical example of its usage.

A singleton class in java can have only one instance and hence all its methods and variables belong to just one instance. Singleton class concept is useful for the situations when there is a need to limit the number of objects for a class.
The best example of singleton usage scenario is when there is a limit of having only one connection to a database due to some driver limitations or because of any licensing issues.
Q6 Four main principles of OOPS language?

• Inheritance
• Polymorphism
• Data Encapsulation
• Abstraction

Q7 What is inheritance?
The process by which one class acquires the properties and functionalities of another class. Inheritance brings reusability of code in a java application.
Q8 Does Java support Multiple Inheritances?
When a class extends more than one classes then it is called multiple inheritance. Java doesn’t support multiple inheritance whereas C++ supports it, this is one of the difference between java and C++.  Refer this: Why java doesn’t support multiple inheritance?
Q9  What is Polymorphism and what are the types of it?
Polymorphism is the ability of an object to take many forms. The most common use of polymorphism in OOPs is to have more than one method with the same name in a single class. There are two types of polymorphism: static polymorphism and dynamic polymorphism,
Q10 What is the method overriding?
It is a feature using which a child class overrides the method of parent class. It is only applicable when the method in child class has the signature same as parent class.