What are the principle concepts of OOPS?
There are four principle concepts upon which object oriented design and programming rest. They are:
- Encapsulation (i.e. easily remembered as A-PIE).
What is Abstraction?
Abstraction refers to the act of representing essential features without including the background details or explanations.
What is Encapsulation?
Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing outside access only as appropriate. It prevents other objects from directly altering or accessing the properties or methods of the encapsulated object.
What is the difference between abstraction and encapsulation?
- Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it’s inside view, where the behavior of the abstraction is implemented.
- Abstraction solves the problem in the design side while Encapsulation is the Implementation.
- Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer
What is Inheritance?
- Inheritance is the process by which objects of one class acquire the properties of objects of another class.
- A class that is inherited is called a superclass.
- The class that does the inheriting is called a subclass.
- Inheritance is done by using the keyword extends.
- The two most common reasons to use inheritance are:
- To promote code reuse
- To use polymorphism
What is Polymorphism?
Polymorphism is briefly described as “one interface, many implementations.” Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts – specifically, to allow an entity such as a variable, a function, or an object to have more than one form.
Explain the different forms of Polymorphism.
There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance and interface.
Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:
- Method overloading
- Method overriding through inheritance
- Method overriding through the Java interface
What is method overloading?
Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type.
- Overloaded methods MUST change the argument list
- Overloaded methods CAN change the return type
- Overloaded methods CAN change the access modifier
- Overloaded methods CAN declare new or broader checked exceptions
- A method can be overloaded in the same class or in a subclass
What is method overriding?
Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type.
- The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected).
- You cannot override a method marked final
- You cannot override a method marked static
What is an Interface?
An interface is a description of a set of methods that conforming implementing classes must have.
- You can’t mark an interface as final.
- Interface methods must be static.
- An Interface cannot extend anything but another interfaces.
What is an abstract class?
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation.
- If even a single method is abstract, the whole class must be declared abstract.
- Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
What is Constructor?
- A constructor is a special method whose task is to initialize the object of its class.
- It is special because its name is the same as the class name.
- They do not have return types, not even void and therefore they cannot return values.
- They cannot be inherited, though a derived class can call the base class constructor.
- Constructor is invoked whenever an object of its associated class is created.
Question: What is synchronized keyword? In what situations you will Use it?
Answer: Synchronization is the act of serializing access to critical sections of code. We will use this keyword when we expect multiple threads to access/modify the same data. To understand synchronization we need to look into thread execution manner.