OOPconcepts.md

Object Oriented Programming Concepts

Class

A class is a group of objects that has common properties. It is a template or blueprint from which objects are created.

Classcode example

Object

An entity that has state and behavior is known as an object. Object is an instance of a class.

Objectcode example

Constructor

Constructor is a block of code, which runs when you use new keyword in order to instantiate an object.

Rules for creating java constructor: 1. Constructor name must be same as its class name. 2. Constructor must have no explicit return type.

Types of java constructors: 1. Default constructor (no-arg constructor). 2. Parameterized constructor

Constructorcode example

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Polymorphism

Polymorphism is the capability of a method to do different things based on the object that it is acting upon. In other words, polymorphism allows you define one interface and have multiple implementations.

Different types of polymorphism in java:

• Method Overloading: Define two or more methods of same name in a class, provided that there argument list or parameters are different.

• Method Overriding: Child class has the same method as of base class. In such cases child class overrides the parent class method without even touching the source code of the base class.

Method overloadingcode example

Method overridingcode example

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Inheritance

Inheritance allows a class to use the variables and methods of another class. In other words, the derived class inherits the states and behaviors from the base class. The derived class is also called subclass and the base class is also known as superclass. The derived class can add its own additional variables and methods. These additional variables and methods differentiate the derived class from the base class.

Inheritancecode example

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Encapsulation

Encapsulation in Java is a mechanism of wrapping the data (variables) and code acting on the data (methods) together as a single unit. In encapsulation the variables of a class will be hidden from other classes, and can be accessed only through the methods of their current class, therefore it is also known as data hiding.

Encapsulationcode example

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Static Method & Static Variable

A static method belongs to the class rather than object of a class. A static method can be invoked without the need for creating an instance of a class. Static method can access static data member and can change the value of it.

Static methodcode example

The static variable can be used to refer the common property of all objects (that is not unique for each object). The static variable gets memory only once in class area at the time of class loading.

Static variablecode example

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Abstract Class & Abstract Method

A class that is declared using “abstract” keyword is known as abstract class. It may or may not include abstract methods which means in abstract class you can have concrete methods (methods with body) as well along with abstract methods ( without an implementation, without braces, and followed by a semicolon). An abstract class can not be instantiated (you are not allowed to create object of Abstract class).

Abstract method has no body and always end the declaration with a semicolon(;). It must be overridden. An abstract class must be extended and in a same way abstract method must be overridden. Abstract method must be in a abstract class.

Abstract class & methodcode example

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Interface

Interface looks like class but it is not a class. An interface can have methods and variables just like the class but the methods declared in interface are by default abstract (only method signature, no body). Also, the variables declared in an interface are public, static and final by default.

Interfacecode example

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Access Modifiers

Determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control:

At the top level—public, or package-private (no explicit modifier).

At the member level—public, private, protected, or package-private (no explicit modifier).

Access modifierscode example

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Packages

Packages in Java is a mechanism to encapsulate a group of classes, interfaces and sub packages. Many implementations of Java use a hierarchical file system to manage source and class files. It is easy to organize class files into packages. All we need to do is put related class files in the same directory, give the directory a name that relates to the purpose of the classes, and add a line to the top of each class file that declares the package name, which is the same as the directory name where they reside.

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Final

• Final variable: Final variables are nothing but constants. We cannot change the value of a final variable once it is initialized.
• Final method: A final method cannot be overridden. Which means even though a sub class can call the final method of parent class without any issues but it cannot override it.
• Final class: We cannot extend a final class.

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This

• This keyword can be used to refer current class instance variable.
• This() can be used to invoke current class constructor.
• This keyword can be used to invoke current class method(implicitly).
• This can be passed as an argument in the method call.
• This can be passed as argument in the constructor call.
• This keyword can also be used to return the current class instance.

Thiscode example

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Super

• Super.<variable_name> refers to the variable of variable of parent class.
• Super() invokes the constructor of immediate parent class.
• Super.<method_name> refers to the method of parent class.

Supercode example

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