Class and Object

Java is an object oriented programming language, each program is represented in terms of object. So an object is any real world object for e.g. a Car, Computer, Apple etc.

To understand it better, lets say we are given a program to read file from the system from a given specified location. So how do we represent a file and how do we represent its behavior ? Like a file is supposed to read, write or possible delete ? how do we represent all of this ?

In an OOP (Object oriented programming) we would represent a File and its behavior in a class. For e.g.

class File {
String fileName;

File(String fileName){
this.fileName = fileName;
}

public void read(){
 ...
}

public void write() {
 ..
}

}

So a class represents everything a file object can do. So in our example, a file can read and write.

So if we have to read a file, we have to create an instance of the file object, like below.

File myfile = new File("sample.txt")
myfile.read();

So now we have created a new instance of the File class which is represented by variable “myfile”. The variable “myfile” uniquely represents the file “sample.txt” and when we can call read/write method on the “myfile” instance and it will execute the code inside the methods read()/write() of the file class.

So the green box represents “Class” and all the yellow boxes represent “Object/Instances” of the class “File”. We can have as many objects of the class File as we want.