Java Object Oriented Programming

Check this code to indentify attribute and object. How we can access one attribute ?
package my_proj;

public class my_first {
int i=20; // attribute created. 

public static void main(String[] args) {
 my_first my_num=new my_first(); // Object Created
 System.out.println(my_num.i); // 20 
}

}
In above code we have created one variable int i =20;.
This variable is also known as attribute of the class my_first.

object

Check this line of above code.
my_first my_num=new my_first();
in above code we creates an object by name my_num. We can say my_num is an object of class my_first.
Objects can inherit all the variables and methods of the class.

object and attribute

We can access any attribute of the class my_first by using dot ( . ) notation. In the last line we have used our object my_num and dot notation to get the value of attribute i of my_first. Output is 20 here.
System.out.println(my_num.i); // 20 

method

We have created attributes ( variables within a class ) and accessed the same by using objects of the class in above code. Now we will create methods inside a class. By using methods we can perform some tasks. ( try to understand difference between attribute and method )


Let us try to add two numbers by using one method and name it as adder. We will create an object of the class my_first and use the same to execute the method. Here is the code.
package Pack1;
class my_first {
	int i =20;
	int j=10;
	public void adder() { // Method is created. 
		System.out.println(i+j); 
	}
	
	
	public static void main(String args[])
	{
	my_first obj1= new my_first();
	System.out.println(obj1.i);//20
	obj1.j=40; 
	obj1.adder();//60
	}

}
In above example you can identify the following points.

attributes, class, method, Object


Check this line
obj1.j=40; 
Here we have overriding the existing value of the attribute. Now the value of j=40 ( not 10 ). The output of above code after using the method adder() is 60.

final

In above code we have updated the value of attribute j to 40 by using our object obj1. We may not allow this updating or everriding of attributes by using final while declaring the attribute. Update this line like this.
final int j=10;
Now we can't update the value of j to any other value. This the exceptions we will get when we try to update the value of j.
 The final field my_first.j cannot be assigned
We can create class in a separate file in same package and use the class.
package Pack1;

class my_first {
	int i =20;
	int j;
	void adder()
	{
		System.out.println(i+j); // 50
	}
	public static void main(String args[])
	{
		my_first obj1 = new my_first();
		System.out.println(obj1.i); // 20
		obj1.j=30;
		obj1.adder();
	}

}
my_second.java
package Pack1;

public class my_second {
	public static void main(String args[])
	{
		my_first obj2 = new my_first();
		obj2.j=10;
		obj2.adder();//30
	}

}
This way we can use different methods.

Let us try one class where we can use addition , subtraction , multiplication and division of two numbers.

my_second.java
package Pack1;

public class my_second {
	public static void main(String args[])
	{
		my_first obj2 = new my_first();
		obj2.j=20;
		obj2.i=40;
		obj2.adder();
		obj2.my_subtract();
		obj2.my_multiply();
		obj2.my_devide();
		
	}

}
my_first.java
package Pack1;
class my_first {
	int i =20;
	int j;
	void adder()
	{
		System.out.println(i+j); 
	}
	
	public void my_subtract() {
		System.out.println(i-j); 
	}
	public void my_multiply() {
		System.out.println(i*j); 
	}
	public void my_devide() {
		System.out.println(i/j); 
	}
	public static void main(String args[])
	{
		
	}

}
The output is here
60
20
800
2

public and private methods

In above code if we change any method to private then we will get exceptions like this

The method my_subtract() from the type my_first is not visible
private void my_subtract() {
		System.out.println(i-j); 
	}

static and public methods

We can access any public method by using object, however we can access any static method without creating any object.

Here my_static is an static object so we can access it directly. However my_public() is an public method so we need to create an object ( obj1 ) to access my_public() method.
package Pack1;

public class my_second {
	static void my_static(){
		//System.out.println(i*j);
		System.out.println("Hi this is my_static");
	}
	public void my_public() {
		System.out.println("Hi this is my_public");
		
	}
	public static void main(String args[])
	{
		//my_public(); // this will generate Exception 
		my_second obj1=new my_second(); // Object is created 
		obj1.my_public();    // method is executed. 
		
		// Can access without object
		my_static(); 		
	}
}
Output
Hi this is my_public
Hi this is my_static

Example

Java Class and object

my_test_class.java
public class my_test_class {

	public static void main(String[] args) {
		// Declare objects here 
		my_add myObj=new my_add();
		
		int z = myObj.adding(12,13);
		System.out.println(z);

	}

}
my_add.java
public class my_add {
int adding(int x, int y){
	
	return x+y;
}
}
Example 1 Example2 forName()



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