Packages & Its types

Packages

• A package as the name suggests is a pack(group) of classes, interfaces and other packages. 
• We can use packages to organize our classes and interfaces. 

• How to declare packages?
• Syntax:
• package package_name;
• Example:
• package ampics

• How to import packages?
• Syntax:
• import package_name.class_name or *;
• Example:
• import ampics.MCA2;
• import ampics.*;
• MCA2 means only specific class.
• * indicates all members of that ampics.

We have two types of packages in Java: 

1. Built-in packages (also known as ready-made package)

• The already defined package like java.io.*, java.lang.* etc are known as built-in packages.
• Example:
• import java.util.Scanner
• Here, java is a top level package
• util is a sub package
• Scanner is a class

2. User-defined packages.

• The package we create is called user-defined package.
• Example: ampics.MCA2
• Here, ampics is a package
• MCA2 is a class.

Advantages of using a package in Java

• Reusability: Using packages, you can create such things in form of classes inside a package and whenever you need to perform that same task, just import that package and use the class.

• Better Organization: It is always required to group the similar types of classes in a meaningful package name so that you can organize your project better and when you need something you can quickly locate it and use it, which improves the efficiency.

• Name Conflicts: We can define two classes with the same name in different packages so to avoid name collision, we can use packages.

2. User-defined packages.
Full Example:
Calculator.java

package letmecalculate;
public class Calculator {
   public int add(int a, int b){
return a+b;
   }
   public static void main(String args[]){
Calculator obj = new Calculator();
System.out.println(obj.add(10, 20));
   }
}

Demo.java
import letmecalculate.Calculator;
public class Demo{
   public static void main(String args[]){
Calculator obj = new Calculator();
System.out.println(obj.add(100, 200));
   }
}

Full Example: Using fully qualified name 

Calculator.java

package letmecalculate;
public class Calculator {
   public int add(int a, int b){
return a+b;
   }
   public static void main(String args[]){
Calculator obj = new Calculator();
System.out.println(obj.add(10, 20));
   }
}

Example.java
//Declaring a package
package anotherpackage;
public class Example{
   public static void main(String args[]){
        //Using fully qualified name instead of import
letmecalculate.Calculator obj =  new letmecalculate.Calculator();
System.out.println(obj.add(100, 200));
   }
}