Packages

A package is a collection of related classes.

It helps Organize your classes into a folder structure and make it easy to locate and use them.

More importantly,It helps improve re-usability.

Syntax:-

package <package_name>;

The following video takes you through the steps of creating a package.


Assignment 1:To create a package

Step 1) Copy the following code into an editor

package p1;
class c1{
public void m1(){

System.out.println("Method m1 of Class c1");
}
public static void main(String args[]){
c1 obj = new c1();
obj.m1();
}
}



Step 2) Save the file as Demo.java. Compile the file as, javac – d . Demo.java



Step 3) Run the code as java p1.c1



Assignment 2) To create a sub-package



Step1) Copy the following code into an editor


package p1.p2;

class c2

{
public void m2(){
System.out.println("Method m2 of Class c2");

}
public static void main(String args[]){
c2 obj = new c2();
obj.m2();
}
}





Step 2) Save the file as Demo2.java. Compile the file as javac – d . Demo2.java




Step 3) Run the code as java p1.p2.c2



importing a package



To create an object of a class (bundled in a package), in your code, you have to use its fully qualified name.

Ex.


java.awt.event.actionListner object = new java.awt.event.actionListner();


But , it could become tedious to type in the long dot-separated package path name for every class you want to use. Instead it is recommended you use the import statement.


Syntax

import <package_name>;

Once imported , you can use the class without mentioning its fully qualified name.

Ex:
import java.awt.event.*; // * signifies all classes in this package
import javax.swing.JFrame // here only the JFrame class is imported
//Usage
JFrame f = new JFrame; // without fully qualified name.



Assignment 3: To import package

Step 1) Copy the code into an editor.

// Using packages created in earlier assignment
package p3;
import p1.*; //imports classes only in package p1 and NOT in the sub-package p2

class c3{
public void m3(){
System.out.println("Method m3 of Class c3");
}
public static void main(String args[]){
c1 obj1 = new c1();
obj1.m1();
p1.p2.c2 obj2 = new p1.p2.c2();
obj2.m2();
}
}



Step 2) Save the file as Demo2.java . Compile the file using the command javac –d .Demo2.java



Step3) Execute the code using the command java p3.c3



Packages - points to note:


To avoid naming conflicts packages are given names of the domain name of the company in reverse Ex : com.guru99. com.microsoft, com.infosys etc.
When a package name is not specified , a class is into the default package (the current working directory) and the package itself is given no name. Hence you were able to execute assignments earlier.
While creating a package, care should be taken that the statement for creating package must be written before any other import statements


// not allowed
import package p1.*;
package p3;


//correct syntax
package p3;
import package p1.*;