Java Tutorial 21 – Package

package;

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

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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 ;

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.*;

the java.lang package is imported by default for any class that you create in Java.  You can study it more in detail here.

The java API is very extensive , contains classes which can perform almost all your programming tasks right from Data Structure Manipulation til Networking. More often than not , you will be using API files in your code. You can see the API documentation here.