Java Package

 Introduction

    A java package is a group of similar types of classes, interfaces and sub-packages. Package in java can be categorized in two form, built-in package and user-defined package. There are many built-in packages such as java, lang, awt, javax, swing, net, io, util, sql etc. Here, we will have the detailed learning of creating and using user-defined packages.

Simple example of java package

The package keyword is used to create a package in java.

//save as Simple.java  

package mypack;  

public class Simple{  

 public static void main(String args[]){  

    System.out.println("Welcome to package");  

   }  

}  

How to access package from another package?

There are three ways to access the package from outside the package.

1. import package.*;

2. import package.classname;

3. fully qualified name.

1. Using packagename.*

    If you use package.* then all the classes and interfaces of this package will be accessible but not subpackages. The import keyword is used to make the classes and interface of another package accessible to the current package.

Example:

//save by A.java  

package pack;  

public class A{  

  public void msg(){System.out.println("Hello");}  

}  

//save by B.java  

package mypack;  

import pack.*;  

  

class B{  

  public static void main(String args[]){  

   A obj = new A();  

   obj.msg();  

  }  

}  

Output:

Hello

2. Using packagename.classname

If you import package.classname then only declared class of this package will be accessible.

Example:

//save by A.java  

  

package pack;  

public class A{  

  public void msg(){System.out.println("Hello");}  

}  

//save by B.java  

package mypack;  

import pack.A;  

  

class B{  

  public static void main(String args[]){  

   A obj = new A();  

   obj.msg();  

  }  

}  

Output:

Hello

3. Using fully qualified name

If you use fully qualified name then only declared class of this package will be accessible. Now there is no need to import. But you need to use fully qualified name every time when you are accessing the class or interface. It is generally used when two packages have same class name e.g. java.util and java.sql packages contain Date class.

Example:

//save by A.java  

package pack;  

public class A{  

  public void msg(){System.out.println("Hello");}  

}  

//save by B.java  

package mypack;  

class B{  

  public static void main(String args[]){  

   pack.A obj = new pack.A();//using fully qualified name  

   obj.msg();  

  }  

}  

Output:

Hello

Note:Sequence of the program must be package then import then class.