Packages

Package?

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.

Advantages Of Packages

• Java package is used to categorize the classes and interfaces so that they can be easily maintained.
• Java package provides access protection.
• Java package removes naming collision.

Accessing a package from other packages

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

• import package.*;
• import package.classname;
• fully qualified name.

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.

//save by A.java  

package pack;  

public class A{  

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

}  

//save as B.java

package mypack;  

import pack.*;  //Importing the package named pack which contains class A.

class B{  

  public static void main(String args[]){  

   A obj = new A();  

   obj.msg();  

  }  

} 

Using packagename.classname

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

//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;  //Importing class A from packagename pack which gives us access to only A.

class B{  

  public static void main(String args[]){  

   A obj = new A();  

   obj.msg();  

  }  

}  

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.

//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();  

  }  

}