Package

• These are the containers for the classes.
• They are used to keep the class name space compartmentalized means we can use same name of the class which is in one package can also define in another package.
• It provides the both a naming and a visibility control mechanism means we can define a class inside a package that are not acceptable to the outside of the package also we can define a class members which can be accessed by other members of the same package

Defining a Package

• By including a package command as the first statement in the java source file.
• The classes defined in that file are belong to the specific package.
• If we did not used the package it will store in default package which don't have name.

General form

package package_name;

ex: package MyPackage;

This statement creates the package called Mypackage.

• Java uses file system to store packages as it stores. class file for any classes in the directory which name is exactly same as package name.

• We can not rename the packages without renaming the directory in which classes are stored.

• We can create a hierarchy of packages which can done by . (period) symbol.

General form

package pkg1[.pkg2[.pkg3]];

Ex: package.java.awt.image;

Finding Packages and Class path

• As java uses directory to store packages then the java run-time environment should know where the packages are created .

• Three ways It will automatically checks the directory where the source file is.

• If not it will uses the class path where that package created.

Ex: If the directory path is C:\MyPrograms\Java\MyPack

then class path is C:\MyPrograms\Java

1. By using- classpath option with java and javac to specify the path to your classes.

Access Protection

Java provides many levels of protection to allow fine-grained control over the visibility of the variables and methods within classes, sub classes, packages. Java addresses four categories of visibility for class members:

• Sub classes in the same package
• Non-sub classes in the same package
• Subclasses in different packages
• Classes that are neither in the same package nor sub classes

These categories can be done by private, public, protected, default access modifiers.

private- Only to the members of the class.

public- Can be accessed from any where.

default - Accessed by the sub classes as well as other classes in the same package.

protected - If we want to access to the sub classes in outside of the package but sub classes of the class.

//protected access to the non-sub classes in the same package
class A 
{
protected int x;
  }


class B {
        B() {
              A a=new A();
              a.x=9;//why we can access this field ?
              System.out.println("The value of x is::"+a.x);
        }
  }

public class Demo
{
  public static void main(String args[])
  {
    B b = new B();

  }
}

Output:

The value of x is::9

Importing Packages

• Java includes the import statement to bring certain classes, or entire packages, into visibility.

• In a Java source file, import statements occur immediately following the package statement (if it exists) and before any class definitions.

import pkg1 [.pkg2].(classname | *);

package pack_name; (optional)

class ClassName

{

//members of class

}

import java.util.*;   //All classes in the sub package util
import java.util.Scanner; //Only Scanner class in the util package.
class MyClass extends Scanner
{
//members of class
}

//OR                                                                 

class MyClass extends java.util.Scanner
{
//members;
}

As we can access the public members only to the outside package non-sub class members so Scanner should be declared as public.