Packages - Java

Packages

========

Packages act as "containers" for classes.

Two types of packages:

          Java API packages

          User defined packages.

Package name                Constents

java.lang                        Language support classes.  These are classes that java compiler itself uses and therefore they are automatically imported.  They include classes for primitive types,strings,math functions,threads and exceptions.

java.util                          Language utility classes such as vectors,hash tables,random numbers,date,etc.

java.io                            Input/Output support classes.  They provide facilities for the input and output of data.

java.awt                         Set of classes for implementing graphical user interface.  They include classes for windows,buttons,lists,menus and so on.

java.net                          Classes for networking.  They include classes for communicating with local computers as well as with internet servers.

java.applet                     Classes for creating and implementing applets.

Using System Packages:

========================

          java.awt.*; - will bring all classes of java.awt package.

          or

          java.awt.

                                                Color

                                                Graphics

                                                Font

                                                .

                                                .

                                                Image

                                      That the package named java contains the package awt

                                      package containing classes

                                      classes containing methods

syntax:

                   import packagename.classname;

                   or

                   import packagename.*;

import statement must appear at the top of the file, before any class declarations,

import is a keyword.

Naming Conventions

===================

packages begin with lowercase letters.

class name begin with an uppercase letter.

                             double y=java.lang.Math.sqrt(x);

                             lang  - package name

                             Math - class name

                             sqrt(x)         - method name

Creating Packages

---------------------

We must first declare the name of the package using the package keyword followed by a package name. This must be first statement in a java source file(Except for comments and white spaces).

ex:

          package firstPackage;

          public class FirstClass

          {

                   ...........

                   ...........

                   ...........

          }

          saved as a file called FirstClass.java, and located in a directory named firstPackage.  When the source file is compiled,  Java will create a .class file and store it in the same directory.

          The .class files must be located in a directory that has the same name as the package, and this directory should be a subdirectory of the directory where classes that will import the package are located.

Own package involves the following steps:

1.       Declare the package at the beginning of a file using the form

                   package packagename;

2.       Define the class that is to be put in the package and declare it public.

3.       Create a subdirectory under the directory where the main source files are stored.

4.       Store the listing as the classname.java file in the subdirectory created.

5.       Compile the file.  This creates .class file in the subdirectory.

Accessing a Package

====================

syntax:

                   import package1 [.package2] [.package3].classname.*;

                   Here package1 is the name of the top level package, package2 is the name of the package that is inside the package1, and so on.

                   The statement must end with semicolon(;).

Using a package

==================

package package1;

public class ClassA

{

          public void displayA()

          {

                   System.out.println("Class A");

          }

}

-------------

import package1.*;

class PackageTest1

{

          public static void main(String args[])

          {

                   ClassA a=new ClassA();

                   a.displayA();

          }

}