java package in java.. in java packages.

PRESENTATION
ON
JAVA PACKAGES
R.HANNAH ROSELINE
ASSISTANT PROFESSOR
SRI RAMAKRISHNA COLLEGE OF ARTS & SCIENCE

CONTENTS
 Introduction
 Types Of Packages
 Pre-Defined Packages or Built in Packages
 Pre-Defined Packages & Their Classes
 User Defined Packages.
 Creating User Defined Packages
 Steps For Creating Package
 Accessing a Package
o Advantages Of Packages

INTRODUCTION
In java, programmers can create several classes &
Interface. After creating these classes and
interface, it is better if they are divided into some
groups depending on their relationship. Thus, the
classes and interface which handle similar or same
task are put into the same directory or folder,
which is also known as package.
Packages act as “containers” for classes. A package
represents a directory that contain related group of
classes & interface.

TYPES OF PACKAGES
There are basically only 2 types of java packages.
They are as follow :
 System Packages or Java API
 User Defined Packages.

Pre-Defined Packages or Built in Packages
As there are built in methods , java also provides
inbuilt packages which contain lots of classes &
interfaces. These classes inside the packages are
already defined & we can use them by importing
relevant package in our program.
Java has an extensive library of packages, a
programmer need not think about logic for doing
any task. For everything, there are the methods
available in java and that method can be used by
the programmer without developing the logic on
his own. This makes the programming easy.

JAVA SYSTEM PACKAGES & THEIR CLASSES
 java.lang
Language Support classes. These are classes that java compiler itself
uses & therefore they are automatically imported. They include classes
for primitive types, strings, maths function, threads &exception.
 java .util
Language Utility classes such as vector, hash tables ,random
numbers, date etc.
 java.io
Input /Output support classes. They provide facilities for the input &
output of data
 java.awt
Set of classes for implementing graphical user interface. They include
classes for windows, buttons, list, menus & 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 & implementing applets.

2. USER DEFINED PACKAGES :
The users of the Java language can also create their own packages. They
are called user-defined packages. User defined packages can also be
imported into other classes & used exactly in the same way as the Built in
packages.
i) Creating User Defined Packages
Syntax :
package packageName;
public class className
{
- - - - - - - - - - - - -
// Body of className
- - - - - - - - - - - -
}
We must first declare the name of the package using the package keyword
followed by the package name. This must be the first statement in a Java
source file. Then define a classes as normally as define a class.

Example :
package myPackage;
public class class1
{
- - - - - - - - - - - - -
// Body of class1
}
In the above example, myPackage is the name of the package. The class
class1 is now considered as a part of this package.
This listing would be saved as a file called class1.java & located in a
directory named mypackage. When the source file is compiled, java will
create a .class file & store it in the same directory.
The .class files must be located in a directory that has the same name as
the package & this directory should be a subdirectory of the directory
where classes that will import the package are located.

STEPS FOR CREATING PACKAGE :
To create a user defined package the following steps should
be involved :-
1: Declare the package at the beginning of a file using
the syntax :-
package packageName;
2: Define the class that is to be put in the package &
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 create .class file in the
subdirectory.

Java also supports the concept of package hierarchy.
This is done by specifying multiple names in a package
statement, seprated by dots (.).
Ex :- package firstPackage.secondPackage;
This approach allows us to group related classes into a
package and their group related package into a larger
package. Store this package in a subdirectory named
firstpackage/secondPackage.
A java package file can have more than one class
definition. In such cases, only one of the classes may be
declared public & that class name with .java extension
is the source file name. When a source file with more
than one class definition is compiled, java creates
independent .class files for those classes.

ACCESSING A PACKAGE
Java package can be accessed either using a fully qualified class
name or using a shortcut approach through the import statement.
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 package & so on. We
can have any number of packages in a package hierarchy. Finally
the explicit classname is specified. The import statement must
end with a semicolon (;). The import startment should appear
before any class definitions in a source file. Multiple import
statements are allowed.
Ex :
import firstpackage.secondPackage.Myclass; or
import firstpackage;

ADVANTAGES OF PACKAGES
There are several advantages of package some of them are as
follow :-
1: Packages are useful to arrange related classes and interface
into a group. This makes all the classes & interface performing
the same task to put together in the same package.
2: Packages hide the classes & interfaces in a seprate
subdirectory, so that accidental deletion of classes & interfaces
will not take place.
3: The classes & interfaces of a packages are isolated form the
classes & interfaces of another packages. This means that we
can use same names for classes of two different classes.
4: A group of packages is called a library. The classes &
interface of a package are like books in a library & can
be reused several times. This reusability nature of
packages makes programming easy.

Importing a Package
 If we want to use a package in Java program it is
necessary to import that package at the top of the
program by using the import keyword before the
package name.
 Syntax:
import packageName;

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");
}
}

EXAMPLE OF PACKAGE BY IMPORT
PACKAGE.CLASSNAME
//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

java package in java.. in java packages.

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