Packages in java

Packages in Java
Prepared by
19P61A0555
E Sahithi Reddy

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.

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.

These are the packages which are already available in Java language.
java.lang :
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 etc… .

java.net :
They include classes for communicating with local computers as well
as with internet servers.
java.applet :
Classes for creating & implementing applets.
java.sql :
This package helps to connect to databases,retrive the data from
them and use it in a java program.

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.

Creating a 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.

• 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.

• Java also supports the concept of package hierarchy.
This is done by specifying multiple names in a package
statement, seperated 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 seperate
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.

Packages in java

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