PACKAGES

Putting classes together

Benefits of using packages in java

—   The classes contained in packages of other programs can be easily reused

—   Here, classes can be unique compared with classes in other packages

—   It provide a way to hide classes

—   It provides a way to separate design from coding

Java packages

—   Java API Packages

—   User defined packages

Java API Packages

Java API provides a large amount of classes grouped into different packages according to the functionality

Frequently used API Packages

Java System packages and classes

Using System Packages

—  The packages are organized in a hierarchical structure.

—  Package named java contains the package awt, which in turn contains various classes required for implementing GUI

Hierarchical representation of java. awt package

Accessing classes stored in package

·                  Use fully classified classname of the class that we want to use

              import packagename.classname

                                    import Java.awt.Color

·                  Accessing the whole classes of a particular package

                                    import package name.*;

              import java.awt.*;

Naming conventions

—    Packages begins with lowercase letters

—    Class names begin with uppercase

—    Methods begin with lowercase

Creating packages

—     Declare the name of the package using the keyword package followed by package name

—     This must be the first statement in the code

—     Here package name is firstPackage

—     Should be saved in the name ‘firstClass.java’

—     Located in the directory named ‘firstPackage

—     This directory should be the subdirectory of the directory where classes imports this package located

  

Summary of the process to be done to create a package

—     Declare the package at the beginning

—     Define the class that is to be put in the package and declare public

—     Create a sub directory under the directory where the main source code stored.

—     Store the listing file as classname.java in sub directory created

—     Compile the file

Accessing the package

—    Using import statement

General form:

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

Using package

—     Package named package1 contains a single class ClassA 

—     Souce file should  be saved as ClassA.java

—     Stored in the subdirectory package1

—     Compile this java file

 

Program in the parent directory

—    Imports the class ClassA from package1

—    Source file should be saved as PackageTest1.java

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