1. Packages Written part of final exam Alex Rudniy
CIS 602 Java and the Web
Packages
Written part of final exam
Alex Rudniy

2. Packages
To make types easier to find and use, to avoid naming conflicts, and to control access, programmers bundle groups of related types into packages.
Definition:  A package is a collection of related types providing access protection and name space management. Note that types refers to classes, interfaces, enums, and annotations.
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3. Packages
Provide and enforce a contract between the developer and the user of a collection of resources
Reuse
Organize resources so they can be used in more than one program
Organize the name space
Avoid naming conflicts
Helps to organize the software
Group related resources
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4. Packages Organize smaller components into larger components
Helps to organize work effort
Divide and Conquer a large body of software
Allow multiple programmers to work on same effort
Encapsulation
Package is a collection of related classes or packages
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5. Graphical Notation
Packages are graphically drawn as a rectangle with a tab on the left hand side. Further packages contained within the main package may be drawn in the rectangle. If this is the case, the name of the main package is shown in the tab. Otherwise it is shown in rectangle.
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6. Predefined packages Predefined packages come with Java
Group of all packages known as Java class library or Java API
Two groups of packages in Java API:
Core packages begin with java
import java.util.Vector;
Extension or Extra packages begin with javax
import javax.swing.JOptionPane;
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7. Core Packages
The Java class library is organized into a number of packages:
java.applet - classes necessary and beneficial for creating classes to go on the web
java.awt - classes for defining GUIs
java.awt.datatransfer - for cut and paste
java.awt.event - for handling button presses, mouse moves, etc
java.awt.image - for handling images
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8. Core Packages java.io - handling input/output
java.lang - classes for primitive types other essential classes
java.math - math functions
java.net - networking functions
java.text - text handling functions
java.util - classes not well defined in other classes but very helpful
java.util.zip - package for handling, creating and using zip files, compression
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9. Extension Packages javax.crypto - for cryptographic operations.
javax.net - for networking applications.
javax.print - describe the types of JavaTM Print Service attributes
javax.swing - a set of "lightweight" components that, to the maximum degree possible, work the same on all platforms.
javax.sound.sampled - for capture, processing, and playback of sampled audio data.
javax.security.cert - public key certificates.
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10. Created packages
Software developers organize own classes into packages
Naming conventions
packages are all lower case commonly use reverse of internet domain name to provide a globally unique name
i.e. com.sun.java.swing
The first line of the unit declares the package to which the class belongs:
package mypackage.subpackage.subsubpackage;
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11. Named and unnamed For example, it is possible to declare a package:
package vector;
public class Vector {Object[] vec;}
A compilation unit that has no package declaration is part of an unnamed package. As an example, the compilation unit:
class FirstCall {public static void main(String[] args) { System.out.println(“Hi Mr. Student”); } }
defines a very simple compilation unit as part of an unnamed package.
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12. Import Packages
If you have classes in packages, you can then reference them in different ways :
with the fully qualified domain name
packagename.classname, for example,
geometry.Point myPoint = new geometry.Point();
or use an import statement
import packagename.classname; //one class
import packagename.*; //entire package
Elephant myElephant = new Elephant();
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13. Disambiguating a Name
If by some chance a member in one package shares the same name with a member in another package and both packages are imported, you must refer to each member by its qualified name.
For example, a developer defined a class named Rectangle in the graphics package. The java.awt package also contains a Rectangle class.
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14. Disambiguating a Name
If both graphics and java.awt have been imported, the following is ambiguous.
Rectangle rect;
In such a situation, you have to be more specific and use the member's qualified name to indicate exactly which Rectangle class you want.
graphics.Rectangle rect;
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15. Directory structure
If you have classes in packages, the directories on the file system they reside in must match the package structure.
For example, I have a directory called d:\java
Then my directory structure would have
d:\java\zoo\Elephant.java
after compiling would give us d:\java\zoo\Elephant.class
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16. Tips
Only public classes or interfaces are accessible outside the package in which they are defined.
Packages may be uncompressed (i.e. directories) or compressed (i.e. zip files)
Package directories may be either placed in the current directory, or on the CLASSPATH.
The Java VM automatically imports java.lang and the current package you are using
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17. Compile and Run javac packagenames.classname
To compile you should to move out to the top source directory (above your packages) and type
javac packagenames.classname
d:\java> javac zoo\Elephant.java
to run the program, you use the dot notation instead
d:\java> java zoo.Elephant
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18. Summary
To create a package, put a type (class, interface, enum, or annotation) in it. To put a class or an interface into a package, put a package statement as the first statement in the source file for the class or the interface. The path name of the source and class file of a class or an interface mirrors the name of the package.
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19. Summary
To use a class or an interface that's in a different package, you have three choices:
use the fully qualified name of the class or the interface
import the class or the interface
import the entire package of which the class or the interface is a member.
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20. Summary
You might have to set your class path so that the compiler and the interpreter can find the source and class files for your classes and interfaces.
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21. Bibliography
Jia, Xiaoping, Object Oriented Software Development Using Java. Addison Wesley, 2003
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