Packages allow organization of related classes and interfaces into namespaces to avoid naming conflicts. A package is declared with the package keyword and specifies a group of related classes. Packages are arranged in a hierarchical manner with subpackages defined using dot notation. Key benefits of packages include easy identification of related classes, avoidance of naming clashes, and access protection control.
3. Packages
• What is a package?
• We already know how to organize our
files
• We use directories for doing so
• The advantage is
• Easy maintenance of files
• If we want to organize our java class files
• System provides a facility
• Packages
3
4. Packages Contd..
• Package
• Name provided to a group of classes
• A collection of related classes and
interfaces
• A namespace management tool provided
by Java
4
5. Packages Contd..
• Directory structures that organize classes and
interfaces
• Mechanism for software reuse
• Can be compared to directories or folders
• Files : Directories :: Class files : Packages
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5
6. Naming Convention
• Follows standard Java naming rules
• Case sensitive
• Generally written in small case
• Every name must be unique
• Packages in the Java language itself begin with
java
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7. Naming Convention Contd..
• Prefix a domain name to the preferred package
name
• Avoids name conflict when used in Internet
• for example, com.company.region.package
• com.ibm.in.banking
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8. Packages
Java packages are classified in to two types :
• Java API packages ( system defined packages)
• User defined packages
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9. Packages Contd..
•The packages are organized in a hierarchical structure as
shown in the figure
java. awt
Package containing awt package
COLOUR
GRAPHICS
Package containing AWT classes
FONT
…
IMAGE AWT classes containg methods
Hierarchical representation of java.awt
package. The package named java contains the
package awt , which in turn contains various
Fig. 26.1 classes required for implementing graphical
user interface
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10. System defined packages
• Some examples
• java.io
• java.util
• java.lang
• java.awt
• java.awt.event
• java.sql
• java.net
• java.math
• java.io
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11. Package Statement
• Used to create a package
• Should be the first statement in a Java source
file
• Syntax
– package nameOfPackage;
Keyword
User-defined name
• eg. : package cis; // cis – college information system
• nameOfPackage is a sequence of names separated by .
(dot)
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12. Package Statement Contd..
• Java supports the package hierarchy
• Specified by multiple names in a package
statement, separated by . (dot)
– eg. : package firstPackage.secondPackage;
• Advantage is
– Group related classes into a package and
then group the related packages into a larger
package
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13. Packages
Example :
package firstPpackage ; // package declanation
public class firstClass // class definition
{
----------- ( body of class )
-----------
}
• Here the package name is ‘firstPackage’
• The class ‘firstClass’ is now considered a part of this
package
• This listing would be saved as a file called firstClass.java
• When the source file is compiled, java compiler will
create a class file and store it in the ‘firstPackage’
directory 13
14. Example Program
// A simple package
package coreBank;
class Balance { package statement
String name;
double bal;
Balance(String n, double b) {
name = n;
bal = b;
}
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15. Example Program Contd..
void show() {
if(bal<0)
System.out.print("-->> ");
System.out.println(name + ": $" + bal);
}
}
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16. Example Program Contd..
package coreBank;
public class AccountBalance {
public static void main(String args[]) {
Balance current[] = new Balance[3];
current[0] = new Balance("K. J. Fielding", 123.23);
current[1] = new Balance("Will Tell", 157.02);
current[2] = new Balance("Tom Jackson", -12.33);
for(int i=0; i<3; i++) current[i].show();
}
}
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17. Compiling and Executing
• On compilation
• The package statement creates
directory(folder) with the name of the package
• That is, folder named ‘coreBank’ is created
• While executing
• Qualify the class name with its package name
• That is, coreBank. AccountBalance
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18. Compiling and Executing Contd..
• Compiling
• javac AccountBalance.java
• Executing
• java coreBank.AccountBalance
Class name to be qualified with its package
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19. Default Package
• System provides when you do not create one
• It is the current working directory
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20. Advantages of Packages
• Easily determine how classes and
interfaces are related
• Avoids namespace conflict
• Implement application-wise access
protection
• Provides a mechanism for class reuse
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21. Summary
• In this class we have discussed
• Package
• Naming convention
• Creating a package
• Concepts of classpath
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22. Frequently Asked Questions
1. What is a package ?
2. What are Java’s system defined packages ?
3. What are the advantages of packages ?
4. How to set classpath for your Java
application ?
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23. Quiz
1.An package is a collection of
1. classes
2. Interfaces
3. editing tools
4. classes and interfaces
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24. Quiz Contd..
2.Which keyword is used to define package
1. packages
2. package
3. packaging
4. directory
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25. Quiz Contd..
3.A class path specifies
1. A list of paths for java source files
2. A list of paths for image files
3. A list of paths for the required classes and
zip files
4. None
CM604.26 25
26. Quiz Contd..
4.Which of the following statements is true ?
1. There can be any number of package
statements in a java file
2. Package statement can be any where in the
java file
3. There can be only one package statement in
a java file
4. None
CM604.26 26
