Packages(9 cm604.26)

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Packages(9 cm604.26)

  1. 1. Packages and ClasspathObjective : On completion of this period, you would be able to learn • Packages • Concept of classpath 1
  2. 2. RecapFinal class• Classes that can not be inherited 2
  3. 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. 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. 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 5 5
  6. 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 6
  7. 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 7
  8. 8. PackagesJava packages are classified in to two types :• Java API packages ( system defined packages)• User defined packages 8
  9. 9. Packages Contd..•The packages are organized in a hierarchical structure asshown 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 9
  10. 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 10
  11. 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) 11
  12. 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 12
  13. 13. PackagesExample :package firstPpackage ; // package declanationpublic 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. 14. Example Program// A simple packagepackage coreBank;class Balance { package statement String name; double bal; Balance(String n, double b) { name = n; bal = b; } 14
  15. 15. Example Program Contd..void show() { if(bal<0) System.out.print("-->> "); System.out.println(name + ": $" + bal); }} 15
  16. 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(); }} 16
  17. 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 17
  18. 18. Compiling and Executing Contd..• Compiling • javac AccountBalance.java• Executing • java coreBank.AccountBalance Class name to be qualified with its package 18
  19. 19. Default Package• System provides when you do not create one• It is the current working directory 19
  20. 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 20
  21. 21. Summary• In this class we have discussed • Package • Naming convention • Creating a package • Concepts of classpath 21
  22. 22. Frequently Asked Questions1. 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 ? 22
  23. 23. Quiz1.An package is a collection of 1. classes 2. Interfaces 3. editing tools 4. classes and interfaces 23
  24. 24. Quiz Contd..2.Which keyword is used to define package 1. packages 2. package 3. packaging 4. directory 24
  25. 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. 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

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