Java interfaces

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Java interfaces

• What vs How

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Example List interface

• public interface List
• public Iterator iterator()
• public Object get( int index )
• public add( Object o )

3
Examples

• List l new List () //Wont work
• List l new ArrayList ()
• List l new LinkedList ()

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In a nutshell

• Code
• A class with only methods, methods which have
  no body.
• Cannot be instantiated
• A class implements an interface
• Concept
• A contract If you implement the interface, you
  must implement the methods defined by it.
• Reward Other classes can treat you as if you are
  an instance of the interface.
• Interface What. Implementation How.

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Interfaces Overview
Implementation 1
Application
Interface
Implementation 2
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A Student Register system

• I want to be able to create, retrieve, update and
  delete (CRUD) Students.
• The contract
• A method which takes a Student object and returns
  its new id in the registery.
• A method which takes an id and returns a
  corresponding Student object from the registry.
• A method which takes an id and a Student object
  and updates the corresponding student data in the
  registry.
• A methods which takes an id, removes the
  corresponding student data and returns a Student
  object with the removed data.

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Example StudentRegister

• public interface StudentRegister
• int addStudent( Student s )
• Student getStudent( int id )
• void updateStudent( int id, Student s )
• Student removeStudent( int id )
• //JavaDocs!

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Implementation

• File system?
• Database?
• Memory?
• Web service?

9
Example FileStudentRegister

• public FileStudentRegister
• implements StudentRegister
• private File file
• public FileStudentRegister ( String file )
• //Setup file
• public int addStudent( Student s )
• //Store Student data to file
• public Student getStudent( int id )
• //Get Student data from file
• public void updateStudent( int id, Student s
  )
• //Update student data in file
• public Student removeStudent( int id )
• //Remove student data from file

10
Example DatabaseStudentRegister

• public DatabaseStudentRegister
• implements StudentRegister
• private JDBC db .
• public FileStudentRegister ( String host,
  String user, String passwd, String db )
• //Setup db
• public int addStudent( Student s )
• //Store Student data in database
• public Student getStudent( int id )
• //Get Student data from database
• public void updateStudent( int id, Student s
  )
• //Update student data in database
• public Student removeStudent( int id )
• //Remove student data from database

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Example StudentApplication

• public class StudentApplication
• StudentRegister register new
• FileStudentRegister ( myFile )

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Example StudentApplication 2

• public class StudentApplication
• StudentRegister register new
• DatabaseStudentRegister (
• host, user, passwd, db )

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Where to choose the implementation?

• Hard code it into the application (highly
  coupled)
• Let a controlling class choose the implementation
  for the application. (Better, but still coupled)
• Use a factory class or a registry (Need to
  recompile).
• Use a configuration file (even better ?).