Computer Science 209

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Computer Science 209

• The Criteria of a Good Interface

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What Is an Interface?

• The set of public methods of a class
• Well, not exactly, because a class can implement
  several interfaces
• Better A set of methods that supports a single
  concept or purpose

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The Importance of Interfaces

• Interfaces are the glue that holds a software
  system together. They allow software components
  to
• be developed independently
• be reused in other applications
• have several different implementations

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The Importance of Interfaces

• Interfaces separate the concerns of users and
  implementers
• If users and implementers obey this contract, the
  rest is easy
• We should strive to program to interfaces, not
  implementations

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Criteria for a Good Interface

• Cohesion
• Completeness
• Convenience
• Carity
• Consistency

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Cohesion

• A class is an abstraction of a single concept and
  should fulfill a coherent purpose
• Methods in the interface should all deal with
  that purpose
• Irrelevant methods split into another class

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Completeness

• Provide all of the methods required to serve the
  classs purpose
• Dont leave out essential methods

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Convenience

• Make common tasks easy to perform
• Dont force users to jump through hoops to
  perform simple tasks
• Examples methods to examine the number of books
  checked out by a patron and the current status of
  a book

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Clarity

• Methods should have behavior and results that are
  simple to describe and understand
• Complex behavior should be split into two or more
  methods

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Consistency

• Use conventional method names, parameters, return
  types, and behavior wherever possible
• Examples equals, toString, and compareTo

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Operations for Patron
Operations Create a Patron Examine the
name Examine the password Examine the
number of books checked out Increment the
number of books checked out Decrement the
number of books checked out Compare for
equality Return a string representation
Coherent? Complete? Convenient? Clear?

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Methods for Patron
Methods public Patron(String, String)
public String getName() public String
getPassword() public int getBooksOut()
public void incBooksOut() public void
decBooksOut() public boolean equals(Object
other) public String toString()
Consistent?
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Operations for Book
Operations Create a Book Examine the title
Examine the author Examine the current
borrower Examine the current status (checked
out or not) Borrow for a given patron
Return for a given patron Compare for
equality Return a string representation
Coherent? Complete? Convenient? Clear?

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Methods for Book
Methods public Book(String, String) public
String getTitle() public String getAuthor()
public boolean checkedOut() public Patron
getBorrower() public String borrowBook(Patron
p) public String returnBook(Patron p)
public String toString()
Consistent?
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The Library
Library

Book
0..3
0..1

Patron
PatronQueue


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Operations for Library
Operations Create a Library Add a patron
Remove a patron Add a book Remove a book
Borrow a book for a given patron Return a book
for a given patron Return a string
representation
Coherent? Complete? Convenient? Clear?

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Methods for Library
Methods public Library() public String
add(Patron p) public boolean remove(Patron p)
public String add(Book b) public boolean
remove(Book b) public String borrowBook(String
title, String patronName,
String password) public String
returnBook(String title, String patronName,
String password) public
String toString()
Consistent?