MINS 116

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MINS 116

• Spring 2000
• Interfaces

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Interface Presentation Objectives

• Interface Concepts
• Interface Hierarchy
• Design by Contract
• UML Constructs
• Java Interface Constructs

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Concepts

• Q What is an interface (in general)?
• (Hint Think about the interfaces you deal with
  in your daily life
• Give some examples )

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Concepts

• Everything we encounter while existing has an
  interface.
• People we encounter have interfaces.
• What are some of these?
• Interfaces are a necessary part of daily
  existence.
• Remember the existentialists argument regarding
  the Brain in a Vat.

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Concepts

• The Real World is composed of very well defined
  interfaces.
• These interfaces are well defined because of
  their physical nature (e.g. your interface with a
  tree or sidewalk can be easily explained).
• What are some poorly defined interfaces we deal
  with daily?

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Concepts

• An interface is a point of contact with something
  allowing us to gain better understanding of
• What the thing is.
• What it does.
• How it behaves.
• Given a certain stimulus, what kind of response
  do we get back?

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Concepts

• Q What is an interface in terms of computer
  based technology?

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Concepts

• There are direct and indirect interfaces
  associated with computer based technology as
  related to a computer user.
• Direct interface are easy for us to see.
• Indirect indirect interfaces live somewhere
  behind the scenes.

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Interface Hierarchy

• Interface Pyramid

GUI - High User Visibility Low Code
Volume UI - Medium User Visibility
Medium Code Volume I - Low or no User
Visibility High Code Volume
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Interface Hierarchy

• What we, as direct computer users (e.g. users of
  browsers, computer screens, etc.) see most of is
  the GUI.
• What we, as indirect computer users (e.g. placing
  a phone call, using a vending machine, etc.)
  interface with, possibly without seeing, is the
  UI.
• What goes on behind the scenes, without user
  knowledge, is the I.

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Interface Hierarchy

• The largest volume of code written involves the
  I.
• The code given least attention and testing
  involves the I.
• The hardest code to debug and correct involves
  the I.
• The code with the longest reaching effects is
  involves the I.

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Interface Hierarchy

• Software Crisis
• The code used most often, has the largest volume,
  is hardest to pin down, tested least, the most
  difficult to debug, hardest to correct, and has
  farthest reaching effects.

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Interface Hierarchy

• Crisis? What Crisis?

We can alleviate some of the problems inherent in
software production by employing Design by
Contract, introduced by Bertrand Meyer.
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Design by Contract

• An interface can be well defined by specifying
• 1) Preconditions - What is the necessary state of
  the system prior to entering the interface.
• 2) Postconditions - What is the necessary state
  of the system upon interface exit.
• 3) Invariant - What is the necessary stable state
  of the system throughout the lifetime of the
  system execution.

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Design by Contract

• By defining preconditions, postconditions, and
  invariants related to an interface, we are
  defining, to any potential user of the interface
• 1) What we expect of them before the use the
  interface.
• 2) What they should expect as a result of using
  the interface.
• 3) What the steady state of the system is with
  respect the the interface.

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UML Constructs

• In UML, an interface is just a class which has
  been stereotyped as an interface.
• An interface should only have behavior (methods)
  defined. An interface can not have state
  (attributes) defined.

Stereotype
Class Name
No Attributes
Methods
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UML Constructs

• After the interface is properly defined, classes
  can be defined which will implement the
  interface.

Customer defines the interface. eCustomer and
Store Customer are both classes in which the
implementation code must provide the logic to
carry out the interface operation.
Interface Class
Implements symbol
Implementing Classes
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Java Interface Constructs

• Java constructs which directly support this
  important concepts are defined by the keywords
• interface - provide an interface definition.
• implements - provide an implementation of an
  interface definition.

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Java Interface Constructs

• For the Customer example .java code
• //
• // public class ICustomer is an example
• // .java interface.
• public interface ICustomer
• // orderItem - place an item into a customers
• // order area.
• //
• // return value The inventory item ID of the
  ordered
• // item of order was successful.
  -1 if the
• // item has been back ordered.
• Next slide..

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Java Interface Constructs

• //
• // precondition The customer must have
  defined an order area.
• //
• // postcondition If the item was successfully
  ordered, the
• // item number will be removed
  from the inventory
• // list.
• //
• // invariant The maximum number of
  ordered items per customer
• // is set to MAX_CUSTITEMS.
• //
• public abstract int orderItem( int itemID )
• ....
• Also define for receiveInvoice() and
  payInvoice()
• ....

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Java Interface Constructs

• By having an interface definition with
  preconditions, postconditions, and invariants
  defined, we have taken all the guessing work out
  of the actual implementation.
• now, lets take a quick look at an
  implementation

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Java Interface Constructs

• //
• // public class eCustomer is an example
• // of an implementation of the .java interface.
• // ICustomer.
• public class eCustomer implements ICustomer
• // orderItem - place an item into a customers
• // order area.
• //
• // return value The inventory item ID of the
  ordered
• // item of order was successful.
  -1 if the
• // item has been back ordered.
• //
• next slide...

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Java Interface Constructs

• // precondition The customer must have
  defined an order area.
• //
• // postcondition If the item was successfully
  ordered, the
• // item number will be removed
  from the inventory
• // list.
• //
• // invariant The maximum number of
  ordered items per customer
• // is set to MAX_CUSTITEMS.
• //
• public int orderItem( int itemID )
• // Check for invariant violation. Return a
  failure
• // if a violation occurred.
• ...
• // Check the precondition. Throw an
  excpetion
• // if not met.
• next slide...

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Java Interface Constructs

• // Perform the required logic.
• ...
• // Check the postcondition. Throw an
  exception
• // if not met.
• ....
• etc.
• ....

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Concluding Remarks

• Interfaces apply to all areas of software
  development, not just the GUI.
• If we can define the general interface well (the
  I in this presentation), we can define all
  aspects of software well.
• Design by Contract provides some good
  guidelines for interface (I) design and
  definition.

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Concluding Remarks

• Java directly supports the concept of interface
  definition through the use of interface and
  implements.
• Preconditions, postconditions, and invariants
  should be checked in the implementation code.

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and now...

• Enough about general software interfaces.
• On to GUI design..
• Last Words - The best interfaces are the ones we
  dont even know we are using.