CSC1401 Classes 1

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CSC1401Classes - 1
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Learning Goals

• Computing concepts
• Identifying objects and classes
• Declaring a class
• Declaring fields
• Default field values

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Objects and Classes

• We have been using them all semester
• Picture beachscene new Picture(beach.jpg)
• A big advantage of the picture class was that any
  method we wrote could be used in any program that
  had a picture object!
  
• We will explore how to create a class from
  scratch, rather than modifying an existing class

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Identifying Objects and Classes

• Object-oriented programs
• Consist of interacting objects
• Which are defined by and created by classes
• To identify the objects in a task
• What are the things that are doing the work or
  being acted upon?
  
• How do you classify them?
• What data (fields) do they need to know to do the
  task?
  
• What procedures (methods) do they need?

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Identifying the Objects and Classes

• Say that we want to write a program to do a slide
  show
  
• A series of pictures shown one after the other
  with some time waiting between the pictures
  
• One way to start is to underline the nouns
• Slide show, picture, wait time
• A slide show has pictures and a time to wait
  between pictures

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How to create a slide show?

• We can use an array of pictures
• Then, we can loop through the pictures,
  displaying each picture

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An array of Pictures

• Recall how we had arrays of Pixels (when first
  examining pictures
  
• We can do the same thing here
• Picture pictureList

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An array of pictures allocating memory

• To have the picture list contain 5 pictures
• pictureList new Picture 5
• To set the first element in the picture list to
  be the beach scene
  
• pictureList0 new Picture (beach.jpg)

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The resulting code

• public static void main(String args)
• Picture pictureList new Picture5
  pictureList0 new Picture(23beach.jpg")
  pictureList1 new Picture(23blueShrub.jpg")
  pictureList2 new Picture(23church.jpg")
  pictureList3 new Picture(23redDoor.jpg")
  pictureList4 new Picture(23butterfly.jpg"
  ) int index for (index 0 index index ) pictureListindex.show()
  

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A problem

• All 5 pictures show up at once
• In a slide show, the pictures are supposed to
  show up one at a time, and there is a brief pause
  to wait while the picture is being displayed

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Causing a wait for a Slide Show

• Use Thread.sleep(waitTime) to wait for waitTime
  number of milliseconds
  
• For wait time we can use integer to hold the
  number of milliseconds to wait
  
• This can cause an exception so write the method
  to throw Exception by adding throw Exception

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The modified code

• public static void main(String args) throws
  Exception
  
• Picture pictureList new Picture5
  pictureList0 new Picture(23beach.jpg")
  pictureList1 new Picture(23blueShrub.jpg")
  pictureList2 new Picture(23church.jpg")
  pictureList3 new Picture(23redDoor.jpg")
  pictureList4 new Picture(23butterfly.jpg"
  ) int index for (index 0 index index ) pictureListindex.show()
• Thread.sleep(2000) pictureListindex.hide()
  
  

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Creating a class

• This slide show is neat, but I can only use it in
  the current program
  
• Id like to be able to reuse it, like we did with
  the methods in the Picture class
  
• So, lets create a slide show class!

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Class Definition

• Each class is defined in a file
• With the same name as the class SlideShow.java
• Class names
• Are singular (SlideShow not SlideShows)
• Start with an uppercase letter
• The rest of the word is lowercase
• Uppercase the first letter of each additional
  word
  
• The syntax for a class definition is
• visibility class Name
• Inside the class definition goes
• Fields, constructors, and methods

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Class Declaration

• To declare a SlideShow class
• Click on the File-New-Java button in jGRASP
• Type in
• public class SlideShow
• Save it in SlideShow.java
• Click on File then Save
• Compile the file

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Class contents

• Classes contain variables (state) and methods
• We have been writing methods for classes (e.g.
  the Picture class)
  
• What variables might our class need?
• Two different kinds of variables within a class
• Local to a method
• Class-wide

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Declaring Fields

• Syntax
• visiblity type name
• visibility type name expression
• Usually use private for the visibility
• So that other classes cant access it directly
• The type is any of the primitive types, a class
  name , or an interface name
  
• Arrays are declared with after the type or
  after the name
  
• type name or type name
• Names start with a lowercase letter
• The first letter of each additional word is
  uppercased

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Default Field Values

• If you dont specify an initial value for a field
  
  
• It will get one anyway when it is created
• Numbers 0
• Objects null (not referring to any object yet)
• boolean false
• public class SlideShow
• //////////////// fields ////////////////////////
  ///////////////////
  
• private Picture pictureArray
• private int waitTime

Initial value will be null
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Testing the SlideShow Class

• Add the fields to the class definition and
  compile it
  
• Try the following
• SlideShow slideShowObj new SlideShow()
• Dont worry, the class isnt supposed to do
  anything yet!
  

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Next

• We need to decide what methods/functions this
  class will have, and to write them

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All Classes Inherit from Object

• If you dont specify the parent class when you
  declare a class
  
• The class with inherit from java.lang.Object
• You can specify the parent class
• Add extends Parent to the class declaration
• public class SlideShow extends Object
• A declaration of
• public class SlideShow
• Is the same as
• public class SlideShow extends Object

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Summary

• Object-oriented programs
• Have interacting objects
• To decide what classes to create
• Identify the objects doing the action or being
  acted upon
  
• And classify them (what type of thing are they?)
• To declare a class
• public class SlideShow
• To declare a field
• private type fieldName

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Assignment

• Read Media Computation Chapter 11, Sections 1-2