CS242 Advanced Programming Concepts in Java

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CS242Advanced Programming Concepts in Java

• 8/30/07
• Java Basics

Prof. Searleman jets_at_clarkson.edu
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Announcements

• Clarkson Student Chapter of the ACM
• interest meeting next week
• Clarkson ACM SIGGRAPH IDEA
• first meeting Monday, 9/3/07

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Outline

• Java Basics
• public methods constructors, accessors,
  mutators, etc.
• instance variable vs. argument vs. local variable
• Class variables/constants static
• Javadoc
• Numbers literals
• Design Issues
• HW1 posted due Thursday, 9/6/07, in class
• Read OODP textbook, Chapter 1

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Recap

• In Java
• Everything (except a primitive) is an object
• An object has an interface

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A simple Circle class

• representation
• center Point
• radius float (primitive type)
• public interface
• constructor(s) Circle(x,y) unit circle
• Circle(radius,x,y)
• float getRadius() Point getCenter()
• void setRadius(float) void setCenter(Point)
• float computeArea()

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Unified Modeling LanguageUML description is a
contract
Type name
Public interface
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• Circle c1 new Circle(1.58f, 10,20)
• float z c1.getRadius()
• Circle c2 new Circle(z, 57, 42)
• float y c2.computeArea()
• Circle c3
• c2 c1
• What is c1? c1 is a reference to an
• instance of the class Circle so is c2
• c3 is a null reference

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Implementing Circle

• private instance variables
• private float radius
• private Point center
• public methods
• constructor(s)
• accessor methods (get)
• mutator methods (set)
• etc.

note must repeat keyword private for each
instance variable
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Class Circle

• Implementing constructors a common problem
  occurs when a parameter is the same name as an
  instance variable e.g.
• public Circle(float radius, int x, int y)
• radius ???
• solution use keyword this

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Constructors

• public Circle(float radius, int x, int y)
• this.radius radius
• center new Point(x,y)
• Its typical to have multiple ways to create an
  object the constructors are distinguished by
  their signatures

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Accessor methods

• Retrieves the current value of an instance
  variable
• public float getRadius()
• return radius

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Mutator methods

• changes the value of an instance variable
• public void setRadius(float newRadius)
• this.radius newRadius

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Other Methods

• Implementing computeArea()
• need to calculate
• how? Make good use of Javadoc

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PI

• javadoc used to find builtin value for PI
• public static final double PI
• class constant
• (1 per class, not per instance)
• Math.PI note Math is a classname
• Theres a problem in computeArea() with
• return radius radius Math.PI
• Why?

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computeArea method

• public float computeArea()
• return radiusradius((float)Math.PI)

Java is strongly typed the return type must be
compatible with the computed expression
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Instance Variables vs.local variables
vs.arguments

• public class Circle
• public float radius // instance variables
• public Point center
• public void draw(int x, int y) // x,y args
• int temp // local variable

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Good OOP Design

• The following rules of thumb are essential to a
  good design
• "Program to an interface, not to an
  implementation."
• Hide and abstract as much of your implementation
  as possible.
• "Favor object composition over inheritance."
• Minimize relationships among objects and organize
  related objects in packages.

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Die Class

• constructor Die()
• int roll()
• int topFace()
• Consider implementing roll(). Need a
• random number generator, so

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Class Random

• java.util.Random is a class
• constructors Random(), Random(long),
• methods int next(int), int nextInt(), int
  nextInt(int),
• so, we can write
• Random rng new Random()
• System.out.println( rng.nextInt() )

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• import java.util.
• public main()
• Random rng new Random()
• / print random int between 1 and 6 /
• System.out.println( rng.nextInt(6) 1 )
• constants in Java
• final int MAX 6
• System.out.println( rng.nextInt(MAX) 1 )

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Design question Should there be 1 RNG used by
every Die instance, or 1 per die?

• public class Die
• // instance vars numSides topFace
• private Random rng new Random()

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• public static void main(String args)
• System.out.println("Roll the first die")
• Die die1 new Die()
• for(int i 1 i lt 10 i)
• System.out.println(die1.rollDie())
• System.out.println("Roll the second die")
• Die die2 new Die()
• for(int i 1 i lt 10 i)
• System.out.println(die2.rollDie())

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Alternative class variable

• public class Die
• // instance vars for numSides topFace
• static Random rng new Random()
• A static (or class) variable is created and
  initialized when a class is first loaded (at
  runtime). There is just one, and it is referred
  to by its class name
• examples of class variables
• Math.PI approximates PI
• System.out standard output stream

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Some design issues for Die class

• Should there be 1 RNG used by every Die instance,
  or 1 RNG for the Die class shared by all
  instances?
• private Random rng new Random()
• or
• static Random RNG new Random()

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Some design issues for Die class

• What is the initial state of a die?
• same number on top face every time
• or random number
• How to test the class?
• separate test program
• main() as a test program
• JUnit?

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Testing

• be thorough
• test boundary conditions
• test all methods
• test both typical and unlikely combinations

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Coding conventions

• Make it easier for class designers to modify the
  source
• Part of the grade on your programs will be for
  good programming style, coding conventions,
  testing, comments, etc.

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Comments

• Write javadoc comments to an applications
  programmer (client)
• Write // and // comments to people who will be
  reading your source code (explain why, not how)

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Need / for javadoc, not /

• /
• Represents a circle by a center point a
  radius
• _at_author Janice T. Searleman
• /
• public class Circle
• private float radius
• private Point center
• /
• Constructs a circle with radius "aRadius"
• and whose center is given by the point (x,y)
• _at_param aRadius length of the radius
• _at_param x x-coordinate of the center
• _at_param y y-coordinate of the center
• /
• public Circle(float aRadius, int x, int y)

Must immediately precede the class heading
Must immediately precede the method heading
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Grading Criteria Style

• Incorrect file organization
• Identifiers have misleading or meaningless names
• Missing or inadequate source file comment
• Missing class/interface comment
• Implementation comments are inadequate or
  redundant
• Naming conventions not followed (file, class,
  method, etc.)
• Poor indentation/formatting/use of white space
• Code is overly complex or redundant
• Magic numbers used instead of constants
• Unnecessary import statements

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Grading Criteria OO Design

• Did not implement given interface
• Unnecessary classes
• Incorrect use of inheritance/composition
• Improper access modifiers
• Insufficient code reuse
• Missing or incorrect methods (including
  constructors)
• Nonportable code
• Poor use of static method or variable

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Grading Criteria Correctness Testing

• Die Class
• Does not model 6-sided die adequately, Random
  number generator missing or inadequate, etc.
• TestDie Class
• Inadequate testing for Die class, Test runs
  missing
• General
• Poor or missing javadoc comments
• Source files missing in /afs/cu/class/cs242

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Things to remember

• You manipulate objects with references
• When all references to an object are gone, it is
  a candidate for garbage collection
• You must create an object before it can be used
• You never need to destroy an object