Review

1
Review
2
Class Design

• Why is PizzaNode a subclass of FoodItemNode?
• It would be easier to make it a subclass of Node
• (see the graphed2 framework of Horstmann)

3
Move Common Behavior Into a Superclass

• class PizzaNode extends Node
• private FoodItem item
• public getFoodItem( ) return item
• ...
• class DrinkNode extends Node
• private FoodItem item
• public getFoodItem( ) return item
• ...

4
Move Common Behavior (2)

• class FoodNode extends Node
• protected FoodItem item
• public getFoodItem() return item
• class PizzaNode extends FoodNode
• // item moved to FoodNode
• ...
• class DrinkNode extends FoodNode
• // item moved to FoodNode
• ...

5
Patterns
6
What Pattern is This?

• Problem I want scrollbars on my textarea

JTextArea textarea new JTextArea( 8 /rows/,
40 / columns / ) JScrollPane pane new
JScrollPane( textarea ) pane.setVerticalScrollbar
Policy(JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED
) // add to contents contentpane.add ( pane )
7
Give Example of Observer Pattern
8
Review of Java and OO Concepts
9
Student (sorry, this is boring)
T
ltltinterfacegtgt Comparable compareTo( other )

• Implement the Student class described at right.
• Use the appropriate visibility for all members
  (attributes and methods).
• Compare students by id
• Write a toString that can be used to produce
  output like this
• 48541111 Bill Gates

ltTStudentgt
Student id firstname lastname Student( first,
last, id ) Student( first, last ) compareTo(
student ) toString( )
10
Write a StudentReader class

• Purpose reads student data from an InputStream.
• Each line of data contains
• student_id firstname lastname
• 48541111 Bill Gates
• Provides an IteratorltStudentgt to return one
  student object for each line of data read.
• Questions
• what methods does an Iterator have?
• how do you show this in UML?
• why use an InputStream? Why not a File?

11
StudentReader
T
ltltinterfacegtgt Iterator hasNext( ) boolean
next( ) T remove( )

• Write the code

ltTStudentgt
Student - id - firstname - lastname Student(
first, last, id ) compareTo( s )
StudentReader StudentReader( InputStream )
"has a"
12
Draw a Sequence Diagram

• void test2( )
• String data new String( "3 5 8 13" )
• Scanner in new Scanner( data )
• int sum 0
• while ( in.hasNext( ) )
• int n in.nextInt( )
• sum n
• System.out.println("Sum is " sum )

13
Draw a Sequence Diagram

• void test3( )
• CouponCalc calc new MyCouponCalc( )
• CouponUI ui new CouponUI( calc )
• ui.run( )

14
Coupon Assistant

• Why not design Coupon Assistant like this?

CouponUI - calc MyCouponCalc
MyCouponCalc MyCouponCalc( )
public CouponUI( ) calc new MyCouponCalc(
) public static void main( ... ) ui new
CouponUI( ) ui.run( )
15
What does this diagram mean?

• Why did we design the Coupon Assistant like this?
• What is the benefit of this?

ltltinterfacegtgt CouponCalc methods
CouponUI - calc CouponCalc run( )
MyCouponCalc MyCouponCalc( )
Main main( )
calc new MyCouponCalc( ) ui new CouponUI(
calc ) ui.run( )
16
JOptionPane
What is showConfirmDialog ? Is it a static or
instance method? What is JOptionPane.NO_OPTION

• int choice
• choice JOptionPane.showConfirmDialog( null,
• "Are you awake?",
• "This is a Confirm Dialog",
• JOptionPane.YES_NO_CANCEL_OPTION )
• if ( choice JOptionPane.NO_OPTION )
• JOptionPane.showMessageDialog( null, "Liar!!")

17
Magic Numbers
Is there any problem with this code?

• int choice
• choice JOptionPane.showConfirmDialog( null,
• "Do you want to quit?",
• "Confirm Quit",
• JOptionPane.OK_CANCEL_OPTION )
• if ( choice 0 ) System.exit( 0 )

How would you improve it?
18
Comparator

• Arrays.sort can accept a 2nd parameter
• Arrays.sort( array, comparator )

array to sort
sort will call comparator.compare(a,b)
T
ltltinterfacegtgt Comparator compare( a T, b T
) equals( other )
Why is this useful? Is compare( ) an instance or
static method?
19
Sort Students by First Name

• Write a Comparator that sorts students by first
  name.
• We can't use "Student.compareTo" because it
  already sorts Students by ID.

/ Define a comparator for ordering students by
first name / public class StudentComparator
implements ComparatorltStudentgt int compare(
Student a, Student b ) // return if after
b, - if a before b, 0 if a and b have same
order return a.getFirstName(
).compareToIgnoreCase( b.getFirstName() )
20
Using the Comparator

• // Example sort an array of students
• Student students Registrar.getStudents( )
• ComparatorltStudentgt comp new StudentComparator()
  
• Arrays.sort( students, comp )

/ Define a comparator for comparing students by
first name / public class StudentComparator
implements ComparatorltStudentgt int compare(
Student a, Student b ) // return if after
b, - if a before b, 0 if a and b have same
order return a.getFirstName( ).compareToIgnoreCa
se( b.getFirstName() )
21
How to create using anonymous class

• Student students Registrar.getStudents( )
• ComparatorltStudentgt comp new ComparatorltStudentgt
  ( )
• // anonymous class definition for a
  ComparatorltStudentgt object
• int compare( Student a, Student b )
• // return if after b, - if a before b, 0 if
  a and b have same order
• return a.getFirstName( ).compareToIgnoreCase(
  b.getFirstName() )
• // end of anonymous class and end of
  assignment ( )
• Arrays.sort( students, comp )

22
Comparator Factory Method

• Create a static method in the Student class that
  creates the comparator object (previous slide)
  using an anonymous class.
• Named the method getStudentNameComparator( )

public class Student implements
ComparableltStudentgt ... public static
ComparatorltStudentgt getStudentNameComparator ( )
return new ComparatorltStudentgt( ) public
int compareTo( Student a, Student b )
return a.getFirstName().compareTo(
b.getFirstName() ) // end of the
statement // end of the method
23
Designing Classes

• Choose responsibilities and attributes.
• Design the public "interface" of the class.
• Guides for class design
• cohesion - should be high
• coupling to other classes - lower is better
• Guides for interface
• completeness
• convenience
• clarity
• consistency

24
BigDecimal Arithmetic

• Write an average( ) method that computes the
  average of an array of BigDecimal objects.

• public BigDecimal average( BigDecimal x )
• BigDecimal sum new BigDecimal( 0.0 )
• for( BigDecimal value x )
• sum sum.add(value)
• return sum.divide( new BigDecimal( x.length ) )

25
BigDecimal Arithmetic

• Write a max( ) method that returns the maximum
  value in an array of BigDecimal objects, using
  only the compareTo method -- don't use
  BigDecimal.max( ).

• public BigDecimal max( BigDecimal x )
• BigDecimal max x0
• //... find the max in x
• for( int k1 k lt x.length k )
• //if ( xk gt max ) max xk
• if ( xk.compareTo(max) gt 0 ) max xk
• return max

26
BigDecimal More Arithmetic

• Write a static sqrt( ) method for BigDecimal
  objects using Euler's method of finding square
  roots.
• Find sqrt(2) accurate to 100 decimal places.

Let y the value you want the square root
of tolerance allowed error in the
answer Algorithm x 1.0 // initial guess
repeat x0 x x ( x y/x ) / 2
until ( x - x0 lt tolerance ) x is
approximately the square root of y.
This is useful practice for how to use method
calls, but it throws a runtime exception because
some values have infinitely long decimal form.
You must use another form of BigDecimal methods
that has a remainder.
27
How to Write a Clone Method

• Clone the Student class (next slide)

28
Deep copy of student

• public class Student implements Cloneable,
  Comparable
• private String id
• private Date birthday // date is mutable
• private ListltCoursegt courses
• public Object clone()
• //1. make a new clone object and clone
  superclasses
• Student clone (Student) super.clone( )
• //2. copy all the atrributes to the clone
• clone.id new String( this.id )
• clone.birthday this.birthday.clone()
• clone.courses new ArrayListltCoursegt( )
• for( course this.courses )
  clone.courses.add( course.clone() )
• //3. return the clone
• return clone

29
Variable Scope and try - catch

• int a in.nextInt( )
• int b in.nextInt( )
• try
• int c a / b
• catch ( Exception e)
• System.err.println("bad division")
• System.out.println("a / b " c )