CS1101: Programming Methodology http://www.comp.nus.edu.sg/~cs1101x/

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CS1101 Programming Methodology
http//www.comp.nus.edu.sg/cs1101x/
Aaron Tan
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This is Week 13

• This week
• Chapter 12 Aggregation, Composition, and
  Inheritance
• On your own
• Read up Chapter 13 Inheritance and Polymorphism
• Read up on Recursion
• Textbook Appendix 8 and lecture website
• The above are not in the scope of the final
  examination.

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Issues not discussed

• There are certain issues we have not discussed,
  such as
• The four access modifiers private, public,
  protected and default.
• Abstract classes and interfaces

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Overriding Methods (1/6)

• You are aware that every class has a default
  constructor, when you do not write one.
• Such a constructor is inherited from the Object
  class, which is the superclass of all classes.
• (I never understand why they give such a name
  Object to this class. This is so confusing!)
• What other things are inherited from the Object
  class? The toString() method and the equals()
  method.
• This means that you can use toString() and
  equals() in your class, even though you have not
  written them.
• However, most likely they dont work properly
  because they are not customised for that class.

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Overriding Methods (2/6)

• Refer to Ball.java and BallDriver.java
• In BallDriver.java, we make use of equals()
  method, and also toString() method (in the last
  two println() statements), even though we didnt
  define them in Ball.java. Heres a sample run

• But they dont work (to your expectation),
  because the inherited equals() method compares
  the addresses of the Ball objects, and the
  toString() method displays the hashcode of the
  address.

Enter 1st ball's colour red Enter 1st ball's
radius 2.5 Enter 1st ball's centre 3 8 Enter
2nd ball's colour red Enter 2nd ball's radius
2.5 Enter 2nd ball's centre 3 8 They are not
equal. 1st ball Ball_at_1d8957f 2nd ball
Ball_at_3ee284
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Overriding Methods (3/6)

• Refer to BallV2.java and BallV2Driver.java
• We write our own toString() method, which
  overrides the inherited toString() method. This
  is called overriding method.
• We also write our own equals() method. But this
  is not an overriding method. (To be explained.)

Enter 1st ball's colour red Enter 1st ball's
radius 2.5 Enter 1st ball's centre 3 8 Enter
2nd ball's colour red Enter 2nd ball's radius
2.5 Enter 2nd ball's centre 3 8 They are
equal. 1st ball red, 2.5, java.awt.Pointx3,y8
2nd ball red, 2.5, java.awt.Pointx3,y8

• Now they work.

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Overriding Methods (4/6)

• Why is the equals() method in BallV2 not an
  overriding method?
• Because it has this heading (signature)

public boolean equals(BallV2 ball)

• The inherited equals() method, which comes from
  the Object class, has this heading (signature)

public boolean equals(Object o)
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Overriding Methods (5/6)

• If we truly want to provide an overriding method
  for equals(), we must stick to the second
  heading.
• The code needs to be changed. See BallV3.java

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Overriding Methods (6/6)

• The instanceof operator checks whether o is
  indeed a reference to a BallV3 object.
• You may omit the if statement if you are sure
  you always pass a BallV3 reference to o.
• Why do we need the (BallV3) cast?

public boolean equals(Object o) if (o
instanceof BallV3) String colour1
this.getColour() double radius1
this.getRadius() Point centre1
this.getCentre() BallV3 ball (BallV3)
o String colour2 ball.getColour()
double radius2 ball.getRadius() Point
centre2 ball.getCentre() return
(colour1.equals(colour2)) (radius1
radius2) (centre1.equals(centre2
)) else return false
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Examinations (1/3)

• CS1101 Exam
• 27 November 2008, Thursday
• 5 7 pm
• Venue to be announced by Registrars Office
• Shall we wear yellow again? ?
• Format
• No MCQ
• Some short-answer questions and some programming
  questions

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Examinations (2/3)

• Scope
• Chapters 1 15 (except 12 and 13)
• Everything you learned in labs and discussion
  sessions.
• For Chapter 15 Files, only on text files.
• Topics that appear quite often in past-years
  papers but are not tested (because we didnt
  cover them this time)
• Regular expressions
• Inheritance and polymorphism
• Recursion

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Examinations (3/3)

• Examination Directory
• http//www.nus.edu.sg/registrar/event/examdir.html
  
• Preparing for exams
• http//www.cdtl.nus.edu.sg/examprep/
• Tips on managing study and exam stress
• http//www.nus.edu.sg/uhwc/counselling/selfhelp/in
  dex.html

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Announcement/Reminder

• Lab and PE marks
• Please check (later this week) http//www.comp.nus
  .edu.sg/cs1101x/3_ca/ca_marks.html
• Consultation
• 13 November, 2008, Thursday, 2 4 pm
• My office (COM1-03-12)

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Whats next? (1/2)

• We have come to the end of CS1101! ? or ??
• Have you learned everything about programming?
• No. There are issues such as inheritance/polymorph
  ism, efficiency, etc.
• No. Even for issues we have discussed, you will
  need more practice. For example, design aspect.
• A correct program may not be a good program!

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Whats next? (2/2)

• CS1102
• Data Structures and Algorithms
• Textbook Data Abstraction and Problem Solving
  with JAVA Walls and Mirrorsby Frank M. Carrano
  and Janet J. Pitchard
• Companion website ftp//ftp.aw.com/cseng/authors/
  carrano/java/
• Prepare yourself well!

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• ALL THE BEST!

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