CS1101: Programming Methodology
http://www.comp.nus.edu.sg/~cs1101x/
Aaron Tan
How was your mid-term test?
 Which of the following best describes your feeling
about the test?
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This is Week 8
 Week 7:
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Revision on Modular programming
Chapter 6: Object-Oriented Programming
Chapter 7: OOP Additional Details
Survey
 This week:
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Chapter 7: OOP Additional Details (cont.)
About String
Exercises
Chapter 8: Software Engineering
Chapter 9: Classes with Class Members
Exercises
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Chapter 7 OOP Additional Details
 Let’s continue chapter 7 from where we
left off last week…
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Writing good programs
 Now that we know basic programming,
we should not be contended with merely
writing correct programs.
 We have to write good programs
 By following good programing practice and
methodology
 By writing elegant and efficient programs
(this requires more practice and is the
subject for follow-up modules, but no harm
starting early!)
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Overloaded Constructors
 Refer to Fraction.java in the textbook.
 We can write more constructors. Examples:
public Fraction( ) {
this(0, 1); // creates a fraction 0/1
}
New
public Fraction(Fraction frac) {
this(frac.numerator, frac.denominator); // copies frac
}
public Fraction(int n) {
this(n, 1); // creates a fraction n/1
}
Given
public Fraction(int n, int d) {
this.numerator = n;
this.denominator = d;
this.quotient = (double) this.numerator / this.denominator;
}
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String class
 After the tracing exercises last week,
some students tried question 4 on
strings, and found that the result doesn’t
match my explanation.
 Why? The reason is that String is a very
special class.
 Example: We do not need the “new”
statement to create a String object!
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String is immutable

In Java, a String object is immutable



This means once a String object is created, it
cannot be changed, such as replacing a character
with another character or removing a character
The String methods we have used so far do not
change the original string. They create a new string
from the original. For example, substring creates a
new string from a given string.
The String class is defined in this manner for
efficiency reason.
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Effect of Immutability
We can do this
because String
objects are
immutable.
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String: Examples

Example 1:
String s = "abc";
s = s + "def";
new string "abcdef" is created for s in the
second statement.
Example 2:
A

String s1 = "abc";
String s2 = "abc";
s1 = "xyz";
 s2

still refers to "abc". (Imagine the horror if this is
not so!)
Hence, strings behave differently from what I explained
last week, about reference variable of an object or an
array.
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Class Activity: Ball class
 Last week, we created a Ball class with these
members:
 Instance variables (data members)

 colour (String)
 radius (double)
Methods: constructors, accessors and mutators
 Overloaded constructors: Ball() and Ball(String, double)
 Accessors: getColour() and getRadius()
 Mutators: setColour(String) and setRadius(double)
 Let’s download the files Ball.java and BallDriver.java
and study them
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Ball: Writing equals() method
 Run BallDriver and enter the same data for
both objects myBall1 and myBall2
 What is the truth value of (myBall1 ==
myBall2)?
Why is it so?

 The equals() method isn’t working because
we haven’t written it! Write the equals()
method.
 Where should it be? In Ball.java or BallDriver.java?
 Write the equals() method now.
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Ball: Writing toString() method (1/2)
 The following is a possible output of the program:
1st ball: [blue, 2.5]
2nd ball: [orange, 3.1]
 Can you replace the output statements in
BallDriver.java with the following?
System.out.print("1st ball: ");
System.out.println(myBall1);
System.out.print("2nd ball: ");
System.out.println(myBall2);
 What’s the output? (Actual output may differ from
below.)
1st ball: Ball@471e30
2nd ball: Ball@10ef90c
Hashcodes
(page 514)
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Ball: Writing toString() method (2/2)
 To make it work, you need to write the toString()
method.
 The toString() method returns a string, which is the
string representation of the data in an object.
 Note that after the toString() method is written, you
may print myBall1 object in any of these two ways:
System.out.println( myBall1 );
or
System.out.println( myBall1.toString() );
 Write the toString() method now.
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Modularisation (1/2)
 Observe that there are duplicate codes in the input section of
BallDriver.java. The blue statements and red statements are
almost the same. If we need to create more Ball objects, the code
gets even longer.
System.out.print("Enter 1st ball's colour: ");
colour = stdIn.next();
System.out.print("Enter 1st ball's radius: ");
radius = stdIn.nextDouble();
Ball myBall1 = new Ball(colour, radius);
System.out.print("Enter 2nd ball's colour: ");
colour = stdIn.next();
System.out.print("Enter 2nd ball's radius: ");
radius = stdIn.nextDouble();
Ball myBall2 = new Ball(colour, radius);
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Modularisation (2/2)
 Can you ‘modularise’ your BallDriver.java program so

that you call a method each time you want to create a
Ball object?
You can then replace the code in the previous slide
with this:
Ball myBall1 = readBall(stdIn, "1st");
Ball myBall2 = readBall(stdIn, "2nd");
 You need to write the readBall() method.
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Chapters 8 and 9
 We’ll take a break from the Ball class and
return to it later.
 Let’s go to Chapters 8 and 9 now.
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Point class (1/3)
 I want to introduce the Point class to you because…
 it appears quite often in past year’s papers
 I want to add an instance variable for a Point object inside the Ball
class (so that we have an object within an object)
 Refer to API: http://java.sun.com/j2se/1.5.0/docs/api/java/awt/Point.html
Field Summary
int x
int y
The x coordinate.
The y coordinate.
Constructor Summary
Point()
Constructs and initializes a point at the origin (0, 0) of the coordinate space.
Point(int x, int y)
Constructs and initializes a point at the specified (x, y) location in the coordinate space.
Point(Point p)
Constructs and initializes a point with the same location as the specified Point object.
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Point class (2/3)
Method Summary
boolean
equals(Object obj)
Determines whether or not two points are equal.
Point
getLocation()
Returns the location of this point.
double
getX()
Returns the X coordinate of the point in double precision.
double
getY()
Returns the Y coordinate of the point in double precision.
void
move(int x, int y)
Moves this point to the specified location in the (x, y) coordinate plane.
void
setLocation(double x, double y)
Sets the location of this point to the specified double coordinates.
void
setLocation(int x, int y)
Changes the point to have the specified location.
void
setLocation(Point p)
Sets the location of the point to the specified location.
String
toString()
Returns a string representation of this point and its location in the (x, y) coordinate space.
This is not a complete list. Refer to the API page.
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Point class (3/3)
 Let’s do some programs on Point class so that you are
familiar with it.
 Download PointDriver.java and study it. Note how a
Point object is displayed.
 Now, let’s do something more complex. Download
MyRectangle.java, MyRectangleDriver.java and
MyRectangleDriverV2.java and study them.
 The computeArea() method in MyRectangle.java is a stub.
Complete it.
 Should we make area another data member of the
MyRectangle class? Think about it. If we do make it a data
member, what changes do we need to make in
MyRectangle.java? We’ll discuss this next week.
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Other Point related classes
 Check out other related classes on the
API
 Point2D.Double
 Point2D.Float
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Ball class: centre (1/2)
 Now that we understand the Point class, let’s
include a new data member for the Ball class
 centre: of type Point, to represent the centre of a

Ball object
Centre is supposed to be a 3D point, since balls are
3D objects, but some students are frightened of 3D,
so we shall just make centre a 2D point
 You need to update/add constructors,add
appropriate accessor and mutator methods,
and update the toString() method in Ball.java.
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Ball class: centre (2/2)
 In your BallDriver.java program, create two Ball
objects, and check whether they overlap each other.
(Two balls overlap when their centres are too close to
each other.)
 You need to add an appropriate method to do the
above.
 What should be the name of the method?
 Where should that method be, in Ball.java or BallDriver.java?
 What is its return type?
 What is/are its parameter(s)?
 This is your take-home exercise
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Next week
 Next week, I will present BallV2.java and
BallV2Driver.java that include all the things
we’ve discussed today
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Announcement/Reminder
 Lab #3
 Deadline: 8 October (Wednesday), 2359hr.
 Identical codes
 Please do not share codes for your lab
assignments!
 Mid-term test
 Results will be released on the IVLE
gradebook. Watch out for the announcement.
 Your answer sheets will be returned to you
next week in your discussion session.
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This is Week 8
 Next week?
 Chapter 10 Arrays and ArrayList(sections
10.7 – 10.14)
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End of file
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