Exercises on Classes
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Ex: The Circle Class (An Introduction to Classes and Instances)
This first exercise shall lead you through all the basic concepts in OOP.
A class called circle
is designed as shown in the following class diagram. It contains:
- Two
private
instance variables:radius
(of the typedouble
) andcolor
(of the typeString
), with default value of1.0
and 'red
', respectively. - Two overloaded constructors - a default constructor with no argument, and a constructor which takes a double argument for radius.
- Two
public
methods:getRadius()
andgetArea()
, which return the radius and area of this instance, respectively.
The source codes for Circle.java
is as follows:
Compile 'Circle.java
'. Can you run the Circle
class? Why?
This Circle
class does not have a main()
method. Hence, it cannot be run directly. This Circle
class is a 'building block' and is meant to be used in another program.
Let us write a test program called TestCircle
(in another source file called TestCircle.java
) which uses the Circle
class, as follows:
Now, run the TestCircle
and study the results.
- Constructor: Modify the class
Circle
to include a third constructor for constructing aCircle
instance with two arguments - adouble
forradius
and aString
forcolor
. Modify the test programTestCircle
to construct an instance ofCircle
using this constructor. - Getter: Add a getter for variable
color
for retrieving thecolor
of this instance. Modify the test program to test this method. - public vs. private: In
TestCircle
, can you access the instance variableradius
directly (e.g.,System.out.println(c1.radius)
); or assign a new value toradius
(e.g.,c1.radius=5.0
)? Try it out and explain the error messages. - Setter: Is there a need to change the values of
radius
andcolor
of aCircle
instance after it is constructed? If so, add twopublic
methods called setters for changing theradius
andcolor
of aCircle
instance as follows: Modify theTestCircle
to test these methods, e.g., - Keyword 'this': Instead of using variable names such as
r
(forradius
) andc
(forcolor
) in the methods' arguments, it is better to use variable namesradius
(forradius
) andcolor
(forcolor
) and use the special keyword 'this
' to resolve the conflict between instance variables and methods' arguments. For example, Modify ALL the constructors and setters in theCircle
class to use the keyword 'this
'. - Method toString(): Every well-designed Java class should contain a
public
method calledtoString()
that returns a description of the instance (in the return type ofString
). ThetoString()
method can be called explicitly (viainstanceName.toString()
) just like any other method; or implicitly throughprintln()
. If an instance is passed to theprintln(anInstance)
method, thetoString()
method of that instance will be invoked implicitly. For example, include the followingtoString()
methods to theCircle
class: Try callingtoString()
method explicitly, just like any other method:toString()
is called implicitly when an instance is passed toprintln()
method, for example,
The final class diagram for the Circle
class is as follows:
Ex: Yet Another Circle Class
Ex: The Rectangle Class
Ex: The Employee Class
Ex: The InvoiceItem Class
Ex: The Account Class
Ex: The Date Class
Ex: The Time Class
Ex: The Ball Class
Exercises on Composition
Ex: The Author and Book Classes (An Introduction to OOP Composition)
This first exercise shall lead you through all the concepts involved in OOP Composition.
A class called Author
(as shown in the class diagram) is designed to model a book's author. It contains:
- Three
private
instance variables:name
(String
),email
(String
), andgender
(char
of either'm'
or'f'
); - One constructor to initialize the
name
,email
andgender
with the given values; (There is no default constructor for Author, as there are no defaults for name, email and gender.) public
getters/setters:getName()
,getEmail()
,setEmail()
, andgetGender()
;
(There are no setters forname
andgender
, as these attributes cannot be changed.)- A
toString()
method that returns 'Author[name=?,email=?,gender=?]
', e.g., 'Author[name=Tan Ah Teck,email=ahTeck@somewhere.com,gender=m]
'.
Write the Author
class. Also write a test driver called TestAuthor
to test all the public
methods, e.g.,
A class called Book
is designed (as shown in the class diagram) to model a book written by one author. It contains:
- Four
private
instance variables:name
(String
),author
(of the classAuthor
you have just created, assume that a book has one and only one author),price
(double
), andqty
(int
); - Two constructors:
- public methods
getName()
,getAuthor()
,getPrice()
,setPrice()
,getQty()
,setQty()
. - A
toString()
that returns 'Book[name=?,Author[name=?,email=?,gender=?],price=?,qty=?
'. You should reuseAuthor
'stoString()
.
Write the Book
class (which uses the Author
class written earlier). Also write a test driver called TestBook
to test all the public
methods in the class Book
. Take Note that you have to construct an instance of Author
before you can construct an instance of Book
. E.g.,
Take note that both Book
and Author
classes have a variable called name
. However, it can be differentiated via the referencing instance. For a Book
instance says aBook
, aBook.name
refers to the name
of the book; whereas for an Author
's instance say auAuthor
, anAuthor.name
refers to the name
of the author. There is no need (and not recommended) to call the variables bookName
and authorName
.
TRY:
- Printing the
name
andemail
of the author from aBook
instance. (Hint:aBook.getAuthor().getName()
,aBook.getAuthor().getEmail()
). - Introduce new methods called
getAuthorName()
,getAuthorEmail()
,getAuthorGender()
in theBook
class to return thename
,email
andgender
of the author of the book. For example,
Ex (Advanced): Book and Author Classes Again - An Array of Objects as an Instance Variable
In the earlier exercise, a book is written by one and only one author. In reality, a book can be written by one or more author. Modify the Book
class to support one or more authors by changing the instance variable authors
to an Author
array.
Notes:
- The constructors take an array of
Author
(i.e.,Author[]
), instead of anAuthor
instance. In this design, once aBook
instance is constructor, you cannot add or remove author. - The
toString()
method shall return 'Book[name=?,authors={Author[name=?,email=?,gender=?],......},price=?,qty=?]
'.
You are required to:
- Write the code for the
Book
class. You shall re-use theAuthor
class written earlier. - Write a test driver (called
TestBook
) to test theBook
class.
Hints:
Ex: The MyPoint Class
A class called MyPoint
, which models a 2D point with x
and y
coordinates, is designed as shown in the class diagram. It contains:
A Book Driver Jobs
- Two instance variables
x
(int
) andy
(int
). - A default (or 'no-argument' or 'no-arg') constructor that construct a point at the default location of
(0, 0)
. - A overloaded constructor that constructs a point with the given
x
andy
coordinates. - Getter and setter for the instance variables
x
andy
. - A method
setXY()
to set bothx
andy
. - A method
getXY()
which returns the x and y in a 2-elementint
array. - A
toString()
method that returns a string description of the instance in the format '(x, y)
'. - A method called
distance(int x, int y)
that returns the distance from this point to another point at the given(x, y)
coordinates, e.g., - An overloaded
distance(MyPoint another)
that returns the distance from this point to the givenMyPoint
instance (calledanother
), e.g., - Another overloaded
distance()
method that returns the distance fromthis
point to the origin(0,0)
, e.g.,
You are required to:
- Write the code for the class
MyPoint
. Also write a test program (calledTestMyPoint
) to test all the methods defined in the class.
Hints: - Write a program that allocates
10
points in an array ofMyPoint
, and initializes to(1, 1)
,(2, 2)
, ...(10, 10)
.
Hints: You need to allocate the array, as well as each of the 10MyPoint
instances. In other words, you need to issue 11new
, 1 for the array and 10 for theMyPoint
instances.
Notes: Point is such a common entity that JDK certainly provided for in all flavors.
Ex: The MyCircle and MyPoint Classes
A class called MyCircle
, which models a circle with a center
(x,y)
and a radius
, is designed as shown in the class diagram. The MyCircle
class uses an instance of MyPoint
class (created in the previous exercise) as its center
.
The class contains:
- Two
private
instance variables:center
(an instance ofMyPoint
) andradius
(int
). - A constructor that constructs a circle with the given center's (
x
,y
) andradius
. - An overloaded constructor that constructs a
MyCircle
given aMyPoint
instance ascenter
, andradius
. - A default constructor that construct a circle with center at
(0,0)
and radius of1
. - Various getters and setters.
- A
toString()
method that returns a string description of this instance in the format 'MyCircle[radius=r,center=(x,y)]
'. You shall reuse thetoString()
ofMyPoint
. getArea()
andgetCircumference()
methods that return the area and circumference ofthis
circle indouble
.- A
distance(MyCircle another)
method that returns the distance of the centers fromthis
instance and the givenMyCircle
instance. You should useMyPoint
'sdistance()
method to compute this distance.
Write the MyCircle
class. Also write a test driver (called TestMyCircle
) to test all the public methods defined in the class.
Hints:
Ex: The MyTriangle and MyPoint Classes
A class called MyTriangle
, which models a triangle with 3 vertices, is designed as shown. The MyTriangle
class uses three MyPoint
instances (created in the earlier exercise) as its three vertices.
It contains:
- Three
private
instance variablesv1
,v2
,v3
(instances ofMyPoint
), for the three vertices. - A constructor that constructs a
MyTriangle
with three set of coordinates,v1=(x1, y1)
,v2=(x2, y2)
,v3=(x3, y3)
. - An overloaded constructor that constructs a
MyTriangle
given three instances ofMyPoint
. - A
toString()
method that returns a string description of the instance in the format 'MyTriangle[v1=(x1,y1),v2=(x2,y2),v3=(x3,y3)]
'. - A
getPerimeter()
method that returns the length of the perimeter in double. You should use thedistance()
method ofMyPoint
to compute the perimeter. - A method
printType()
, which prints 'equilateral
' if all the three sides are equal, 'isosceles
' if any two of the three sides are equal, or 'scalene
' if the three sides are different.
Write the MyTriangle
class. Also write a test driver (called TestMyTriangle
) to test all the public
methods defined in the class.
Ex: The MyRectangle and MyPoint Classes
Design a MyRectangle
class which is composed of two MyPoint
instances as its top-left and bottom-right corners. Draw the class diagrams, write the codes, and write the test drivers.
Ex: The Customer and Invoice classes
The Customer
class models a customer is design as shown in the class diagram. Write the codes for the Customer
class and a test driver to test all the public
methods.
The Invoice
class, design as shown in the class diagram, composes a Customer
instance (written earlier) as its member. Write the codes for the Invoice
class and a test driver to test all the public
methods.
Ex: The Customer and Account classes
The Customer
class models a customer is design as shown in the class diagram. Write the codes for the Customer
class and a test driver to test all the public
methods.
The Account
class models a bank account, design as shown in the class diagram, composes a Customer
instance (written earlier) as its member. Write the codes for the Account
class and a test driver to test all the public
methods.
More Exercises on Classes
Ex: The MyComplex class
A class called MyComplex
, which models complex numbers x+yi
, is designed as shown in the class diagram. It contains:
- Two instance variable named
real
(double
) andimag
(double
) which stores the real and imaginary parts of the complex number, respectively. - A constructor that creates a
MyComplex
instance with the given real and imaginary values. - A default constructor that create a MyComplex at
0.0 + 0.0i
. - Getters and setters for instance variables
real
andimag
. - A method
setValue()
to set the value of the complex number. - A
toString()
that returns '(x + yi)
' wherex
andy
are the real and imaginary parts, respectively. - Methods
isReal()
andisImaginary()
that returnstrue
if this complex number is real or imaginary, respectively.
Hints: - A method
equals(double real, double imag)
that returnstrue
ifthis
complex number is equal to the given complex number (real, imag).
Hints: - An overloaded
equals(MyComplex another)
that returnstrue
if this complex number is equal to the givenMyComplex
instanceanother
.
Hints: - A method
magnitude()
that returns the magnitude of this complex number. - Methods
argument()
that returns the argument of this complex number in radians (double
). Note: TheMath
library has two arc-tangent methods,Math.atan(double)
andMath.atan2(double, double)
. We commonly use theMath.atan2(y, x)
instead ofMath.atan(y/x)
to avoid division by zero. Read the documentation ofMath
class in packagejava.lang
. - Methods
add(MyComplex right)
andsubtract(MyComplex right)
that adds and subtract the givenMyComplex
instance (calledright
), into/fromthis
instance and returnsthis
instance. Hints: - Methods
addNew(MyComplex right)
andsubtractNew(MyComplex right)
that adds and subtractthis
instance with the givenMyComplex
instance calledright
, and returns a newMyComplex
instance containing the result.
Hint: - Methods
multiply(MyComplex right)
anddivide(MyComplex right)
that multiplies and dividesthis
instance with the givenMyComplex
instanceright
, and keeps the result inthis
instance, and returns this instance. - A method
conjugate()
that operates onthis
instance and returnsthis
instance containing the complex conjugate.
You are required to:
- Write the
MyComplex
class. - Write a test driver to test all the
public
methods defined in the class. - Write an application called
MyComplexApp
that uses theMyComplex
class. The application shall prompt the user for two complex numbers, print their values, check for real, imaginary and equality, and carry out all the arithmetic operations.
Take note that there are a few flaws in the design of this class, which was introduced solely for teaching purpose:
- Comparing
double
s inequal()
using ' may produce unexpected outcome. For example,(2.2+4.4)6.6
returnsfalse
. It is common to define a small threshold calledEPSILON
(set to about10^-8
) for comparing floating point numbers. - The method
addNew()
,subtractNew()
produce new instances, whereasadd()
,subtract()
,multiply()
,divide()
andconjugate()
modifythis
instance. There is inconsistency in the design (introduced for teaching purpose).
Also take note that methods such as add()
returns an instance of MyComplex
. Hence, you can place the result inside a System.out.println()
(which implicitly invoke the toString()
). You can also chain the operations, e.g., c1.add(c2).add(c3)
(same as (c1.add(c2)).add(c3))
, or c1.add(c2).subtract(c3)
.
Ex: The MyPolynomial Class
A class called MyPolynomial
, which models polynomials of degree-n
(see equation), is designed as shown in the class diagram.
It contains:
- An instance variable named
coeffs
, which stores the coefficients of the n-degree polynomial in adouble
array of sizen+1
, where c0 is kept at index 0. - A constructor
MyPolynomial(coeffs:double...)
that takes a variable number of doubles to initialize the coeffs array, where the first argument corresponds to c0.
The three dots is known as varargs (variable number of arguments), which is a new feature introduced in JDK 1.5. It accepts an array or a sequence of comma-separated arguments. The compiler automatically packs the comma-separated arguments in an array. The three dots can only be used for the last argument of the method.
Hints: - A method
getDegree()
that returns the degree of this polynomial. - A method
toString()
that returns 'cnx^n+cn-1x^(n-1)+...+c1x+c0'. - A method
evaluate(double x)
that evaluate the polynomial for the givenx
, by substituting the givenx
into the polynomial expression. - Methods
add()
andmultiply()
that adds and multiplies this polynomial with the givenMyPolynomial
instanceanother
, and returnsthis
instance that contains the result.
Write the MyPolynomial
class. Also write a test driver (called TestMyPolynomial
) to test all the public
methods defined in the class.
Question: Do you need to keep the degree of the polynomial as an instance variable in the MyPolynomial
class in Java? How about C/C++? Why?
Ex: Using JDK's BigInteger Class
Recall that primitive integer type byte
, short
, int
and long
represent 8-, 16-, 32-, and 64-bit signed integers, respectively. You cannot use them for integers bigger than 64 bits. Java API provides a class called BigInteger
in a package called java.math
. Study the API of the BigInteger
class (Java API ⇒ From 'Packages', choose 'java.math' ' From 'classes', choose 'BigInteger' ' Study the constructors (choose 'CONSTR') on how to construct a BigInteger
instance, and the public methods available (choose 'METHOD'). Look for methods for adding and multiplying two BigIntegers
.
Write a program called TestBigInteger
that:
- adds '11111111111111111111111111111111111111111111111111111111111111' to '22222222222222222222222222222222222222222222222222' and prints the result.
- multiplies the above two number and prints the result.
Hints:
Ex: The MyTime Class
A class called MyTime
, which models a time instance, is designed as shown in the class diagram.
It contains the following private
instance variables:
hour
: between 0 to 23.minute
: between 0 to 59.Second
: between 0 to 59.
You are required to perform input validation.
It contains the following public
methods:
setTime(int hour, int minute, int second)
: It shall check if the givenhour
,minute
andsecond
are valid before setting the instance variables.
(Advanced: Otherwise, it shall throw anIllegalArgumentException
with the message 'Invalid hour, minute, or second!'.)- Setters
setHour(int hour)
,setMinute(int minute)
,setSecond(int second)
: It shall check if the parameters are valid, similar to the above. - Getters
getHour()
,getMinute()
,getSecond()
. toString()
: returns 'HH:MM:SS
'.nextSecond()
: Update this instance to the next second and return this instance. Take note that thenextSecond()
of23:59:59
is00:00:00
.nextMinute()
,nextHour()
,previousSecond()
,previousMinute()
,previousHour()
: similar to the above.
Write the code for the MyTime
class. Also write a test driver (called TestMyTime
) to test all the public
methods defined in the MyTime
class.
Ex: The MyDate Class
A class called MyDate
, which models a date instance, is defined as shown in the class diagram.
The MyDate
class contains the following private
instance variables:
year
(int
): Between1
to9999
.month
(int
): Between1
(Jan) to12
(Dec).day
(int
): Between1
to28|29|30|31
, where the last day depends on the month and whether it is a leap year for Feb (28|29
).
It also contains the following public
static final
variables (drawn with underlined in the class diagram):
MONTHS
(String[]
),DAYS
(String[]
), and DAY_IN_MONTHS
(int[]
):static
variables, initialized as shown, which are used in the methods.
The MyDate
class has the following public
static
methods (drawn with underlined in the class diagram):
isLeapYear(int year)
: returnstrue
if the givenyear
is a leap year. A year is a leap year if it is divisible by 4 but not by 100, or it is divisible by 400.isValidDate(int year, int month, int day)
: returnstrue
if the givenyear
,month
, andday
constitute a valid date. Assume thatyear
is between1
and9999
,month
is between1
(Jan) to12
(Dec) andday
shall be between1
and28|29|30|31
depending on themonth
and whether it is a leap year on Feb.getDayOfWeek(int year, int month, int day)
: returns the day of the week, where0
for Sun,1
for Mon, ...,6
for Sat, for the given date. Assume that the date is valid. Read the earlier exercise on how to determine the day of the week (or Wiki 'Determination of the day of the week').
The MyDate
class has one constructor, which takes 3 parameters: year
, month
and day
. It shall invoke setDate()
method (to be described later) to set the instance variables.
The MyDate
class has the following public
methods:
setDate(int year, int month, int day)
: It shall invoke thestatic
methodisValidDate()
to verify that the givenyear
,month
andday
constitute a valid date.
(Advanced: Otherwise, it shall throw anIllegalArgumentException
with the message 'Invalid year, month, or day!'.)setYear(int year)
: It shall verify that the givenyear
is between1
and9999
.
(Advanced: Otherwise, it shall throw anIllegalArgumentException
with the message 'Invalid year!'.)setMonth(int month)
: It shall verify that the givenmonth
is between1
and12
.
(Advanced: Otherwise, it shall throw anIllegalArgumentException
with the message 'Invalid month!'.)setDay(int day)
: It shall verify that the givenday
is between1
anddayMax
, wheredayMax
depends on themonth
and whether it is a leap year for Feb.
(Advanced: Otherwise, it shall throw anIllegalArgumentException
with the message 'Invalid month!'.)getYear()
,getMonth()
,getDay()
: return the value for theyear
,month
andday
, respectively.toString()
: returns a date string in the format 'xxxday d mmm yyyy
', e.g., 'Tuesday 14 Feb 2012'.nextDay()
: updatethis
instance to the next day and returnthis
instance. Take note thatnextDay()
for31 Dec 2000
shall be1 Jan 2001
.nextMonth()
: updatethis
instance to the next month and returnthis
instance. Take note thatnextMonth()
for31 Oct 2012
shall be30 Nov 2012
.nextYear()
: updatethis
instance to the next year and returnthis
instance. Take note thatnextYear()
for29 Feb 2012
shall be28 Feb 2013
.
(Advanced: throw anIllegalStateException
with the message 'Year out of range!' if year > 9999.)previousDay()
,previousMonth()
,previousYear()
: similar to the above.
Write the code for the MyDate
class.
Use the following test statements to test the MyDate
class:
Write a test program that tests the nextDay()
in a loop, by printing the dates from 28 Dec 2011
to 2 Mar 2012
.
Ex: Bouncing Balls - Ball and Container Classes
A class called Ball
is designed as shown in the class diagram.
The Ball
class contains the following private
instance variables:
x
,y
andradius
, which represent the ball's center(x, y)
co-ordinates and the radius, respectively.xDelta
(Δx
) andyDelta
(Δy
), which represent the displacement (movement) per step, in thex
andy
direction respectively.
The Ball
class contains the following public
methods:
- A constructor which accepts
x
,y
,radius
,speed
, anddirection
as arguments. For user friendliness, user specifiesspeed
(in pixels per step) anddirection
(in degrees in the range of(-180°, 180°]
). For the internal operations, thespeed
anddirection
are to be converted to(Δx, Δy)
in the internal representation. Note that the y-axis of the Java graphics coordinate system is inverted, i.e., the origin(0, 0)
is located at the top-left corner. - Getter and setter for all the instance variables.
- A method
move()
which move the ball by one step. reflectHorizontal()
which reflects the ball horizontally (i.e., hitting a vertical wall)reflectVertical()
(the ball hits a horizontal wall).toString()
which prints the message 'Ball at (x, y) of velocity (Δx, Δy)
'.
Write the Ball
class. Also write a test program to test all the methods defined in the class.
A class called Container
, which represents the enclosing box for the ball, is designed as shown in the class diagram. It contains:
- Instance variables
(x1, y1)
and(x2, y2)
which denote the top-left and bottom-right corners of the rectangular box. - A constructor which accepts
(x, y)
of the top-left corner,width
andheight
as argument, and converts them into the internal representation (i.e.,x2=x1+width-1
).Width
andheight
is used in the argument for safer operation (there is no need to check the validity ofx2>x1
etc.). - A
toString()
method that returns 'Container at (x1,y1) to (x2, y2)
'. - A
boolean
method calledcollidesWith(Ball)
, which check if the givenBall
is outside the bounds of the container box. If so, it invokes theBall
'sreflectHorizontal()
and/orreflectVertical()
to change the movement direction of the ball, and returnstrue
.
Use the following statements to test your program:
Ex: The Ball and Player Classes
The Ball
class, which models the ball in a soccer game, is designed as shown in the class diagram. Write the codes for the Ball
class and a test driver to test all the public
methods.
The Player
class, which models the players in a soccer game, is designed as shown in the class diagram. The Player interacts with the Ball (written earlier). Write the codes for the Player
class and a test driver to test all the public
methods. Make your assumption for the kick()
.
Can you write a very simple soccer game with 2 teams of players and a ball, inside a soccer field?
Exercises on Inheritance
Ex: The Circle and Cylinder Classes
This exercise shall guide you through the important concepts in inheritance.
In this exercise, a subclass called Cylinder
is derived from the superclass Circle
as shown in the class diagram (where an an arrow pointing up from the subclass to its superclass). Study how the subclass Cylinder
invokes the superclass' constructors (via super()
and super(radius)
) and inherits the variables and methods from the superclass Circle
.
You can reuse the Circle
class that you have created in the previous exercise. Make sure that you keep 'Circle.class
' in the same directory.
Write a test program (says TestCylinder
) to test the Cylinder
class created, as follow:
Method Overriding and 'Super': The subclass Cylinder
inherits getArea()
method from its superclass Circle. Try overriding the getArea()
method in the subclass Cylinder
to compute the surface area (=2π×radius×height + 2×base-area) of the cylinder instead of base area. That is, if getArea()
is called by a Circle
instance, it returns the area. If getArea()
is called by a Cylinder
instance, it returns the surface area of the cylinder.
If you override the getArea()
in the subclass Cylinder
, the getVolume()
no longer works. This is because the getVolume()
uses the overriddengetArea()
method found in the same class. (Java runtime will search the superclass only if it cannot locate the method in this class). Fix the getVolume()
.
Hints: After overridding the getArea()
in subclass Cylinder
, you can choose to invoke the getArea()
of the superclass Circle
by calling super.getArea()
.
TRY:
Provide a toString()
method to the Cylinder
class, which overrides the toString()
inherited from the superclass Circle
, e.g.,
Try out the toString()
method in TestCylinder
.
Note: @Override
is known as annotation (introduced in JDK 1.5), which asks compiler to check whether there is such a method in the superclass to be overridden. This helps greatly if you misspell the name of the toString()
. If @Override
is not used and toString()
is misspelled as ToString()
, it will be treated as a new method in the subclass, instead of overriding the superclass. If @Override
is used, the compiler will signal an error. @Override
annotation is optional, but certainly nice to have.
Ex: Superclass Person and its subclasses
Ex: Point2D and Point3D
Ex: Point and MovablePoint
Ex: Superclass Shape and its subclasses Circle, Rectangle and Square
Write a superclass called Shape
(as shown in the class diagram), which contains:
- Two instance variables
color
(String
) andfilled
(boolean
). - Two constructors: a no-arg (no-argument) constructor that initializes the
color
to 'green' andfilled
totrue
, and a constructor that initializes thecolor
andfilled
to the given values. - Getter and setter for all the instance variables. By convention, the getter for a
boolean
variablexxx
is calledisXXX()
(instead ofgetXxx()
for all the other types). - A
toString()
method that returns 'A Shape with color of xxx and filled/Not filled
'.
Write a test program to test all the methods defined in Shape
.
Write two subclasses of Shape
called Circle
and Rectangle
, as shown in the class diagram.
The Circle
class contains:
- An instance variable
radius
(double
). - Three constructors as shown. The no-arg constructor initializes the radius to
1.0
. - Getter and setter for the instance variable
radius
. - Methods
getArea()
andgetPerimeter()
. - Override the
toString()
method inherited, to return 'A Circle with radius=xxx, which is a subclass of yyy
', whereyyy
is the output of thetoString()
method from the superclass.
The Rectangle
class contains:
- Two instance variables
width
(double
) andlength
(double
). - Three constructors as shown. The no-arg constructor initializes the
width
andlength
to1.0
. - Getter and setter for all the instance variables.
- Methods
getArea()
andgetPerimeter()
. - Override the
toString()
method inherited, to return 'A Rectangle with width=xxx and length=zzz, which is a subclass of yyy
', whereyyy
is the output of thetoString()
method from the superclass.
Write a class called Square
, as a subclass of Rectangle
. Convince yourself that Square
can be modeled as a subclass of Rectangle
. Square
has no instance variable, but inherits the instance variables width and length from its superclass Rectangle.
hour
: between 0 to 23.minute
: between 0 to 59.Second
: between 0 to 59.
You are required to perform input validation.
It contains the following public
methods:
setTime(int hour, int minute, int second)
: It shall check if the givenhour
,minute
andsecond
are valid before setting the instance variables.
(Advanced: Otherwise, it shall throw anIllegalArgumentException
with the message 'Invalid hour, minute, or second!'.)- Setters
setHour(int hour)
,setMinute(int minute)
,setSecond(int second)
: It shall check if the parameters are valid, similar to the above. - Getters
getHour()
,getMinute()
,getSecond()
. toString()
: returns 'HH:MM:SS
'.nextSecond()
: Update this instance to the next second and return this instance. Take note that thenextSecond()
of23:59:59
is00:00:00
.nextMinute()
,nextHour()
,previousSecond()
,previousMinute()
,previousHour()
: similar to the above.
Write the code for the MyTime
class. Also write a test driver (called TestMyTime
) to test all the public
methods defined in the MyTime
class.
Ex: The MyDate Class
A class called MyDate
, which models a date instance, is defined as shown in the class diagram.
The MyDate
class contains the following private
instance variables:
year
(int
): Between1
to9999
.month
(int
): Between1
(Jan) to12
(Dec).day
(int
): Between1
to28|29|30|31
, where the last day depends on the month and whether it is a leap year for Feb (28|29
).
It also contains the following public
static final
variables (drawn with underlined in the class diagram):
MONTHS
(String[]
),DAYS
(String[]
), and DAY_IN_MONTHS
(int[]
):static
variables, initialized as shown, which are used in the methods.
The MyDate
class has the following public
static
methods (drawn with underlined in the class diagram):
isLeapYear(int year)
: returnstrue
if the givenyear
is a leap year. A year is a leap year if it is divisible by 4 but not by 100, or it is divisible by 400.isValidDate(int year, int month, int day)
: returnstrue
if the givenyear
,month
, andday
constitute a valid date. Assume thatyear
is between1
and9999
,month
is between1
(Jan) to12
(Dec) andday
shall be between1
and28|29|30|31
depending on themonth
and whether it is a leap year on Feb.getDayOfWeek(int year, int month, int day)
: returns the day of the week, where0
for Sun,1
for Mon, ...,6
for Sat, for the given date. Assume that the date is valid. Read the earlier exercise on how to determine the day of the week (or Wiki 'Determination of the day of the week').
The MyDate
class has one constructor, which takes 3 parameters: year
, month
and day
. It shall invoke setDate()
method (to be described later) to set the instance variables.
The MyDate
class has the following public
methods:
setDate(int year, int month, int day)
: It shall invoke thestatic
methodisValidDate()
to verify that the givenyear
,month
andday
constitute a valid date.
(Advanced: Otherwise, it shall throw anIllegalArgumentException
with the message 'Invalid year, month, or day!'.)setYear(int year)
: It shall verify that the givenyear
is between1
and9999
.
(Advanced: Otherwise, it shall throw anIllegalArgumentException
with the message 'Invalid year!'.)setMonth(int month)
: It shall verify that the givenmonth
is between1
and12
.
(Advanced: Otherwise, it shall throw anIllegalArgumentException
with the message 'Invalid month!'.)setDay(int day)
: It shall verify that the givenday
is between1
anddayMax
, wheredayMax
depends on themonth
and whether it is a leap year for Feb.
(Advanced: Otherwise, it shall throw anIllegalArgumentException
with the message 'Invalid month!'.)getYear()
,getMonth()
,getDay()
: return the value for theyear
,month
andday
, respectively.toString()
: returns a date string in the format 'xxxday d mmm yyyy
', e.g., 'Tuesday 14 Feb 2012'.nextDay()
: updatethis
instance to the next day and returnthis
instance. Take note thatnextDay()
for31 Dec 2000
shall be1 Jan 2001
.nextMonth()
: updatethis
instance to the next month and returnthis
instance. Take note thatnextMonth()
for31 Oct 2012
shall be30 Nov 2012
.nextYear()
: updatethis
instance to the next year and returnthis
instance. Take note thatnextYear()
for29 Feb 2012
shall be28 Feb 2013
.
(Advanced: throw anIllegalStateException
with the message 'Year out of range!' if year > 9999.)previousDay()
,previousMonth()
,previousYear()
: similar to the above.
Write the code for the MyDate
class.
Use the following test statements to test the MyDate
class:
Write a test program that tests the nextDay()
in a loop, by printing the dates from 28 Dec 2011
to 2 Mar 2012
.
Ex: Bouncing Balls - Ball and Container Classes
A class called Ball
is designed as shown in the class diagram.
The Ball
class contains the following private
instance variables:
x
,y
andradius
, which represent the ball's center(x, y)
co-ordinates and the radius, respectively.xDelta
(Δx
) andyDelta
(Δy
), which represent the displacement (movement) per step, in thex
andy
direction respectively.
The Ball
class contains the following public
methods:
- A constructor which accepts
x
,y
,radius
,speed
, anddirection
as arguments. For user friendliness, user specifiesspeed
(in pixels per step) anddirection
(in degrees in the range of(-180°, 180°]
). For the internal operations, thespeed
anddirection
are to be converted to(Δx, Δy)
in the internal representation. Note that the y-axis of the Java graphics coordinate system is inverted, i.e., the origin(0, 0)
is located at the top-left corner. - Getter and setter for all the instance variables.
- A method
move()
which move the ball by one step. reflectHorizontal()
which reflects the ball horizontally (i.e., hitting a vertical wall)reflectVertical()
(the ball hits a horizontal wall).toString()
which prints the message 'Ball at (x, y) of velocity (Δx, Δy)
'.
Write the Ball
class. Also write a test program to test all the methods defined in the class.
A class called Container
, which represents the enclosing box for the ball, is designed as shown in the class diagram. It contains:
- Instance variables
(x1, y1)
and(x2, y2)
which denote the top-left and bottom-right corners of the rectangular box. - A constructor which accepts
(x, y)
of the top-left corner,width
andheight
as argument, and converts them into the internal representation (i.e.,x2=x1+width-1
).Width
andheight
is used in the argument for safer operation (there is no need to check the validity ofx2>x1
etc.). - A
toString()
method that returns 'Container at (x1,y1) to (x2, y2)
'. - A
boolean
method calledcollidesWith(Ball)
, which check if the givenBall
is outside the bounds of the container box. If so, it invokes theBall
'sreflectHorizontal()
and/orreflectVertical()
to change the movement direction of the ball, and returnstrue
.
Use the following statements to test your program:
Ex: The Ball and Player Classes
The Ball
class, which models the ball in a soccer game, is designed as shown in the class diagram. Write the codes for the Ball
class and a test driver to test all the public
methods.
The Player
class, which models the players in a soccer game, is designed as shown in the class diagram. The Player interacts with the Ball (written earlier). Write the codes for the Player
class and a test driver to test all the public
methods. Make your assumption for the kick()
.
Can you write a very simple soccer game with 2 teams of players and a ball, inside a soccer field?
Exercises on Inheritance
Ex: The Circle and Cylinder Classes
This exercise shall guide you through the important concepts in inheritance.
In this exercise, a subclass called Cylinder
is derived from the superclass Circle
as shown in the class diagram (where an an arrow pointing up from the subclass to its superclass). Study how the subclass Cylinder
invokes the superclass' constructors (via super()
and super(radius)
) and inherits the variables and methods from the superclass Circle
.
You can reuse the Circle
class that you have created in the previous exercise. Make sure that you keep 'Circle.class
' in the same directory.
Write a test program (says TestCylinder
) to test the Cylinder
class created, as follow:
Method Overriding and 'Super': The subclass Cylinder
inherits getArea()
method from its superclass Circle. Try overriding the getArea()
method in the subclass Cylinder
to compute the surface area (=2π×radius×height + 2×base-area) of the cylinder instead of base area. That is, if getArea()
is called by a Circle
instance, it returns the area. If getArea()
is called by a Cylinder
instance, it returns the surface area of the cylinder.
If you override the getArea()
in the subclass Cylinder
, the getVolume()
no longer works. This is because the getVolume()
uses the overriddengetArea()
method found in the same class. (Java runtime will search the superclass only if it cannot locate the method in this class). Fix the getVolume()
.
Hints: After overridding the getArea()
in subclass Cylinder
, you can choose to invoke the getArea()
of the superclass Circle
by calling super.getArea()
.
TRY:
Provide a toString()
method to the Cylinder
class, which overrides the toString()
inherited from the superclass Circle
, e.g.,
Try out the toString()
method in TestCylinder
.
Note: @Override
is known as annotation (introduced in JDK 1.5), which asks compiler to check whether there is such a method in the superclass to be overridden. This helps greatly if you misspell the name of the toString()
. If @Override
is not used and toString()
is misspelled as ToString()
, it will be treated as a new method in the subclass, instead of overriding the superclass. If @Override
is used, the compiler will signal an error. @Override
annotation is optional, but certainly nice to have.
Ex: Superclass Person and its subclasses
Ex: Point2D and Point3D
Ex: Point and MovablePoint
Ex: Superclass Shape and its subclasses Circle, Rectangle and Square
Write a superclass called Shape
(as shown in the class diagram), which contains:
- Two instance variables
color
(String
) andfilled
(boolean
). - Two constructors: a no-arg (no-argument) constructor that initializes the
color
to 'green' andfilled
totrue
, and a constructor that initializes thecolor
andfilled
to the given values. - Getter and setter for all the instance variables. By convention, the getter for a
boolean
variablexxx
is calledisXXX()
(instead ofgetXxx()
for all the other types). - A
toString()
method that returns 'A Shape with color of xxx and filled/Not filled
'.
Write a test program to test all the methods defined in Shape
.
Write two subclasses of Shape
called Circle
and Rectangle
, as shown in the class diagram.
The Circle
class contains:
- An instance variable
radius
(double
). - Three constructors as shown. The no-arg constructor initializes the radius to
1.0
. - Getter and setter for the instance variable
radius
. - Methods
getArea()
andgetPerimeter()
. - Override the
toString()
method inherited, to return 'A Circle with radius=xxx, which is a subclass of yyy
', whereyyy
is the output of thetoString()
method from the superclass.
The Rectangle
class contains:
- Two instance variables
width
(double
) andlength
(double
). - Three constructors as shown. The no-arg constructor initializes the
width
andlength
to1.0
. - Getter and setter for all the instance variables.
- Methods
getArea()
andgetPerimeter()
. - Override the
toString()
method inherited, to return 'A Rectangle with width=xxx and length=zzz, which is a subclass of yyy
', whereyyy
is the output of thetoString()
method from the superclass.
Write a class called Square
, as a subclass of Rectangle
. Convince yourself that Square
can be modeled as a subclass of Rectangle
. Square
has no instance variable, but inherits the instance variables width and length from its superclass Rectangle.
- Provide the appropriate constructors (as shown in the class diagram). Hint:
- Override the
toString()
method to return 'A Square with side=xxx, which is a subclass of yyy
', whereyyy
is the output of thetoString()
method from the superclass. - Do you need to override the
getArea()
andgetPerimeter()
? Try them out. - Override the
setLength()
andsetWidth()
to change both thewidth
andlength
, so as to maintain the square geometry.
Exercises on Composition vs Inheritance
They are two ways to reuse a class in your applications: composition and inheritance.
Ex: The Point and Line Classes
Let us begin with composition with the statement 'a line composes of two points'.
Complete the definition of the following two classes: Point
and Line
. The class Line
composes 2 instances of class Point
, representing the beginning and ending points of the line. Also write test classes for Point
and Line
(says TestPoint
and TestLine
).
The class diagram for composition is as follows (where a diamond-hollow-head arrow pointing to its constituents):
Instead of composition, we can design a Line
class using inheritance
. Instead of 'a line composes of two points', we can say that 'a line is a point extended by another point', as shown in the following class diagram:
Let's re-design the Line
class (called LineSub
) as a subclass of class Point
. LineSub
inherits the starting point from its superclass Point
, and adds an ending point. Complete the class definition. Write a testing class called TestLineSub
to test LineSub
.
Summary: There are two approaches that you can design a line, composition
or inheritance
. 'A line composes two points' or 'A line is a point extended with another point''. Compare the Line
and LineSub
designs: Line
uses composition and LineSub
uses inheritance. Which design is better?
Ex: The Circle and Cylinder Classes Using Composition
Try rewriting the Circle-Cylinder
of the previous exercise using composition (as shown in the class diagram) instead of inheritance. That is, 'a cylinder is composed of a base circle and a height'.
Which design (inheritance or composition) is better?
Exercises on Polymorphism, Abstract Classes and Interfaces
Ex: Abstract Superclass Shape and Its Concrete Subclasses
Rewrite the superclass Shape
and its subclasses Circle
, Rectangle
and Square
, as shown in the class diagram.
In this exercise, Shape
shall be defined as an abstract
class, which contains:
- Two
protected
instance variablescolor
(String
) andfilled
(boolean
). Theprotected
variables can be accessed by its subclasses and classes in the same package. They are denoted with a'#'
sign in the class diagram. - Getter and setter for all the instance variables, and
toString()
. - Two
abstract
methodsgetArea()
andgetPerimeter()
(shown in italics in the class diagram).
The subclasses Circle
and Rectangle
shall override the abstract
methods getArea()
and getPerimeter()
and provide the proper implementation. They also override the toString()
.
Write a test class to test these statements involving polymorphism and explain the outputs. Some statements may trigger compilation errors. Explain the errors, if any.
What is the usage of the abstract
method and abstract
class?
Ex: Polymorphism
Examine the following codes and draw the class diagram.
Explain the outputs (or error) for the following test program.
Ex: Interface Movable and its implementations MovablePoint and MovableCircle
Suppose that we have a set of objects with some common behaviors: they could move up, down, left or right. The exact behaviors (such as how to move and how far to move) depend on the objects themselves. One common way to model these common behaviors is to define an interface called Movable
, with abstract
methods moveUp()
, moveDown()
, moveLeft()
and moveRight()
. The classes that implement the Movable
interface will provide actual implementation to these abstract
methods.
Let's write two concrete classes - MovablePoint
and MovableCircle
- that implement the Movable interface.
The code for the interface Movable
is straight forward.
For the MovablePoint
class, declare the instance variable x
, y
, xSpeed
and ySpeed
with package access as shown with '~'
in the class diagram (i.e., classes in the same package can access these variables directly). For the MovableCircle
class, use a MovablePoint
to represent its center (which contains four variable x
, y
, xSpeed
and ySpeed
). In other words, the MovableCircle
composes a MovablePoint
, and its radius
.
Write a test program and try out these statements:
Write a new class called MovableRectangle
, which composes two MovablePoints
(representing the top-left and bottom-right corners) and implementing the Movable
Interface. Make sure that the two points has the same speed.
What is the difference between an interface and an abstract class?
Ex: Interfaces GeometricObject and Resizable
- Write the
interface
calledGeometricObject
, which declares twoabstract
methods:getParameter()
andgetArea()
, as specified in the class diagram.
Hints: - Write the implementation class
Circle
, with a protected variableradius
, which implements the interfaceGeometricObject
.
Hints: - Write a test program called
TestCircle
to test the methods defined inCircle
. - The class
ResizableCircle
is defined as a subclass of the classCircle
, which also implements an interface calledResizable
, as shown in class diagram. The interfaceResizable
declares anabstract
methodresize()
, which modifies the dimension (such asradius
) by the given percentage. Write the interfaceResizable
and the classResizableCircle
.
Hints: - Write a test program called
TestResizableCircle
to test the methods defined inResizableCircle
.
More Exercises on OOP
Ex: The Discount System
You are asked to write a discount system for a beauty saloon, which provides services and sells beauty products. It offers 3 types of memberships: Premium, Gold and Silver. Premium, gold and silver members receive a discount of 20%, 15%, and 10%, respectively, for all services provided. Customers without membership receive no discount. All members receives a flat 10% discount on products purchased (this might change in future). Your system shall consist of three classes: Customer
, Discount
and Visit
, as shown in the class diagram. It shall compute the total bill if a customer purchases $x of products and $y of services, for a visit. Also write a test program to exercise all the classes.
The class DiscountRate
contains only static
variables and methods (underlined in the class diagram).
Ex: Polyline of Points with ArrayList
A polyline is a line with segments formed by points. Let's use the ArrayList
(dynamically allocated array) to keep the points, but upcast to List
in the instance variable. (Take note that array is of fixed-length, and you need to set the initial length).
Exercises on Data Structures
Ex: MyIntStack
A stack is a first-in-last-out queue. Write a program called MyIntStack
, which uses an array to store the contents, restricted to int
.
Write a test program.
Try:
- Modify the
push()
method to throw anIllegalStateException
if the stack is full. - Modify the
push()
to returntrue
if the operation is successful, orfalse
otherwise. - Modify the
push()
to increase the capacity by reallocating another array, if the stack is full.
Exercise (Nodes, Link Lists, Trees, Graphs):
[TODO]
- Study the existing open source codes, including JDK.
- Specialized algorithms, such as shortest path.
Drivers Training Book Online
Exercise (Maps):
[TODO]
- Representation of map data.
- Specialized algorithms, such as shortest path.
Exercise (Matrix Operations for 3D Graphics):
[TODO]
- Study the existing open source codes, including JDK's 2D Graphics and JOGL's 3D Graphics.
- Efficient and specialized codes for 3D Graphics (4D matrices). Handle various primitive types such as
int
,float
anddouble
efficiently.
Fast Ladies by Jean-François Bouzanquet is a fascinating look back at the female racing drivers who made their mark on the world from the time the first races were taking place right the way through to 1970.
Female racing drivers have always faced additional challenges, they were often banned from races outright or had to make do racing in ladies only events and in ladies only land speed record attempts.
Despite this many remarkable characters rose to prominence over the years including legendary villains like Violette Morris who ended up working with the Nazis and being assassinated by the French resistance, and heroes like Kay Petre who defeated all before her, she set multiple speed records, racing on multiple continents and battled the men in the 24 Hours of Le Mans.
Above Image: Violette Morris behind the wheel.
The life of Violette Morris sounds truly cinematic, she was an openly gay racing driver who eventually had her breasts removed to more easily fit behind the large steering wheels of the race cars of the era.
She frequently beat men on the track and in the boxing ring, much to their chagrin. During WWII she sided with the Nazis and became a valuable collaborator and source of information.
Above Image: Violette Morris earlier in her career as a boxer.
Morris became a strategic asset inside France during the war, she was nicknamed 'The hyena of the Gestapo' and she was a special guest in Hitler's private box during the 1936 Olympics in Berlin.
She was eventually killed during the war by a British-led commando raid with the French Resistance specifically sent out to find and assassinate her.
Above Image: Kat Petre with her 1924 Delage – a 10.5 litre V12-engined Land Speed Record car.
Kay Petre was about as different from Morris as it was possible to get, standing at only 4'10' tall she proved utterly fearless – setting women's speed records at the banked Brooklands circuit of 124 mph in a Bugatti and 134.75 in a Delage.
She famously battled against Gwenda Stewart and countless other male and female drivers from the era until her career was cut short in 1937 at Brooklands where Reg Parnell misjudged an overtaking maneuver crashed into Petre's Austin Seven – causing her to crash terribly and very nearly die.
Above Image: From the left, racing drivers Gwenda Stewart, Doreen Evans, Kay Petre, and Elsie Wisdom – September 1935.
Book Drivers Test
Petre was never able to race again, so she carved out a new career as a published motoring and food writer. She would eventually be hired by Leonard Lord of the British Motor Corporation as a style and color consultant, she styled many of the BMC cars of the era and she created the signature fabric patterns used on the original Minis.
Skipping forward to the year 2021 and female racing drivers are a common sight, there's even an entire formula dedicated to them called the W Series.
Fast Ladies is an invaluable resource that documents the rise of the female racing driver, it's available in both Kindle and hard cover versions though pricing varies and it's now out of print.
Ben has had his work featured on CNN, Popular Mechanics, Smithsonian Magazine, Road & Track Magazine, the official Pinterest blog, the official eBay Motors blog, BuzzFeed, and many more.
Silodrome was founded by Ben back in 2010, in the years since the site has grown to become a world leader in the alternative and vintage motoring sector, with millions of readers around the world and many hundreds of thousands of followers on social media.
This article and its contents are protected by copyright, and may only be republished with a credit and link back to Silodrome.com - ©2020