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The University of Hong Kong
Department of Computer Science
COMP2396 Object-oriented Programming and Java
Assignment 1
Deadline: 11:55pm, 14th Feb, 2019.
Overview
This assignment tests your understanding of classes and objects, and their implementations in
Java. You are required to implement 4 classes, namely Shape, Square, Triangle and Circle.
The Square class, Triangle class and Circle class are used to model squares, triangles, and
circles, respectively. They are subclasses of the Shape class which provides an abstraction of
general shapes. For each of the 4 classes mentioned above, you are required to design and
implement a tester class to test the correctness of your implementation. You are also required
to write Javadoc for all public classes and their public class members.
Specifications
The Shape class
The Shape class is used to model general shapes. It has instance variables for storing color,
fill-type, orientation, screen coordinates of the center, and local coordinates of the vertices of
a shape. It has methods for getting the screen coordinates of the vertices of a shape. Below is
a detailed description for the Shape class.
Specification of the Shape class:
instance variables
Color color – a Color object specifying the color of the shape. To use the Color class,
place“import java.awt.Color;”at the top of the source file of your Shape class.
Please refer to http://docs.oracle.com/javase… for details
of the Color class.
boolean filled – a boolean value specifying whether the shape is filled or not filled.
double theta – a double value specifying the orientation (in radians) of the shape in the
screen coordinate system (see Figure 1).
double xc – a double value specifying the x-coordinate of the center of the shape in the
screen coordinate system.
double yc – a double value specifying the y-coordinate of the center of the shape in the
screen coordinate system.
double[] xLocal – an array of double values specifying the x-coordinates of the
vertices (in counter clock-wise order) of the shape in its local coordinate system.
double[] yLocal – an array of double values specifying the y-coordinates of the
vertices (in counter clock-wise order) of the shape in its local coordinate system.
methods:
void setVertices(double d) – a method for setting the local coordinates of the
vertices of a shape. This is a dummy method and is supposed to be overridden in the
subclasses.
void translate(double dx, double dy) – a method for translating the center of the
shape by dx and dy, respectively, along the x and y directions of the screen coordinate
system (i.e., dx and dy should be added to xc and yc respectively).
void rotate(double dt) – a method for rotating the shape about its center by an angle
of dt (in radians) (i.e., dt should be added to theta).
int[] getX() – a method for retrieving the x-coordinates of the vertices (in counter
clock-wise order) of the shape in the screen coordinate system (rounded to nearest
integers).
int[] getY() – a method for retrieving the y-coordinates of the vertices (in counter
clock-wise order) of the shape in the screen coordinate system (rounded to nearest
integers).
- You should make all the instance variables and methods public by using the public access
modifiers.
Figure 1. Relationship between local and screen coordinates of the vertices of a shape.
The screen coordinates of the vertices can be computed from the local coordinates of the
vertices based on the orientation and center of the shape using the following formula:
where (x, y) and (x’, y’) denote the local and screen coordinates of a vertex, respectively, ?
the orientation (in radians) of the shape, and (xc, yc) the screen coordinates of the center of
the shape. - You may use Math.sin() and Math.cos() to compute sine and cosine functions, and
Math.round() to round a number to its nearest integer. Please refer to
https://docs.oracle.com/javas… for details of the Math class.
local coordinate system screen coordinate system
The Square class
The Square class is used to model squares. It is a subclass of the Shape class and it inherits all
the instance variables and methods of the Shape class. The Square class overrides the
setVertices() method for setting the local coordinates of the 4 vertices of a standard square.
Below is a detailed description for the Square class.
Specification of the Square class:
instance variables inherited from Shape:
color, filled, theta, xc, yc, xLocal, yLocal
methods inherited from Shape:
translate, rotate, getX, getY
overriding method:
void setVertices(double d) – a method for setting the local coordinates of the 4
vertices of a standard square. Here, a standard square has its center located at (0, 0) and
its sides being parallel to the x- and y-axes of its local coordinate system. The parameter
d specifies half-the-length of a side of the square. - You should make all the instance variables and methods public by using the public access
modifiers.
Figure 2. The local coordinates of the 4 vertices of a standard square with a side of 2d.
For a standard square with a side of 2d, the local coordinates of its 4 vertices in counter
clockwise order starting from the lower right corner are (d, d), (d, -d), (-d, -d) and (-d, d),
respectively.
The Triangle class
The Triangle class is used to model triangles. Like the Square class, it is a subclass of the
Shape class and it inherits all the instance variables and methods of the Shape class. The
Triangle class overrides the setVertices() method for setting the local coordinates of the 3
vertices of a standard triangle. Below is a detailed description for the Triangle class.
Specification of the Triangle class:
instance variables inherited from Shape:
color, filled, theta, xc, yc, xLocal, yLocal
methods inherited from Shape:
translate, rotate, getX, getY
overriding method:
(d, d)
(-d, -d) (d, -d)
(-d, d)
void setVertices(double d) – a method for setting the local coordinates of the 3
vertices of a standard triangle. Here, a standard triangle is an equilateral triangle having
its center located at (0, 0) and one of its vertex on the positive x-axis of its local
coordinate system. The parameter d specifies the distance from the center of the
triangle to any of its vertices. - You should make all the instance variables and methods public by using the public access
modifiers.
Figure 3. The local coordinates of the 3 vertices of a standard triangle with a distance of d from its center to any of its
vertices.
For a standard triangle with a distance of d from its center to any of its vertices, the local
coordinates of its 3 vertices in counter clockwise order starting from the one on the positive
x-axis are (d, 0), (?? cos ?/3 , ?? sin ?/3) and (?? cos ?/3 , ? sin ?/3), respectively.
The Circle class
The Circle class is used to model circles. Like the Square class and the Triangle class, it is a
subclass of the Shape class and it inherits all the instance variables and methods of the Shape
class. The Circle class overrides the setVertices() method for setting the local coordinates
of the upper left and lower right vertices of an axis-aligned bounding box of a standard circle,
as well as the getX() and getY() methods for retrieving the screen coordinates of the upper
left and lower right vertices of this bounding box. Below is a detailed description for the
Circle class.
Specification of the Circle class:
instance variables inherited from Shape:
color, filled, theta, xc, yc, xLocal, yLocal
methods inherited from Shape:
translate, rotate
overriding methods:
void setVertices(double d) – a method for setting the local coordinates of the upper
left and lower right vertices of an axis-aligned bounding box of a standard circle. Here,
a standard circle is a circle having its center located at (0, 0) of its local coordinate
system. The parameter d specifies the radius of the circle.
int[] getX() – a method for retrieving the x-coordinates of the upper left and lower
right vertices of an axis-aligned bounding box of the circle in the screen coordinate
system (rounded to nearest integers).int[] getY() – a method for retrieving the y-coordinates of the upper left and lower
right vertices of an axis-aligned bounding box of the circle in the screen coordinate
system (rounded to nearest integers). - You should make all the instance variables and methods public by using the public access
modifiers.
Figure 4. The local coordinates of the upper left and lower right vertices of an axis-aligned bounding box of a
standard circle with a radius of d.
For a standard circle with a radius of d, the local coordinates of the upper left and lower right
vertices of its axis-aligned bounding box are (-d, -d) and (d, d), respectively. Their
corresponding screen coordinates are (-d + xc, -d + yc) and (d + xc, d + yc), respectively,
where (xc, yc) denote the screen coordinates of the center of the circle.
The tester classes
The tester classes are used to verify the correctness of your implementation of the above 4
classes. You should design your own tester classes. Generally, your tester classes should
create an object of a class and access all its instance variables and methods using the dot
operator, and print out debugging messages to the console. (You do not need to actually draw
the shapes in your tester classes!)
A simple GUI for visualization
Figure 5. Assign1_GUI.java implements a simple GUI for visualizing the shape classes.
(d, d)
(-d, -d)
Assign1_GUI.java implements a simple GUI for you to test and visualize your shape classes.
The GUI draws a square, a circle and a triangle on the canvas. When the user clicks on a
shape, the shape rotates clockwise 360o
.
Marking Scheme
Marks are distributed as follows: - Implementation of the Shape class and its tester class (40%)
- Implementation of the Square class and its tester class (10%)
- Implementation of the Triangle class and its tester class (10%)
- Implementation of the Circle class and its tester class (20%)
- Javadoc and comments (20%)
Submission
Please pack the source code (*.java) of your shape classes and tester classes into a single zip
file, and submit it to the course Moodle page.
A few points to note: - Always remember to write Javadoc for all public classes and their public class
members. - Always remember to submit the source code files (*.java) but NOT the bytecode
files (*.class). - Always double check after your submission to ensure that you have submitted the
most up-to-date source code files. - Your assignment will not be marked if you have only submitted the bytecode files
(*.class). You will get zero mark for the assignment. - Please submit your assignment on time. Late submission will not be accepted.
~ End ~
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