Using Java to Illustrate Graph
Theory concepts
Carol E. Wolf
Computer Science Department
Pace University
What is Java?
• Java is an object-oriented programming
language developed by Sun Microsystems.
• It is widely used on the Internet and was
designed to be portable.
• The Java Development Kit (JDK) can be
downloaded for free from Sun.
• It contains a compiler, an interpreter, and
many classes.
• Java programs are compiled to bytecode,
which is independent of the operating
system.
• Programs come in two varieties,
applications and applets.
• Java applications are full programs and are
executed by an interpreter on the user’s
computer.
• Java applets require a web browser such as
Netscape Communicator or Internet
Explorer.
• Applets are invoked by an HTML
document. Theoretically, they can then be
viewed on any computer with an
appropriate browser.
• Many animations and graphics on web sites
have been created using Java applets.
• One objection to Java, is that it takes time
both to download the files and then start
them up. Programs in native code would
execute more quickly.
• Java has been adopted by a number of
computer science departments for
programming classes.
• It is easier to learn than C++, has fewer
complications, and is safer.
• It is fully object-oriented, and so students
start off with the current (and better)
programming paradigm.
• However, this is a disadvantage for
instructors who have only been teaching
procedural languages.
Object-Oriented Programming
• An object combines both data and methods
(algorithms) into one package called a class.
• An example is that of an integer (int). An
integer object has a value (data) and
operations on that data (+, -, *, /, %, ==, <=,
etc.)
• Integers are primitive objects and do not
require definition by the programmer.
• Another example is that of a date. A date
has integer data: day, month, and year, and
operations on that data: equal, before (less
than), display, etc.
• Java does not (to my knowledge) have a
built-in date class. However, it provides a
facility for the programmer to create one.
• In fact, Java programs are entirely
composed of classes such as this.
Java and Graphics
• Because Java was designed for viewing in a
browser, it takes full advantage of screen
graphics.
• It provides a full range of drawing
functions, fonts, and colors.
• These are easy for beginners to use and so
provide a convenient introduction to the
language.
• The coordinate system for the screen has the
origin in the upper left hand corner. The xaxis extends to the left, but the y-axis
extends down.
• Measurements on the screen are in terms of
pixels, picture elements.
• Java has some graphics classes, such as a
Point class. These come with JDK.
However, they are simple for a programmer
to construct as well.
class point
{
private int x, y;
point (int x, int y)
{
this.x = x;
this.y = y;
} // point (int x, int y)
int get_x () {return x;}
int get_y () {return y;}
void set_xy (int x, int y)
{
this.x = x;
this.y = y;
} // void set_xy (int x, int y)
}// class point
class circle
{
private point center;
private int radius;
circle (point center, int radius)
{
this.center = center;
this.radius = radius;
} // circle (point center, int radius)
public void drawCircle (Graphics page)
{
int x, y;
x = center.get_x ();
y = center.get_y ();
page.drawOval (x-radius, y-radius, 2*radius, 2*radius);
} // public void drawCircle (Graphics page)
} // class circle
Relationships
• The circle class illustrates the relationship
of has-a. A circle has a center and a radius.
The radius is an integer, a primitive data
type, but the center is a point, a data type
defined by the programmer.
• Two other relationships are is-a and uses-a.
• An is-a relationship is one of inheritance.
Inheritance is an important feature of any
object-oriented language.
class drawn_point extends point
{
protected int radius = 3;
drawn_point (int x, int y)
{
super (x, y);
} // drawn_point (int x, int y)
void drawPoint (Graphics page)
{
page.fillOval (x-radius, y-radius, 2 * radius, 2 * radius);
} // void drawPoint (Graphics page)
} // class drawn_point extends point
• Java, unlike C++, does not support multiple
inheritance. Instead, a class can implement
another class.
• This is an example of a uses-a relationship.
• The principle classes that are used are
listeners. These listen for some event to
take place on the applet.
• Examples are ActionListener,
MouseListener, MouseMotionListener,
ItemListener, and AdjustmentListener.
public class draw_points extends Applet implements MouseListener
{
private static char name_char;
private named_point newPoint;
Graphics page;
public void init ()
{
setBackground (Color.cyan);
setForeground (Color.blue);
page = getGraphics ();
name_char = 'A';
addMouseListener (this);
} // public void init ()
private String get_next_name ()
{
String next_name = "A";
next_name = next_name.replace ('A', name_char);
name_char = (char) ((int) name_char + 1);
return next_name;
}// private String get_next_name ()
public void mouseClicked (MouseEvent event)
{
int x, y;
String next_name = get_next_name ();
x = event.getX ();
y = event.getY ();
newPoint = new named_point (x, y, next_name);
newPoint.drawPoint (page);
} // public void mouseClicked (MouseEvent event)