1903 Fall 2007
Lab 1: Using BlueJ at UofW
The purpose of this lab is to give you some experience with BlueJ. Over the
course of the term, in classes and in labs, you will be given more instruction.
In this lab you will see how to:
Start BlueJ
Open a BlueJ project (a Java program)
Save the BlueJ project using a new name
Compile and run a Java program
Interact with an object system by sending messages
Modify a Java program and compile a Java class
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Lab 1, details:
We discuss a particular example that comes with the BlueJ installation called
shapes. The example deals with objects that are squares, triangles and circles.
As you will see, we design object-oriented systems so objects have specific
responsibilities and will respond to specific commands or messages.
We examine some Java program instructions in the Triangle class in detail.
You are asked to review the Circle and Square classes on your own.
We will not discuss the Canvas class. Its responsibilities are to draw or repaint
the images we see on the screen.
You will compile the shapes classes and interact with Java classes and objects.
As shapes is an interactive program, you will be asked to send specific
messages.
Lastly, you are asked to make a change to the Triangle class to experience
modifying a program and verifying your changes.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Student Identification
Student number
______________________
Last name
______________________
First name
______________________
Workstation user id
______________________
Email address
______________________
Include this slide with your answers to lab questions
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
3
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Starting BlueJ
Logon to a workstation in 3D03 or 3C13 (you may need specific
logon information from your lab demonstrator).
Click Start, then click on Programs, then click on BlueJ, and then
click the BlueJ program
Start >> Programs >> BlueJ >> BlueJ
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
4
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Opening a Java Project/Program
Open the shapes project
that can be found in a
folder named
blueJExamples on the H
drive.
The diagram shown on the
right illustrates the classes
that comprise the Java
program.
We will pay special
attention to the Triangle,
Square and Circle classes
in this tutorial.
Fall 2007
Project>>Open Project>>
ACS-1903 BlueJ
Ron McFadyen
5
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Saving as a Different a Java Project/Program
Click the Project menu, and select Save As. Choose a new name and location to
save your work and your work will not affect the original copy of shapes
Project>>Save As >>
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
6
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Saving as a Different a Java Project/Program
What drive have you used for your copy of shapes?
Answer:
What directory or folder have you used?
Answer:
What name did you give your copy of shapes?
Answer:
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
7
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Reviewing the Java Program
You can view the Java classes by double-clicking on each of the four classes shown in the
class pane.
Double-click on the orange rectangle
named Triangle. This action opens the
class editor.
Double-click
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
8
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Reviewing the Triangle class
Examine the code on your workstation, and note the following :
You should see coloured text in use: some red, some blue, etc. BlueJ uses colour to
denote special aspects of the code.
The first line is a Java import statement.
import java.awt.*;
In this case the program is stating that it will use a Java library known as awt. The editor
shows the word import is in red because it is keyword (this keyword informs the Java
compiler of the Java library that is needed).
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
9
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Reviewing the Triangle class
The blue-coloured text is used for comments.
/**
* A triangle that can be manipulated and that draws itself on a canvas.
*
* @author
Michael Kolling and David J. Barnes
* @version 1.0 (15 July 2000)
*/
Comments are not executed by the computer.
Comments are used to guide humans when they read the program code. As such,
comments should be explanatory.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
10
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Reviewing the Triangle class
Following the comments we have just discussed, you will see the beginning of the
Triangle class.
public class Triangle
{
private int height;
private int width;
private int xPosition;
private int yPosition;
private String color;
private boolean isVisible;
A class always begins with the key words public class followed by its name; in this case
the class is named Triangle.
The above partial listing shows the attributes in Triangle. These attributes will hold
values that describe a particular triangle (its height, width, position, etc).
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
11
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Reviewing the Triangle class
Following the lines giving the name of the class and its attributes, we see a method that is
used whenever we ask for an instance of the Triangle to be created.
/**
* Create a new triangle at default position with default color.
*/
public Triangle()
{
height = 30;
width = 40;
xPosition = 50;
yPosition = 15;
color = "green";
isVisible = false;
}
Later on we will ask for a triangle to be instantiated… when we do, it is the above Java
code that will execute. From the above you know the height, width, and positioning coordinates, amongst other things for the triangle that is created.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
12
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
Where do new triangles position themselves when they are instantiated?
Answer:
What colour is given to a newly instantiated triangle?
Answer:
How high is a newly instantiated triangle?
Answer:
How wide is a newly instantiated triangle?
Answer:
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Reviewing the Triangle class
Following the “constructor” method just described, we have several methods which in
this example begin with the words public void followed by the name of the method.
Each method defines a responsibility that an object has, or a capability it has. In this
example you will see that a triangle can
make itself visible
make itself invisible
move to the right
move to the left
…
The programmer that designed shapes decided that triangles should be responsible for
making themselves visible and invisible, moving themselves right, left, up, or down on
the screen. As you will see, we can ask a triangle to do these things.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
14
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
What things can we ask a triangle to do? (The answer is the names of its methods)
Answer:
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Reviewing the Triangle class
At this time it is only important that you realize a class may have many methods and that
each is designed for a specific purpose: to become visible, to become invisible, to move
up, etc. Do not be concerned with exactly how each of those is done… that will come
later on in the course.
There is a concept called anthropomorphism that is important for object programming.
(Consult a dictionary if needed, or wikipedia.) Java programmers will design systems
with classes where each method is some responsibility that an object of that class is
responsible for.
Consider an ordinary office where people are employed and each person is given specific
responsibilities. When asked, a person will perform a particular duty such as completing
an invoice, issuing a cheque and so on. The same principle is applied to object-oriented
computing systems.
After exploring the Triangle class, we know that triangles can be asked to do very
specific things. We’ll look at that now…
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
16
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Compiling a Java Program
Before we can run a Java program we must compile it. This is a process that creates an
execuable form of a program. At the BlueJ screen where we see the four classes, we
need to select Compile from the Tools menu item.
Tools >> Compile
When the compile is done
you will see the message
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
17
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
Let’s create a triangle …. Remember the constructor method – the triangle will initially
be invisible and so we will ask it make itself visible.
The next few slides show the screens discussed below.
Begin by right-clicking the Triangle class and choose newTriangle(). You will be asked
to name the triangle (or you can go with the default name of triangle1.)
When you click OK for naming the triangle you will see a red object appear on your
screen indicating that you have instantiated an object.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
Right-clicking on a class presents a selection of possibilities … choose newTriangle()
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
The result of creating a triangle.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
Right-click on an object and the messages it can respond to are shown.
Choose makeVisible().
After selecting
makeVisible() look at
the bottom of your
screen. You should see
a new button,
click it and you will see
how Canvas is
displaying your
program results. See
the next slide.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
After selecting makeVisible() look at the bottom of your screen. You should see a new
button,
Click it and you will see how Canvas is displaying your program results. See the next
slide.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
A triangle was created, it was made visible,
and the output becomes viewable by clicking on
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
What happens when you instantiate a second triangle and make it visible?
Answer:
If two triangles occupy the same space (and so it appears as though only one is
there), how can you make the two triangles both visible?
Answer:
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
So far we have only considered the Triangle class.
Examine the Square and Circle classes. To do this you must double-click the
Square and Circle classes.
Find the “constructor” methods named Square() and Circle().
Where do new squares position themselves when they are instantiated?
Answer:
Where do new circles position themselves when they are instantiated?
Answer:
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
What things can a square do? (look for the other methods – those sections of code
beginning public void ….
Answer:
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
What things can a circle do? (look for the other methods – those sections of code
beginning public void ….
Answer:
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
Now… make a square. Make it visible.
Can you align it directly under the triangle? You may need to use the moveHorizontal()
message and give it a specific distance to move. Don’t hesitate to experiment.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Interacting with the shapes program
To instantiate a square and have it align directly under the triangle, you needed to
send various messages asking for certain things to be done. What were these
messages?
Answer:
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Modifying the shapes program
As you progress in this course you will modify programs and you will create programs.
Let’s start with something easy. Let’s modify the code for Triangle so that a triangle is
yellow instead of green when a triangle is instantiated.
We begin by opening the class editor for the Triangle class by double-clicking the orange
rectangle with the name Triangle.
Double-click
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Modifying the shapes program
Below is the code relating to the construction of a new triangle. Look at the second last
Java line. We must modify that line and then recompile the Triangle class. Then we can
test our change and determine if we have made the change correctly or not.
/**
* Create a new triangle at default position with default color.
*/
public Triangle()
{
height = 30;
width = 40;
xPosition = 50;
yPosition = 15;
color = “green";
isVisible = false;
}
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
We need to change the
word green to yellow.
Note that it is only by
chance the word green is
shown in green-coloured
text.
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Modifying the shapes program
Once the change has been made you must choose the
/**
* Create a new triangle at default position with default color.
*/
public Triangle()
{
height = 30;
width = 40;
xPosition = 50;
yPosition = 15;
color = “yellow";
isVisible = false;
}
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
We need to change the
word green to yellow
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Modifying the shapes program
Once the change has been made you must Compile the class.
After clicking Compile you should see the message “Class compiled – no syntax errors” at
the bottom of the screen.
You test your changes by instantiating triangles and seeing the new colour! See next slide.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Modifying the shapes program
Two triangles, one circle, one square. Note that one triangle was moved as both were
positioned at the same coordinates.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen
1903 Fall 2007
Lab 1: Using BlueJ at UofW
Lab 1,the shapes program
Lastly:
When you have produced results similar to the previous slide you can copy them by using
the Ctl-PrintScreen keys together, and then pasting the captured image into a Word
document or a PowerPoint slide. Your lab demonstrator can show you how to do this.
On previous slides you have answered various questions. Hand these in, along with the
Ctl-PrintScreen image, to your instructor. Be sure to staple these pages together, and
include the student identification slide.
Fall 2007
ACS-1903 BlueJ
Ron McFadyen