Download Using jGRASP

9/21/2012
Using jGRASP
An interactive tool
for creating and debugging
Java programs
Start jGRASP
If you haven’t installed jGRASP
on your computer yet, do that
first, then come back to this
presentation.
• Locate the big G icon
on your desktop or go
to the Start Menu, All
Programs, then click
on jGRASP.
• The jGRASP window
will open.
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Navigation
• At the left side
of the screen is
the navigation
pane.
• Here’s where
you see the list
of folders and
files on your
computer.
Navigation
• Navigate to your USB
drive (or wherever
your CS 171G files are
stored) by doubleclicking on the drive.
• Caution: Before you
work on a Java
program, always make
sure that the correct
folder is displayed in
this pane.
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Navigation Buttons
Back button
Move up to
higher
folder level
Refresh the
folder
display
Create a
new folder
Practice
• Create a new
folder (on your
USB or hard drive)
just for your Java
programs
• Navigate to that
folder.
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Practice: Create a Java Applet
• Click the File
Menu, choose
New, then click on
Java
• You’ll begin typing
your program
instructions in the
large, blank area
on the right
Practice: Create a Java Applet
•
•
Type in the Java program
exactly as shown.
jGRASP will color-code the
instructions as you type.
Careful! Java is case-sensitive.
Make sure you get the capital letters
as shown.
•
Save the file with the file
name Example.java
Remember the curly braces from
JavaScript? You're going to use
them in Java, also. Make sure you
have them matched up.
 spelling  punctuation  capitalization
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Practice: Compile a Java Applet
• The Java compiler
will check your
program for errors.
• Error messages will
display in the
bottom area of the
screen.
• Compile your
program by clicking
the
button on the
toolbar.
Practice: Compile a Java Applet
• A new file called
Example.class has been
created.
• Example.java is your
“source” code.
• Example.class contains
"bytecode".
• If the program compiled
with no errors, you will
see "operation
complete".
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Practice: Compiler errors?
• If you see error messages instead of
“Operation Complete”, go back and check
what you typed.
• Gradually, you will learn what each type of
error message means.
• Correct any typing errors that you made,
then click the Compile button again.
• Repeat until you get “Operation Complete”.
Practice: View the Java Applet
• Once you have
successfully
compiled the
program, you can
view the applet.
• Applets need
HTML and a web
browser or
“appletviewer”
• Applets cannot
run by themselves
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Practice: View the Java Applet
• jGRASP has an
easy way to
create a
temporary HTML
file and view the
applet.
• Click the running
man icon
on the toolbar
Note: In old versions of jGrasp, there was an
“apple” button for viewing applets.
Practice: View the Java Applet
•
•
•
•
•
Here’s what you should see.
The applet program you
wrote is displayed in Applet
Viewer.
It draws one, red line from
(0,0) to (100, 100).
When you’re finished
viewing the applet, close
the Applet Viewer with the
X button in the top, right
corner.
The Applet Viewer has to
be closed before you can
make any changes to the
program.
You’ll learn all about drawing, setting colors and
using the coordinate system in the “Graphics” notes
and lab assignment.
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A Second Java Example
Creating an Application
• Close the program (File
Menu, then Close).
• Now create a new Java
program and type in the
program shown here.
• This program is an
application. It can run by
itself, without having
HTML, a web browser, or
applet viewer.
Note: this program has a
few errors. Type it exactly
as shown here. We’ll get rid
of the errors shortly.
A Second Java Example
Debugging
•
•
•
Save the program with the name Example2.java
Compile the program
In the bottom area of the screen, you should see one error message.
•
Java has a multi-level compiler. It checks for very simple “typo” kinds of
errors first, such as this missing semicolon.
Click in the program area and put a semicolon at the end of the line. (In my
program, it’s line 11.)
x = scan.nextInt();
Compile the program again.
•
•
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A Second Java Example
Debugging
•
•
Now you should see one error message, but it’s a different error.
This error is a level 2 compiler error. Although you might think it’s just a
simple typo, Java thinks it’s a more complex error than the missing
semicolon.
•
•
•
•
The level 2 error messages are usually not very helpful. The error
message is “cannot find symbol” which doesn’t tell you much.
The real problem here is that the “s” on “system” should be a capital “S”.
Change it to a capital S.
Scanner scan = new Scanner(System.in);
Compile the program again.
A Second Java Example
Debugging
• Now you should see a successful compile and the message
“Operation Complete”.
• Debugging is the process of finding and fixing errors in a program.
• The compiler helps you find syntax errors… errors in spelling,
punctuation, capitalization, words out of order, etc.
• Even though you fix all of the syntax errors in a program, it doesn’t
necessarily mean there are no more errors.
• The other type of error is called a logic or “runtime” error. When
you run the program (or view an applet), it doesn’t work right.
Runtime errors are even harder to debug than syntax errors.
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A Second Java Example
Running the Application
•
In jGRASP, you can run an application by clicking the Run button
•
Click run.
Notice the cursor next to the two triangles in the
message area. It’s waiting for you to input a number. Type some number
and press Enter.
• When you run an application in jGRASP, the input and
output is done in the “Run I/O” tab near the bottom of the
screen. I/O is short for “Input and Output.
• This program is complete. You can exit from jGRASP now.
Summary
• jGrasp is a simple interactive program that
allows you to edit, compile, and test Java
programs
• To compile a program use the
button.
• To run a program (or view an applet) use the
button.
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