CSI1102
2005 Frequently Asked Questions
Q: In A3 question 3, what kind
of objects should I use in the driver class?
A: In the driver class you can
create objects of an interest_bearing_account class that is derived for the
class account.
Q: In A3 question 3, when do I
compute the interest rate?
A: After you did the withdrawals, because one month has passed. Then display
the new balances.
Q: In A3 question 1, the dialog
window does not close properly.
A:
Add the following line
in your main method:
System.exit(0);
Q: In A3 question 1, how do I
display the table? How do I change the font?
A: Use an array of two JTextArea, and pass
it as second argument to the dialog box:
JTextArea outputArea[] = {new JTextArea(header_string), new
JTextArea(info_string)};
outputArea[0].setFont(new Font("Courier", Font.BOLD,
12));
JOptionPane.showMessageDialog(null, outputArea,
"Rolling a Die 100 Times", JOptionPane.INFORMATION_MESSAGE);
Q: In A3 question 2, how do I hide a button?
A: JButton b = new JButton("text_on_button");
b.setVisible(false); //hides the button b
Q: In A3, how do I associate
the BorderLayout with the container of the JFrame?
A:
layout = new
BorderLayout(); // layout
could be a class variable, previously declared as being of
type BorderLayout
Container c = getContentPane();
c.setLayout(layout);
// make an array of JButton, and add each button to c in the right position
(north, south, etc.)
Q: How do I draw the window for
A2 question 1 (to exit when I press zero)? How do I draw thick lines?
A: See this file and this
file for some code you can use as a model (starter code). One easy way to
simulate thick lines is to draw several thin lines consecutively.
Q: Do I need a separate UPC
class?
A:
This is your design choice. Things to consider: Could you have several objects
of the class? Do you need to pass it as parameter to the constructor of another
class, for example WindowUPC?
Q: How do I extract each digit
in a number?
A: You have several choices. You
can transfrom the number into a String (using Integer.toString for example) and
use methods of the class String. Or you can use arithmentic operations on int
values. For exmaple if int N = 672, N % 10 = 2, N/10 - (N/100)*10 = 67-60 = 7,
N/100 = 6.
Q: How do I draw the colors in the applet for A2 question 2?
A: To draw colors from Red to Yellow you need to vary only one component of the
RGB code (in this case the G component, see the color codes below). An example
of a code fragment of the paint method of the applet is:
public void paint (Graphics page)
{ int loc = 0;
// From Red to Green through Yellow
for (int j=0; j<256; j+=2, loc++){
Color c = new Color(255, j, 0);
page.setColor (c);
page.drawLine (loc, 0, loc, APPLET_HEIGHT);
}
// loops for the rest of the color changes ...
}
The RGB color codes from Figure 2.21 are:
Color.red 255, 0, 0
Color.yellow 255,255,0
Color.green 0, 255, 0
Color.cyan
0, 255, 255
Color.blue 0, 0, 255
Color.magenta 255, 0, 255
Q: How do I generate random
characters?
A: Generate random integers for
the right ASCII codes, and then use cast to print them as characters not
integers. You don’t need to know the ASCII code for letter a, just use ‘a’ and
convert it into an integer.
Q: I see we need to use the
Keyboard class in the labs. Where do I get it? How do I use it?
A: The Keyboard class was provided by the authors of the textbook and is
on the CD. You can download it from here (unzip
the file) or from the Examples
directory (copy the directory cs1 with the file Keybord.class in it).
In RealJ, you need to set the CLASSPATH
variable to the directory that contains the subdirectory cs1 (that contains the
file Keyboard.class). You can do this by selecting "Add folder"
from the window "configure classpath" of your Project.
If the directory cs1 is in
the same directory with your .java files, you do not need this step.
When you run it from RealJ use the menu
"Run with console", not "Run Application" !!!
You could also run your program
from the command line, so you can input the expected values from the keyboard.
For example for the file Echo.class, you type: java Echo if the cs1
subdirectory is in your current directory, or java -classpath
path_to_cs1_directory_; Echo
You use the class by entering import cs1.Keyboard; as the first line of your code. See chapter 2 of the text
book for an explanation; this is also explained in class.
// example of using the keyboard
class
import cs1.Keyboard;
public class Echo
{
// read from the user and print it out
public static void main (String[] args)
{
String message;
int a;
System.out.println("Enter any text: ");
message =
Keyboard.readString();
System.out.println("You entered: \"" + message);
}
}
Q: How to run a Java program (of type application)?
A: There are two ways. From a command line
or using an integrated development environment (such as RealJ).
From RealJ - Use the menu:
Create a (new empty) project.
Add your Java file(s). Use
the Add Files option under the Project menu. Or use the option Add window to
project if the file is already open.
Specify which is the main file (that contains the "main"
method).
Compile by selecting Build from the menu.
Run by selecting Run Application.
From a command line, run the Java compiler (javac) and then the Java
interpreter (java):
javac MyFile.java (This produces MyFile.class
You may need to specify the full path to javac.
You need to be in the directory where your java files are.
If your program has more files, compile each file).
java
MyFile
(assuming that MyFile.java contains the "main" method).