cs111 Frequently Asked Questions (FAQ)

If you have a question you would like answered on this list, or if you are not satisfied with an answer that appears below, please send email to the course instructors.

Table of Contents

The following questions are linked to answers below.

The CS111 server

• I'm having trouble using Fetch/ws_ftp to connect to the CS111 server. What am I doing wrong?
• How do I navigate to a CS111 drop folder in Fetch?
• How do I navigate to a CS111 drop folder in Winsock FTP?
• When I upload files to/download files from the CS111 server using Fetch, files don't have the right name/format. What's the fix?

Java Programming

• How do I change the background color of my applet?
• My applet gets stuck in a flickering mode. What's wrong?
• I invoked a graphics method to draw something on the screen,but it didn't appear in the window. Why?
• Where can I read about arithmetic in Java?
• What are floating-point numbers?
• How can I convert between ints and doubles?
• How can I get a double result from dividing two ints?
• I'm getting a code error for a specific line of my code, but I can't find the error. Why not?

Questions and Answers

The CS111 Server

A: There are many possible mistakes you may be making. Here are some important tips to follow:

• You should be connecting to host cs111.wellesley.edu, not sallie.wellesley.edu.
• Your user name should be no longer than 8 characters long. If your Lucy user name is more than 8 characters long, just use the first 8 characters as your CS111 user name.
• Your user name should be in all lowercase letters.
• Passwords are case sensitive. Make sure that you use the same capitalization that you used when you submitted your password.

A: When you connect to Fetch, you are connected to your home directory (/users111/your-user-name) by default. Your goal is to navigate to the CS111 drop folders, which are located in /usr/users/cs111/drop. You navigate using the Change Directory command. Here's how it works:

• From the menu bar along the top of the window, select Directory and select the Change Directory bar within the menu.
• Type the address of the directory you wish you get to -- in this case, /usr/users/cs111/drop.
• You are now in the cs111 drop directory. Scroll down until you find your personal drop folder, and open it by double-clicking it.

Q: How do I navigate to a CS111 drop folder in Winsock FTP?

A: In Winsock FTP, you have two windows. The window on the left represents your computer. The files and folders there are the ones stored on your own machine. The window on the right shows the files and folders stored on the cs111 server. To get to the drop folder, you need to change your directory on the cs111 server.

• Click on the ChgDir button on the right side of the window (the top button).
• In the window that pops up, type: /usr/users/cs111/drop.
• Scroll down to your personal drop folder, and double-click to open it.

Q: When I upload files to/download files from the CS111 server using Fetch, files don't have the right name/format. What's the fix?

A: If your uploaded files end up having .bin or .hqx extensions, or if downloaded files don't have the right icons, your Fetch preferences are probably incorrect. You can fix them by installing the CS111 patches.

Applets

A: There are numerous reasons why an applet might not run on your home page. Before you do anything else, read the applet invocation notes, and follow the steps explained there. If you still have problems, here are some possible reasons:

• You have modified your <applet ...> and <param ...> tags to be correct, but Netscape hasn't noticed your changes. Reloading your HTML file won't do the trick, because Netscape "remembers" the old applet information. Sometimes creating a new web browser window and loading your HTML file into the new window will notice your changes, but this is not guaranteed. If that fails, quit the Netscape application and relaunch it.
• You don't have a codebase attribute for the <applet ..> tag. For a remote applet, you must have such an attribute, and it must be an absolute (global) URL -- i.e., one beginning with http:...
• Your codebase attribute specifies a file, not a directory. Remember, it is the combination of the codebase and code attributes that specifies the location of the Jave bytecode file.
• Your codebase attribute is specifies the wrong directory. See the instructions for determining the codebase attribute within the applet invocation notes.
• Your codebase attribute is correct, but the references for data resources (images, audio files, etc) are incorrect. See the notes on referencing data resources at the end of the applet invocation notes.

A: If the applet is running, then your <applet ..> tag codebase attribute is correct. But if images are not appearing, it means that the addresses for data resources are not correct. See the notes on referencing data resources at the end of the applet invocation notes for how to fix this.

Java Programming

A: Add the following method to your applet

	public void init() {

	  this.setBackground(bgcolor);

	}

where bgcolor is an expression denoting the background color you want. Important: do not use the method invocation this.setBackground(bgcolor); within an applet's paint method -- this will cause the applet to get stuck in flicker mode.

A: It's likely that you have a method invocation this.setBackground(bgcolor); within the applet's paint method. As noted in the answer to the previous question, this invocation should appear in the applet's init method, not its paint method.

A: Here are several possible causes for missing graphics:

• The pen color of the graphics context is the same as the background color of the window.
• The coordinates for the drawn entity are outside the horizontal or vertical coordinate ranges of the window.
• The entity was drawn in the window, but some other entity was later drawn "over" it.

A: Here are several sources for details on Java arithmetic:

• programming.java, Sections 5.1--5.3.
• Computing Concepts with Java, Chapter 2.
• Core Java, Chapter 3

A: A floating-point number is a number that has a fractional part; it is written with a decimal point, e.g.: 2.5, 3.141, -5.99. Floating point numbers are distinguished from integers, which have no fractional part. In Java, there are two types of floating-point numbers, those of type float and those of type double. Numbers of type double have higher precision than those of type float. By default, we will use double for floating-point numbers in CS111.

A: If exp is an expression denoting an int, it can be converted to a double by means of a so-called "cast" expression:

(double) exp

If exp is an expression denoting a double, it can be converted to an int by means of a "cast" expression that truncates the information after the decimal point:

(int) exp

Here are some examples:

Java Expression	Value
(double) 17	17.0
(int) 3.141	3
(int) 5.99	5
(int) -3.141	-3
(int) -5.99	-5

In practice, converting an int to a double is rarely needed, since most binary operators (e.g. +, -, * , /, <, >, ==, !=) automatically convert an int argument to a double when the other argument is a double. (But see the question about dividing two ints, below.) Here are some examples involving addition:

Java Expression	Value
10 + 3	13
10.9 + 3.141	14.041
10.9 + 3	13.9
10 + 3.141	14.141
((int) 10.9) + 3	13
(int) (10.9 + 3.141)	14.141
(int) 10.9 + 3.141	13.141
2.5 + 3.5	6.0

Note in the example

(int) 10.9 + 3.141

that (int) binds more strongly than +; that is, the above example is treated as:

((int) 10.9) + 3.141

Sometimes truncating the fractional portion of a double is not the desired behavior -- instead, we want to find the floor (round toward negative infinity), find the ceiling (round toward infinity), or round toward the nearest integer. These calculations are performed by the Math.floor, Math.ceil, and Math.round library functions, as shown below:

Java Expression	Value
Math.floor(3.141)	3
Math.ceil(3.141)	4
Math.round(3.141)	3
Math.floor(5.9)	5
Math.ceil(5.9)	6
Math.round(5.9)	6
Math.round(2.5)	3
Math.round(3.5)	4
Math.floor(-3.141)	-4
Math.ceil(-3.141)	-3
Math.round(-3.141)	-3
Math.floor(-5.9)	-6
Math.ceil(-5.9)	-5
Math.round(-5.9)	-6
Math.round(-2.5)	-2
Math.round(-3.5)	-3

See the java.lang.Math API for more information on library math functions.

A: In Java, the division of two numbers of which at least one is a double always yields a double result. However, dividing two ints always yields an int -- this is the result of so-called "integer division", in which any fractional portion of the result is truncated. How can we get a double result from dividing two ints? By first casting one or both of the integer arguments to a double. Here are some examples:

Java Expression	Value
7 / 2	3
7.0 / 2	3.5
7 / 2.0	3.5
7.0 / 2.0	3.5
((double) 7) / 2	3.5
7 / ((double) 2)	3.5
(double) (7 / 2)	3
(double) 7 / 2	3.5
((int) 7.0) / 2	3

A: The compiler reports an error as soon as it reaches a bit of code it cannot execute. Sometimes, when it cannot execute a piece of code, it is because of a mistake made earlier in the method. Check the line before the line that's receiving the error.

• Does it have a semicolon, if one is needed?
• Do you close off all the sets of quotation marks you opened? An open set of quotations could mean your code interprets the first part of your line as text to be saved or printed. If so, it will be trying to execute the part you had in quotes.
• Did you close off all earlier methods before declaring a new one? If you forgot a curly brace, the program sees you trying to declare a method within another method and generates the error message then.