CSci A201/A597
Frequently Asked Questions on Lab #2

• General Questions
• Questions About Comma.java
• Questions About Months.java
• Questions About Typing.java

1. Cafe gives me wierd errors while I'm building my program. What can I do?

   Check the following (in order of priority):

   • Do you have a syntax error? Missing semi-colons, braces, parentheses, etc. would fall in this category. Error messages appear in the Output window. If you double click on the message it will take you directly to the offending line in your source code. Make the appropriate corrections in your code and run your program again.

   • Do you have the latest version of the TextInputStream.java? This file was updated on Sept. 11, and you should have downloaded a fresh copy to your ccj folder.

   • Is your disk in the a: drive full? Cafe attempts to create various auxilary files on your disk. If your disk becomes full, then Cafe will not be able to build your project. It may be time to change disks.

   • Have you forgotten to add your .java source file to the project or did you add the wrong file to the project? Check your project window to see if you can see your .java file there. If it isn't there, ask your AI (or just experiment yourself to find out) how you can add/remove a file to/from an existing project. Alternatively, you could just create a new project, taking care this time to add the file correctly. (This is done in step 3 of ProjectExpress, or by opening the file after opening the project, and saying File-Add to project.)

   • Are your files in the right place? Your disk should have a ccj folder with only ccj files in it. All your own .java and .prj files must be on your disk, outside the ccj directory.

   Strange things do happen sometimes, so make sure you see someone during office hours early in the week (i.e. not on Thursday morning) if you have problems.

2. When I run my program, the DOS window appears, but then suddenly disappears even though I've got the Console.in.readLine(); statement at the end of the program. What is wrong?

   Your program has a run-time, or logical, error in it. When this happens, the program terminates prematurely and it never gets to that Console.in.readLine(); statement at the end.

   Your error is probably due to a substring index out of bounds. The numbers that you pass to the substring method must be legal indices into the string (or one beyond the last index.) For example, all of the following calls to substring will cause run-time errors:

   String capital;
   capital = "Paris";
   System.out.println(capital.substring(-1, 3));
   System.out.println(capital.substring(3, 6));
   System.out.println(capital.substring(6));
   

3. You said you would provide instructions on using the JDK from Sun

   Yes. here is a brief installation and use instruction. This instruction is for Windows (win32) platforms only. You may need to make changes according to your platform.

   • Dowload the file jdk117B-win32.exe or something similar from the URL http://java.sun.com/products/jdk/1.1/.
   • Run this program. It will start the installer after unpacking itself. You should at least install the program files and library and header files. Installing the demo applets is not necessary but recommended. You will not need to install the source. The program will by default install on c:\jdk1.1.7b. The rest of this section assumes you have chosen the default. If you change the path, change the paths in the following steps appropriately.
   • After the installation finishes, you will find all the things in the c:\jdk1.1.7b directory. In particular, you will see two files there called index.html and README - make sure you read them very thoroughly, especially the README file.

     In the README file, there is a section on PATH and CLASSPATH, I am including that here verbatim other than comments within [brackets]:

     -----------------------------------------------------------------------
     MICROSOFT WINDOWS PATH and CLASSPATH
     -----------------------------------------------------------------------
     [snip]
          Windows NT only - If you are using Windows NT, it is  
          preferable to make the following environment variable  
          changes in the Control Panel. Start the Control Panel,  
          select System, then edit the environment variables. 
     
       1. PATH -  Add the absolute path of the "jdk1.1.7B\bin" 
          directory to your PATH statement as follows. 
     
          The PATH statement enables Windows to find the executables 
          (javac, java, javadoc, etc.) from any current directory.
     
          To find out the current value of your PATH, at the
          DOS prompt type:
     
              C:\%gt; path
     
          To change the PATH, open the AUTOEXEC.BAT file and make the
          change to the PATH statement. To edit the AUTOEXEC.BAT
          file in Windows 95: 
     
            i. Start a text editor by choosing "Start", "Programs", 
               "Accessories", and choosing WordPad or NotePad. 
           ii. Choose Open from the File menu and type "c:\autoexec.bat" 
               for the filename This will open the file for editing. 
          iii. Look for the PATH statement. Notice that the PATH statement
               is a series of directories separated by semi-colons (;).  
               Windows looks for programs in the PATH directories in order,
               from left to right. Look for other versions in the PATH.
               There should only be one path to a classes.zip file.
               When in doubt, put the java directory at the end of 
               the path statement. For example, in the following PATH
               statement, we have added the java directory at the end: 
     
                PATH C:\WINDOWS;C:\WINDOWS\COMMAND;C:\;C:\DOS;C:\JDK1.1.7B\BIN 
     
          To make the path take effect, execute the following:
     
                C:\%gt; autoexec.bat
     
     
       2. CLASSPATH Environment Variable - If you follow the default 
          installation, you do not need to set CLASSPATH, because the
          tools automatically set it for you.  If your CLASSPATH has 
          not previously been set, you can skip this step.
     
     [Jit's comments: You can skip this step if this is the first time you
     installed Java in your system.]
     

   • After this, reboot your system, and you should be all set. To see if your installation is working or not, open a DOS window and type the following:

          C:\> java -version
          

     You should see a message saying the version of Java on your system, which should be 1.1.7b. You can now compile a program by typing javac filename.java and run a program by typing java filename. (Note that you don't need to give an extension while running your proram.)

1. How do I read a string from the user? When I use Console.in.readString(), I get an error.

   Do not use the readString() method. You must use Console.in.readLine() or even better, Console.in.readWord() to read the input from the user into a String variable.

2. When I try to get the length of the string that the user inputted, the length seems to be one more than what I would expect

   Are you using Console.in.readLine()? If so, it seems to include the return that the user hits after she imputs the word. Instead of readLine(), use readWord() - this should work as expected.

1. I have no idea how to transform numbers into month names! Can you give me a hint to get me started?

   The textbook suggests that you set up a String variable containing the month names, separated by one or more blanks so that all "fields" are of the same width. Just to make this absolutely clear, I'll write this initial value as an expression joining the twelve different month names. This way, I can write all the names in a column and I can easily verify that each "field" is, indeed, of the same width.

      "January   " +    // month 1
      "February  " +    // month 2
      "March     " +    // month 3
      "April     " +    // month 4
      "May       " +    // month 5
      "June      " +    // month 6
      "July      " +    // month 7
      "August    " +    // month 8
      "September " +    // month 9
      "October   " +    // month 10
      "November  " +    // month 11
      "December  "      // month 12
   

   How wide is each field? Let's count:

   //  1234567890
      "January   " 
   

   In my case, each month takes up 10 characters in the String. (Note that it could also be done with a field width of 9.) Of course, we must keep in mind, that the J in January is at index zero.

   //  0123456789
      "January   " 
   

   So, how does this help? Well, there is a direct correlation between the starting index of the month name and the month number. Consider the following table:

   Month number	Start index
   1	0
   2	10
   3	20
   ...	...

   You try to fill in entries for months 4, 5, 6, etc. Now, looking at the table, you need to try to come up with a formula relating the month number to the starting index of the month name in your string. If you can do this, then you can use the substring method to extract the actual month name and print it.

1. How do I get the time?

   To find out the current time, create a Time object by calling the default constructor (i.e. the constructor with no arguments.) Here's an example:

   Time thisVeryInstant;          // set up a label for the object
   thisVeryInstant = new Time();  // create the actual object by
                                  //   calling the default constructor
   

2. Here's my program. It won't work. What is wrong?

   // ... lots of deleted code ...
   
   time rightNow;
   rightNow = new time();  
   
   // ... lots of deleted code ...
   

   First of all, please provide more information about what is wrong with your program when you mail it to someone for help. If it fails to build because of a syntax error, then provide us with the error message. If it runs, but then produces the wrong results or bombs altogether, then let us know the details. This way we'll be able to identify your problem faster and easier.

   In this particular program, the problem is that time is given in all lowercase. It should be Time, not time. Here's the corrected code fragment:

   Time rightNow;
   rightNow = new Time();  
   

3. Do I have to check if the user types what he is supposed to? How do I do that?

   No, you do not have to check to see if the user types the required sentence properly. This means that he can very easily fool your program into thinking he types really fast. But, right now, we are not concerned with typing accuracy, just speed.