CPSC 170
Exceptions and File I/O

Exceptions Aren't Always Errors

The file CountLetters.java contains a program that reads a word from the user and prints the number of occurrences of each letter in the word. Save it to your directory and study it, then compile and run it to see how it works. In reading the code, note that the word is converted to all upper case first, then each letter is translated to a number in the range 0..25 (by subtracting 'A') for use as an index. No test is done to ensure that the characters are in fact letters.
  1. Run CountLetters and enter a phrase, that is, more than one word with spaces or other punctuation in between. At the end of the first word, this program will throw an ArrayIndexOutOfBoundsException, because a non-letter will generate an index that is not between 0 and 25.  Of course, you could explicitly test the value of the character to see if it is between 'A' and 'Z'. However, an alternative is to treat  the translated character as an index, and catch the ArrayIndexOutOfBoundsException when it occurs. When a non-letter appears, it is effectively the end of the word; you want to ignore the rest of the phrase and display the count. Since your only response to a non-letter is to stop processing, the handler will be empty. Modify this method to do this as follows:

  2. Now modify the body of the catch so that it prints a useful message (e.g., "Not a letter") followed by the exception message  Remember that an exception is an object (which happens to have a toString method!) Compile and run the program.

  3. Although it's useful to print the exception for debugging, when you're trying to smoothly handle a condition that you don't consider erroneous you often don't want to display the exception explicitly. Notice that when the exception was printed it told you generally what the exception was and then there was a more specific message(i.e.which index that was bad).  You can often extract the specifics of an exception using a method from the exception object. Look at the online Java documentation for IndexOutOfBoundsException to see how to extract these details. (Hint: this class inherits its interesting methods from its superclasses, so you might need to look at some of those to find a useful method) Use this information to change the 2nd print statement to only display the character that created the out of bounds index. 
Print a copy of CountLetters.java to turn in.

Placing Exception Handlers

The file ParseInts.java contains a program that does the following: Save ParseInts to your directory, compile and run it. If you give it the input:

            10 20 30 40

It should print
            The sum of the integers on the line is 100.

Now try a line that contains both integers and other values, e.g:

            We have 2 dogs and 1 cat.

You will get a NumberFormatException when it tries to call Integer.parseInt on "We", which is not an integer. One way around this is to put the loop that reads inside a try and catch the NumberFormatException but not do anything with it. This way if it's not an integer it doesn't cause an error; it goes to the exception handler, which does nothing. Do this as follows:  Print a copy of ParseInts.java to turn in.

Throwing Exceptions

The file Factorials.java contains a program that calls the factorial method of the MathUtils.java class to compute the factorials of integers entered by the user. Save these files to your directory and study the code in both, then compile and run Factorials to see how it works. Try several positive integers, then try a negative number. You should find that it works for small positive integers (values < 17), but that it returns a large negative value for larger integers and that it always returns 1 for negative integers. Returning 1 as the factorial of any negative integer is not correct mathematically, the factorial function is not defined for negative integers. To correct this, you could modify your factorial method to check if the argument is negative, but then what? The method must return a value, and even if it prints an error message, whatever value is returned could be misconstrued. Instead it should throw an exception indicating that something went wrong so it could not complete its calculation. You could define your own exception class, but there is already an exception appropriate for this situation IllegalArgumentException, which extends RuntimeException. Modify your program as follows: Returning a negative number for values over 16 also is not correct. The problem is arithmetic overflow the factorial is bigger than can be represented by an int. This can also be thought of as an IllegalArgumentException this factorial method is only defined for arguments up to 16. Modify your code in factorial to check for an argument over 16 as well as for a negative argument. You should throw an IllegalArgumentException in either case, but pass different messages to the constructor so that the problem is clear.

Print a copy of Factorial.java and MathUtils.java to turn in.

Reading a User-Selected File

Write a program that prompts the user for a filename, then opens a Scanner to the file and copies it, a line at a time, to the standard output. If the user enters the name of a file that does not exist, ask for another name until you get one that refers to a valid file. Some things to consider: Print a copy of your program  to turn in.

Reading from and Writing to Text Files

Write a program that will read in a file of student academic credit data and create a list of students on academic warning. The list of students on warning will be written to a file. Each line of the input file will contain the student name (a single String with no spaces), the number of semester hours earned (an integer), the total quality points earned (a double). The following shows part of a typical data file:

      Smith  27 83.7

      Jones  21 28.35

      Walker 96 182.4

      Doe    60 150

The program should compute the GPA (grade point or quality point average) for each student (the total quality points divided by the number of semester hours) then write the student information to the output file if that student should be put on academic warning. A student will be on warning if he/she has a GPA less than 1.5 for students with fewer than 30 semester hours credit, 1.75 for students with fewer than 60 semester hours credit, and 2.0 for all other students.

The file Warning.java contains a skeleton of the program. Do the following:

  1. Set up a Scanner object scan from the input file and a PrintWriter outFile to the output file inside the try clause (see the comments in the program). Note that you ll have to create the PrintWriter from a FileWriter, but you can still do it in a single statement.

  2. Inside the while loop add code to read and parse the input get the name, the number of credit hours, and the number of quality points. Compute the GPA, determine if the student is on academic warning, and if so write the name, credit hours, and GPA (separated by spaces) to the output file.

  3. After the loop close the PrintWriter

  4. Think about the exceptions that could be thrown by this program:
    Add a catch for each of these situations, and in each case give as specific a message as you can. The program will terminate if any of these exceptions is thrown, but at least you can supply the user with useful information.

  5. Test the program. Test data is in the file a students.dat  Be sure to test each of the exceptions as well.
Print a copy of your program  to turn in.