CPSC 170 Lab 5: Exceptions

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:
   • Put the first for loop in a try.
   • Add a catch that catches the exception, but don't do anything with it.
   • Compile and run your program
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 was bad). It would be more useful for debugging if the message printed what character caused the exception. 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 print statement to specify the character that created the out of bounds index.

Placing Exception Handlers

The file ParseInts.java contains a program that does the following:

• prompts for and reads in a line of input
• uses a second Scanner to take the input line one token at a time and parses an integer from each token as it is extracted. This is different -- make sure you understand it
• sums the integers
• prints the sum

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, for example:

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 block 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:

• Modify the program to add a try statement that encompasses the entire while loop. Catch a NumberFormatException and have an empty body for the catch.
• Compile and run the program and enter a line with mixed integers and other values. You should find that it stops summing at the first non-integer, so the line above will produce a sum of 0, and the line "1 fish 2 fish" will produce a sum of 1. This is because the entire loop is inside the try, so when an exception is thrown the loop is terminated. To make it continue, move the try and catch inside the loop. Now when an exception is thrown, the next statement is the next iteration of the loop, so the entire line is processed. The dogs-and-cats input should now give a sum of 3, as should the fish input.

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:

• Modify the header of the factorial method to indicate that it can throw an IllegalArgumentException.
• Modify the body of factorial to check the value of the argument and, if it is negative, throw an IllegalArgumentException. Note that the expression that follows throw is actually an instance of the IllegalArgumentException class, and that the constructor takes a String parameter. Use this parameter to be specific about what the problem is.
• Compile and run the Factorials program after making these changes. Now when you enter a negative number an exception will be thrown, terminating the program. The program ends because the exception is not caught, so it is thrown by the main method, causing a runtime error.
• Modify the main method in your Factorials class to catch the exception thrown by factorial and print an appropriate message, but then continue with the loop. The message should include only the message that you passed with the exception, not the exception itself!

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.

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:

• Remember that you can create a Scanner from a File object, which you can create from the String representing the filename.
• The Scanner constructor that takes a File may throw a FileNotFoundException -- this is how you will know if the file does not exist. Think carefully about how to use the try/catch structure in combination with a loop that asks for a new filename if the current file does not exist.
• Remember that the scope of a variable declared inside a try block does not extend to the following code. Furthermore, the compiler knows that an initialization that occurs inside a try may or may not get executed. So any variable that you will want to use both in and after the try must be declared and initialized before the try.

Hand In: Tar your lab directory and e-mail it to your instructor with cpsc170 lab6 in the subject.