Algorithms and OOD (CSC 207 2013F) : Labs

Laboratory: Exceptions

Summary: In this laboratory, you will begin working with exceptions in Java.


Fork and clone the repository at (You don't really need to fork the repo, but if you don't you can't save your work.)

Start Eclipse.

Import the repository into Eclipse.

Quickly skim the code to see what the purpose is.


Exercise 1: A Reminder About I/O

Run to verify that it works as you might expect (except for very large positive or negative numbers).

Early in the main method for, you should see a line that reads as follows.

PrintWriter pen = new PrintWriter(System.out, true);

PrintWriter provides a constructor that doesn't require the boolean. Find out what happens if you just use

PrintWriter pen = new PrintWriter(System.out);

Then explain the difference in output (if you observe one).

Exercise 2: Exception Basics

a. Remove the throws Exception from the main method and determine what, if any, error messages you get. If Eclipse permits you to do so, try running the program.

After you finish exploring the effects of that removal, reinsert the throws warning.

b. Remove the throws Exception from the definition of readInt and determine what, if any, error messages you get. If Eclipse permits you to do so, try running the program with that warning removed.

Do not reinsert the warning.

c. Within readInt, you should have a sequence of lines (or perhaps a single line) that does something like the following:

String response = br.readLine();
return Integer.parseInt(response);

Enclose those lines in a try/catch clause that returns 0 if an exception is thrown, as in

  try {
      String response = br.readLine();
      return Integer.parseInt(response);
  } catch (Exception e) {
      return 0;

Verify that Eclipse is no longer concerned about errors in your code.

Determine what happens when the user enters an invalid value (such as Hello) in response to a request for an integer.

Exercise 3: Computing A Quadratic Root

To the MathUtils class, add a static smallQuadraticRoot(double a, double b, double c) method that computes the smaller of the two roots of a quadratic expression.

Note that you can use the following formula to compute that root:

(-b - sqrt(b2 - 4ac))/2a

You will, of course, have to translate that mathematical expression into Java code.

b. Write a few simple unit tests for that procedure. Note that you are likely to find it easier of you choose quadratics for which you know the solution.

  • If we multiply x-2 and x+5, we get x2+3-10, we know the roots are 2 and -5.
  • If we multiply 2x+3) and 3x-4), giving us 6x2+x-12, we know that the roots are -3/2 and 4/3.

Exercise 4: Adding User Input

a. Write a new main class, QR, which

  • prompts the user for coefficients of a quadratic function,
  • computes one root of the quadratic,
  • prints that root, and
  • prints the value of the quadratic at that root.

b. Use your class to compute a root of x2-x-2. (The roots of that quadratic are 2 and -1.)

Exercise 5: Erroneous Input

a. Determine what happens if the user enters 0 for the coefficient of x2.

b. Determine what happens if user enters values for which there is no real root.

c. Determine what happens if the user enters values for which the function has only one root (e.g., x2-2x+1 has only one root)?

Exercise 6: Indicating Potential Errors

a. Extend smallerQuadraticRoot to indicate that it may throw an exception. Note that you'll need to change the method head for smallerQuadraticRoot to something like the following

public static double smallerQuadraticRoot(double a, double b, double c)
         throws Exception

b. Can you successfully compile your modified code? If not, make any changes necessary to permit you to compile it.

c. Can you successfully compile QR? If not, get help from a tutor or faculty member.

d. What now happens if you enter the “erroneous” input described in the previous exercise?

Exercise 7: Throwing Exceptions

a. Extend smallerQuadraticRoot so that it throws an exception if a is 0. For example,

if (a == 0) {
    throw new Exception("Cannot compute quadratic roots of linear functions.");
} // if (a == 0)

b. Extend smallerQuadraticRoot so that it throws an exception if the root is not real (i.e., if it has an imaginary component). Note that the root is not real if the thing you're taking a square root of is negative.

c. What now happens if you enter the “erroneous” input described above?

Exercise 8: Catching Exceptions

If you've written your main method using the recommended template, you have the line throws Exception in the head of that method. Remove that line.

a. What effect do you expect removing that line will have?

b. Verify your answer experimentally. Ask a tutor or teacher if you don't understand the results of your experiment.

c. Enclose your call to smallerQuadraticRoot in a try/catch block. For example,

try {
    double root = f.smallerQuadraticRoot(a,b,c);
    pen.println("The smaller root of the polynomial is: " + root);
    pen.println("Experimentally, " + a + "*" + root + "*" + root
            + "+" + b + "*" + root + "+" + c + " = " 
            + (a*root*root + b*root + c));
} catch (Exception e) {
    pen.println("Sorry, I could not compute a root.");
} // try/catch

d. Determine what happens with the problematic inputs described above.

Exercise 9: Specific Exceptions

a. Update smallerQuadraticRoot so that it tries to throw a DivideByZeroException if the coefficient of the quadratic term is 0. You can still throw a generic exception if the result includes an imaginary component.

b. What do you expect to happen when you try to compile the revised program?

c. Verify your answer experimentally.

Exercise 10: Your Own Exceptions

As you should have determined in the previous exercise, Java does not know by default what a DivideByZeroException is. Hence, you'll need to create your own Exception. You do so using the strategy described in the reading.

a. Create and compile a Java file for DivideByZeroException.

b. Verify that the previously-modified code now works.

Exercise 11: Catching Specific Exceptions

a. Extend QR so that it has a catch clause for your new DivideByZeroException before the catch clause for the generic Exception. For example,

try {
} catch (DivideByZeroException dbze) {
    pen.println("Cannot compute a result because the coefficient of the quadratic term is 0.");
} catch (Exception e) {
} // try/catch

b. Determine what happens in each of the problematic cases.

c. What do your results for this problem suggest?

Copyright (c) 2013 Samuel A. Rebelsky.

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