Computing Concepts with Java Essentials
Laboratory Notebook
Chapter 7 - Debugging

Cay S. Horstmann
Geof Pawlicki

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We would like to thank Dr. Leslie Foster for contributing the "Pascal Triangle" debugging exercise.

Lab Objectives

To gain experience with

P1. Unit Tests

Testing a function for run time errors in context of an ongoing project is always difficult. The source of an error may well be obscured by other functions, there could be more than one error, it might arise under unusual circumstances or even appear to go away. Testing a new function in isolation, before adding it to a project gives you an excellent opportunity to locate errors before they become lost in a broken project.

Testing in isolation is called unit testing. It requires a test stub, that is, a function main() that calls your new function with parameters obtained from 1) user input, 2) a file or 3) a function that generates parameter values.

Write two test stubs for the following function. The first test stub takes test values from prompted user input. The second test stub generates test cases randomly.

public class FindFirst
{  /**
    * Search a string for a substring
    * @param s the string to look in
    * @param sub the string to look for
    * @return if found, the integer position of sub in s
              else, -1   

   public static int findFirst(String s, String sub)
   {  int i = 0; 

      while (sub.length() + i <= s.length())
      {  if(s.substring(i, i + sub.length()) == sub)
            return i;
      return -1;

   public static void main(String[] args)
   {  String searchFor;
      String searchIn;
      boolean done = false;

      while (!done)
      {  /* 
             Your work goes here 
             (get test cases consisting of two strings for findFirst())



It is of couse tedious to type in lots of test data every time you find a bug in your code. How can you reduce that workload ?

Did you think of boundary cases? What would be boundary cases in this example?

R1. Finding a bug

Now run the following function through two test procedures (manual and automatic test case generation)

   * Searches for last occurrence of one string in another
   * @param s the string to look in
   * @param sub the string to look for
   * @return if found, the integer position of sub in s
              else, -1   
public static int findLast(String s, String sub)
{  int n = s.length; 

   while (sub.length() <= s.length())
   {  if(s.substring(s.length() - 2 - sub.length(), s.length() - 2) == sub)
         return n - s.length();
         s = s.substr(0, s.length() - 2);
   return -1;

What is the error?

Did one of your test cases catch it? Which?

R2. Selecting Test Cases

Test cases should provide complete coverage of each instruction branch in your function. Every branch, for example both statement blocks in an if - else pairing, should succeed and fail at least once each.

/* Simplified calculator to perform two variable arithmetic

public static void main(String[] args)
{  System.out.println("Enter an arithmetic expression (e.g. 2 + 3):");
   double left =;
   String op =;
   double right =;
   double answer = 0;
   if (op.equals("+"))   
   {  answer = left + right;
   else if (op.equals("-"))   
   {  answer = left - right;
   else if (op.equals("*"))   
   {  answer = left * right;
   else if (op.equals("/")   
   {  if (right != 0)
      {   answer = left / right;
      {  System.out.println("Divide by 0");
   {  System.out.println(op + " is an unknown operation.");
   System.out.println(left + " " + op + " " + right + " = " + answer);;

Give a set of test cases for left, op, and right that provide complete coverage of all branches.

Compile and run the program, then enter the test cases that you supplied. Does each branch work as expected ?

P2. Test Case Evaluation

Having tested the input to a function, how can the output be validated? You can :

Let's test the power() function from page 273

  * Compute an integral power of a number
  * @param a the number to be raised to an integral power
  * @param n the power to which it is to be raised
  * @return a to the nth power
public static double power(double a, int n)
{  double r = 1;
   double b = a;
   int i = n;
   while (i > 0)
   {  if(i % 2 == 0)   /* n is even */
      {  b = b * b;
         i = i /2;
      else             /* n is odd */
      {  r = r * b;
  return r;

  1. Write a program that uses the reciprocal Math.sqrt to test whether Math.sqrt(power(x,2)) == x
  2. If the test succeeds, how much confidence do you have that power is correct ?
  3. Use the fact that xy = ey.log(x). Write a program that computes the power function in another way, and compares the two values.
  4. (Extra credit) Which way is actually faster, power(double a, int n) or xy = ey.log(x) ?

P3. Oracles

Use the fib function in ch12/fibloop.cpp as an oracle to test whether the following function is correct:

 * Compute a Fibonacci number
 * @param n an integer
 * @return the nth Fibonacci number
public static long fibonacci(int n)
{  return (long)(Math.pow((1 + Math.sqrt(5)) / 2, n) / Math.sqrt(5) + 0.5);

public static void main(String[] args)
{  System.out.print("Enter the number of the Fibonacci number to be computed: ");
   int fibNumber =;
      your work goes here

P4. Asserting preconditions

Throw an IllegalArgumentException in the following example to signal that an improper input has occurred and to prevent the bad parameters from being utilized by the function.

import ccj.*;

public class PrecondExercise
{  /**
    * Compute seconds from midnight
    * @param hours hour of a clock time (between 0 and 23)
    * @param min minutes of a clock time (between 0 and 59)
    * @return the number of seconds between midnight and the clock time
   public static int convertToSec(int hours, int min)
   {  /* your work here -- assert that hours and min are within input range */
      return hours * 3600 + min * 60;

    * Compute the number of seconds between two clock times
    * @param hfrom the hours of the earlier time
    * @param mfrom the minutes of the earlier time
    * @param hto the hours of the later time
    * @param mto the minutes of the later time
    * @return the number of seconds between them
   public static int secondsBetween(int hfrom, int mfrom, int hto, int mto)
   {  int secs = convertToSec(hto, mto) - convertToSec(hfrom, mfrom);
      /* your work here -- assert that the from time is before the to time */
      return secs;

   public static void main(String[] args)
   {  System.out.print("Input start time: (hh mm): ");
      int startHr =;
      int startMin =;
      System.out.print("Input end time: (hh mm): ");
      int endHr =;
      int endMin =;

      System.out.println(secondsBetween(startHr, startMin, endHr, endMin)
         + " seconds elapsed");

How does your program handle

R3. Program Traces

A program trace is the result of print statements at known points in a program. It shows that the program has executed commands up to a certain point. For example, the following main() function documents the parameters to power(int a, int n).

public static void main(String[] args)
{  double answer = 0;

   System.out.print("Enter the number to be raised to a power: ");
   double a =;
   System.out.print("Enter the power to which " + a + " is to be raised: ");    
   int n =;       
   System.out.println("Before call to power(a,n), a= " + a +
      " n= " + n + " and answer = " + answer);
   answer = power(a, n);
   System.out.println("After call to power(a,n), a= " + a +
      " n= " + n + " and answer = " + answer);

Add print statements to the preceeding power(int a, int n) function to document all changes to variables r, a and n. Show the resulting trace when power is called with a = 3 and n = 11.

R4. The Debugger

The following portion of the lab is designed to have you practice some of the basics of debugging. Copy the following program listing into a new file called pastri.cpp on your hard drive.

import ccj.*;

public class Pastri
    * skip n spaces on a line
    * @param n - the number of spaces to skip

   public static void skip(int n)
   {  int i;  /* a counter */

      for (i = 0; i <= n; i++ ) 
         System.out.print(" ");

    * calculate n factorial
    * @param n calculate the factorial of n
    * @return n!

   public static int factorial(int n)
   {  int  product; /* accumulator for the running product  */
      int  i; /*  a counter  */

      product = 1;
      for (i = 1; i <= n; i++)
      {  product = product + i ;
      return product;

    * calculate the number of combinations of n things taken
    * k at a time (n choose k)
    * @param n the number of items to choose from
    * @param k the number of items choosen
    * @return  n choose k
   public static int combination(int n, int k)
   {  int comb = factorial(n) / factorial(k) * factorial(n-k);

      return comb;

   public static void main(String[] args)
   {  int nrows; /*  the number of rows to print  */
      int n; /*  a counter for the current row  */
      int k; /*  a counter for the current column  */
      int comb; /* the number of combinations  */
      int spacesToSkip = 36; /* spaces to skip at the beginning of each row */

      System.out.print("Enter the number of rows (<=13) in Pascal's triangle: ");
      nrows =;

      /*  print the title  * /
      System.out.println("THE FIRST " + nrows + "ROWS OF PASCAL'S TRIANGLE");

      / *  start a loop over the number of rows  */

      for (n = 0; n < nrows; n++)
      {  skip(spacesToSkip);                     /* space to make a triangle */
         spacesToSkip = spacesToSkip - 2;

         for (k = 0; k <= n; k++) 
         {  comb = combination(n,k) ;



The program's purpose is to display Pascal's Triangle, a sequence of integers that arises in numerous areas of math and computer science, especially combinatorics and probability. When completed, the program's output should be:

Enter the number of rows (<= 13) in Pascal's Triangle. 5


                      1   1
                    1   2   1
                  1   3   3   1
                1   4   6   4   1

Entries in Pascal's triangle are indexed by integers. n is the number of the row and k is the position from the leftmost member of the row. The indexes in both directions start at zero (0), so in the fifth row listed above, C(4,0) = 1, C(4,1) = 4, and so on.

The values themselves are computed by the formula C(n, k) = n! / ( k! (n-k)!), which is called a combination. n! denotes a factorial, that is, the product n(n-1)(n-2)...(2)(1). The combinations can be interpreted as the number of ways to choose k elements from a collection containing n elements. When described, it is customary to say "n choose k", for instance '4 choose 3 is 4' ".

If 4 objects are numbered 1 through 4, how many ways can you select 2 of them ? Show all the possible pairings here. Then compute C(4, 2) by using the formula.. Does it match the number of selections?

As you will discover with the help of your debugger, the program to compute Pascal's triangle has some mistakes. Each provides an opportunity to learn a debugging concept in context of it's use.

Compile the file

What output is displayed?

Step over / inspect

On what line is the cursor positioned now ?

What is the value for nrows shown in the dialog box?

More stepping

Does the output have the title: TABLE 1: THE FIRST 5 ROWS OF PASCAL'S TRIANGLE ?

That is strange. The program is supposed to print three newlines, then the title. Let's find out why the title doesn't appear.

What line are you on now?

When you changed the positioning of the comment delimiters, did any changes occur to the commented text? This depends on your development environment--describe what happened, if anything.


Does the title now show up on the screen?

How many rows of Pascal's triangle are displayed?

How many lines are now in the watch window?

What values of n and k are in the watch window? (If the watch window isn't present select View Watch.)

What are the values of n and k?

Now what are the values of n and k?

Step into

What value does n have in the window? (Note that k is undefined in the watch window since k is not active in the function factorial.

What value does product have in the inspect window?

Setting breakpoints

It is tedious to have to keep stepping to get inside a program. A more efficient way is to set a breakpoint at a line of interest. Then the program runs at full speed until a breakpoint is reached.

What is the value listed for comb?

Is your Pascal's Triangle correct (for nrows = 5)?


Do the results look OK when you input 13 for the number of rows?

They should now. Exit, saving any files you might wish to keep.

R5. Debugger Commands

Fill in the commands that your debugger uses for the following activities:

Explanation Your debugger
Run Continue running the debugged program, until it reaches the end or a breakpoint

Step Over Go to the next line of code, stepping over any function calls

Step Into Step into the next function call or, if none, go to the next line of code

Output Screen View the program output

Reset Reset the debugger so that the program can be run once again from the beginning

Inspect Inspect a value

Watch Add a value to a window of continuously monitored values

View Watch View the values that are currently being watched

Add Breakpoint Add a breakpoint at the current line

Toggle Breakpoint Add a breakpoint at the current line, or, if there is already a breakpoint, turn it off.

Don't forget to send your answers when you're finished.