1. CSS161: Fundamentals of Computing
Debugging
This slide set was compiled from the Absolute Java textbook slides (Walter Savitch) and the professor’s own class materials.
CSS161: Fundamentals of Computing

2. CSS161: Fundamentals of Computing
Loop Bugs
Infinite Loops
Off-by-one error: one too many or one too few loops
Solution
Use <= or >=
Use integer variables
for ( int count = 0; count != 9; count += 2 ) {
// loop body }
public class OffByOne {
public static void main( String[] args ) {
double count;
for ( count = 0.1; count <= 0.3; count += 0.1 )
System.out.println( "count = " + count );
System.out.println( "last count = " + count ); }
css161]$ java OffByOne
count = 0.1
count = 0.2
last count =
CSS161: Fundamentals of Computing

3. CSS161: Fundamentals of Computing
Tracing Variables
Calculate the sum of 1 through to 10
Output
public class Sum10 {
public static void main( String[] args ) {
int n = 10;
int sum = 10;
while ( n > 1 ) {
System.out.println( "loop begin: n = " + n + ", sum = " + sum );
sum = sum + n;
n--;
System.out.println( "loop end: n = " + n + ", sum = " + sum ); }
System.out.println( "The sum of the integers 1 to 10 is " + sum );
css161]$ java Sum10
loop begin: n = 10, sum = 10
loop end: n = 9, sum = 20
loop begin: n = 9, sum = 20
loop end: n = 8, sum = 29
loop begin: n = 8, sum = 29
loop end: n = 7, sum = 37
loop begin: n = 7, sum = 37
loop end: n = 6, sum = 44
loop begin: n = 6, sum = 44
loop end: n = 5, sum = 50
loop begin: n = 5, sum = 50
loop end: n = 4, sum = 55
loop begin: n = 4, sum = 55
loop end: n = 3, sum = 59
loop begin: n = 3, sum = 59
loop end: n = 2, sum = 62
loop begin: n = 2, sum = 62
loop end: n = 1, sum = 64
The sum of the integers 1 to 10 is 64
css161]$ java Sum10
loop begin: n = 10, sum = 10
loop end: n = 9, sum = 19
loop begin: n = 9, sum = 19
loop end: n = 8, sum = 27
loop begin: n = 8, sum = 27
loop end: n = 7, sum = 34
loop begin: n = 7, sum = 34
loop end: n = 6, sum = 40
loop begin: n = 6, sum = 40
loop end: n = 5, sum = 45
loop begin: n = 5, sum = 45
loop end: n = 4, sum = 49
loop begin: n = 4, sum = 49
loop end: n = 3, sum = 52
loop begin: n = 3, sum = 52
loop end: n = 2, sum = 54
loop begin: n = 2, sum = 54
loop end: n = 1, sum = 55
The sum of the integers 1 to 10 is 55
CSS161: Fundamentals of Computing

4. General Debugging Techniques
Examine the system as a whole – don’t assume the bug occurs in one particular place
Try different test cases and check the input values
Comment out blocks of code to narrow down the offending code
Check common pitfalls
Take a break and come back later
DO NOT make random changes just hoping that the change will fix the problem!
CSS161: Fundamentals of Computing

5. Debugging Example (1 of 10)
Code to get a user input until either ‘a’ or ‘b’ is entered.
Compilation error:
import java.util.Scanner;
public class DebugExample {
public static void main( String[] args ) {
String s = "";
char c = ' ';
Scanner keyboard = new Scanner( System.in );
do {
System.out.println("Enter 'A' for option A or 'B' for option B.");
s = keyboard.next( );
s.toLowerCase( );
c = s.substring( 0, 1 ); }
while ( ( c != 'a' ) || ( c != 'b' ) );
css161]$ javac DebugExample.java
DebugExample.java:13: incompatible types
found : java.lang.String
required: char
c = s.substring( 0, 1 ); ^
1 error
CSS161: Fundamentals of Computing

6. Debugging Example (2 of 10)
Use the “random change” technique to the date type of c to String:
Receive more errors:
import java.util.Scanner;
public class DebugExample {
public static void main( String[] args ) {
String s = "";
String c = ' ';
Scanner keyboard = new Scanner( System.in );
do {
System.out.println("Enter 'A' for option A or 'B' for option B.");
s = keyboard.next( );
s.toLowerCase( );
c = s.substring( 0, 1 ); }
while ( ( c != 'a' ) || ( c != 'b' ) );
css161]$ javac DebugExample.java
DebugExample.java:6: incompatible types
found : char
required: java.lang.String
String c = ' '; ^
DebugExample.java:15: incomparable types: java.lang.String and char
while ( ( c != 'a' ) || ( c != 'b' ) );
3 errors
CSS161: Fundamentals of Computing

7. Debugging Example (3 of 10)
Compiler errors eliminated by s.charAt( 0 )
The program now stuck in an infinite loop
import java.util.Scanner;
public class DebugExample {
public static void main( String[] args ) {
String s = "";
char c = ' ';
Scanner keyboard = new Scanner( System.in );
do {
System.out.println("Enter 'A' for option A or 'B' for option B.");
s = keyboard.next( );
s.toLowerCase( );
c = s.charAt( 0 ); }
while ( ( c != 'a' ) || ( c != 'b' ) );
css161]$ javac DebugExample.java
css161]$ java DebugExample
Enter 'A' for option A or 'B' for option B. C B A
(Control-C)
CSS161: Fundamentals of Computing

8. Debugging Example (4 of 10)
Employ tracing variables.
String S is never changed to lowercase.
import java.util.Scanner;
public class DebugExample {
public static void main( String[] args ) {
String s = "";
char c = ' ';
Scanner keyboard = new Scanner( System.in );
do {
System.out.println("Enter 'A' for option A or 'B' for option B.");
s = keyboard.next( );
System.out.println( "String s = " + s );
s.toLowerCase( );
System.out.println( "Lowercase s = " + s );
c = s.charAt( 0 );
System.out.println( "c = " + c ); }
while ( ( c != 'a' ) || ( c != 'b' ) );
css161]$ java DebugExample
Enter 'A' for option A or 'B' for option B. A
String s = A
Lowercase s = A
c = A
CSS161: Fundamentals of Computing

9. Debugging Example (5 of 10)
toLowerCase( ) returns a lowercase string
While c can now receives ‘a’ or ‘b’, the program can’t get out of the loop.
do {
System.out.println("Enter 'A' for option A or 'B' for option B.");
s = keyboard.next( );
System.out.println( "String s = " + s );
s = s.toLowerCase( );
System.out.println( "Lowercase s = " + s );
c = s.charAt( 0 );
System.out.println( "c = " + c ); }
while ( ( c != 'a' ) || ( c != 'b' ) );
css161]$ java DebugExample
Enter 'A' for option A or 'B' for option B. A
String s = A
Lowercase s = a
c = a B
String s = B
Lowercase s = b
c = b
CSS161: Fundamentals of Computing

10. Debugging Example (6 of 10)
The following patch works but makes the Boolean expression meaningless.
import java.util.Scanner;
public class DebugExample {
public static void main( String[] args ) {
String s = "";
char c = ' ';
Scanner keyboard = new Scanner( System.in );
do {
System.out.println("Enter 'A' for option A or 'B' for option B.");
s = keyboard.next( );
System.out.println( "String s = " + s );
s = s.toLowerCase( );
System.out.println( "Lowercase s = " + s );
c = s.charAt( 0 );
System.out.println( "c = " + c );
if ( c == 'a' )
break;
if ( c == 'c' ) }
while ( ( c != 'a' ) || ( c != 'b' ) );
Then, this could be while ( true )
css161]$ java DebugExample
Enter 'A' for option A or 'B' for option B. A
String s = A
Lowercase s = a
c = a
css161]$
CSS161: Fundamentals of Computing

11. Debugging Example (7 of 10)
Dissect the Boolean expression
|| is always true!
public class DebugExample {
public static void main( String[] args ) {
String s = "";
char c = ' ';
Scanner keyboard = new Scanner( System.in );
do {
System.out.println("Enter 'A' for option A or 'B' for option B.");
s = keyboard.next( );
s = s.toLowerCase( );
c = s.charAt( 0 );
System.out.println( "c = " + c );
System.out.println( "c != 'a' is " + ( c != 'a' ) );
System.out.println( "c != 'b' is " + ( c != 'b' ) );
System.out.println( "( c != 'a' ) || ( c != 'b' ) is "
+ ( ( c != 'a' ) || ( c != 'b' ) ) ); }
while ( ( c != 'a' ) || ( c != 'b' ) );
css161]$ java DebugExample
Enter 'A' for option A or 'B' for option B. A
c = a
c != 'a' is false
c != 'b' is true
( c != 'a' ) || ( c != 'b' ) is true
CSS161: Fundamentals of Computing

12. Debugging Example (8 of 10)
Recall the || and && operators’ truth table
LHS
(c != ‘a’)
RHS
(c != ‘b’) ||
(c != ‘a’) || (c != ‘b’)
&&
(c != ‘a’) && (c != ‘b’)
False
True
CSS161: Fundamentals of Computing

13. Debugging Example (9 of 10)
Replace || with &&
Get a correct result
public class DebugExample {
public static void main( String[] args ) {
String s = "";
char c = ' ';
Scanner keyboard = new Scanner( System.in );
do {
System.out.println("Enter 'A' for option A or 'B' for option B.");
s = keyboard.next( );
s = s.toLowerCase( );
c = s.charAt( 0 );
System.out.println( "c = " + c );
System.out.println( "c != 'a' is " + ( c != 'a' ) );
System.out.println( "c != 'b' is " + ( c != 'b' ) );
System.out.println( "( c != 'a' ) && ( c != 'b' ) is "
+ ( ( c != 'a' ) && ( c != 'b' ) ) ); }
while ( ( c != 'a' ) && ( c != 'b' ) );
css161]$ java DebugExample
Enter 'A' for option A or 'B' for option B. A
c = a
c != 'a' is false
c != 'b' is true
( c != 'a' ) && ( c != 'b' ) is false
css161]$
CSS161: Fundamentals of Computing

14. Debugging Example (10 of 10)
Use a Boolean variable with a meaningful name to control the do-while loop.
import java.util.Scanner;
public class DebugExample {
public static void main( String[] args ) {
String s = "";
char c = ' ';
Scanner keyboard = new Scanner( System.in );
boolean invalidKey;
do {
System.out.println("Enter 'A' for option A or 'B' for option B.");
s = keyboard.next( );
s = s.toLowerCase( );
c = s.charAt( 0 );
if ( c == 'a' )
invalidKey = false;
else if ( c == 'b' )
else
invalidKey = true; }
while ( invalidKey );
CSS161: Fundamentals of Computing

15. CSS161: Fundamentals of Computing
Assertion Checks
Syntax
assert Expression1 : Expression2;
where
Expression1 is a Boolean expression.
Expression2 is an expression that has a value
Usage
Place an assertion to check the state of your program, (i.e., conditions, invariants)
For more usage:
if ( n % 3 == 0 ) {
...;
} else if ( n % 3 == 1 ) {
} else {
assert n % 3 == 2 : n; // we know n % 3 == 2 }
CSS161: Fundamentals of Computing

16. CSS161: Fundamentals of Computing
Preventive Coding
Incremental Development
Write a little bit of code at a time and test it before moving on
Code Review
Have others look at your code
Pair Programming
Programming in a team, one typing while the other watches, and periodically switch roles
CSS161: Fundamentals of Computing

17. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on Textbook p153’s exercises 41 ~ 43.
CSS161: Fundamentals of Computing