Transcript Review Slides 2

Review 2
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Constants: final
• A final variable is a constant
• Once its value has been set, it cannot be changed
• Named constants make programs easier to read and maintain
• Convention: use all-uppercase names for constants
final double QUARTER_VALUE = 0.25;
final double DIME_VALUE = 0.1;
final double NICKEL_VALUE = 0.05;
final double PENNY_VALUE = 0.01;
payment = dollars + quarters * QUARTER_VALUE
+ dimes * DIME_VALUE + nickels * NICKEL_VALUE
+ pennies * PENNY_VALUE;
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Constants: static final
• If constant values are needed in several methods, declare them
together with the instance fields of a class and tag them as
static and final
• Give static final constants public access to enable other
classes to use them
public class Math
{
. . .
public static final double E = 2.7182818284590452354;
public static final double PI = 3.14159265358979323846;
}
double circumference = Math.PI * diameter;
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Syntax 4.2 Constant Definition
In a method:
final typeName variableName = expression;
In a class:
accessSpecifier static final typeName variableName =
expression;
Example:
final double NICKEL_VALUE = 0.05; public static final
double LITERS_PER_GALLON = 3.785;
Purpose:
To define a constant in a method or a class.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Assignment, Increment, and Decrement
• Assignment is not the same as mathematical equality:
items = items + 1;
• items++ is the same as items = items + 1
• items-- subtracts 1 from items
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Assignment, Increment, and Decrement
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Arithmetic Operations
• / is the division operator
• If both arguments are integers, the result is an integer. The
remainder is discarded
• 7.0 / 4 yields 1.75
7 / 4 yields 1
• Get the remainder with % (pronounced "modulo")
7 % 4 is 3
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Arithmetic Operations
final
final
final
final
int
int
int
int
PENNIES_PER_NICKEL = 5;
PENNIES_PER_DIME = 10;
PENNIES_PER_QUARTER = 25;
PENNIES_PER_DOLLAR = 100;
// Compute total value in pennies
int total = dollars * PENNIES_PER_DOLLAR + quarters *
PENNIES_PER_QUARTER + nickels * PENNIES_PER_NICKEL +
dimes * PENNIES_PER_DIME + pennies;
// Use integer division to convert to dollars, cents
int dollars = total / PENNIES_PER_DOLLAR;
int cents = total % PENNIES_PER_DOLLAR;
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
The Math class
• Math class: contains methods like sqrt and pow
• To compute xn, you write Math.pow(x, n)
• However, to compute x2 it is significantly more efficient simply
to compute x * x
• To take the square root of a number, use the Math.sqrt; for
example, Math.sqrt(x)
• In Java,
can be represented as
(-b + Math.sqrt(b * b - 4 * a * c)) / (2 * a)
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Self Check 4.8
What is the value of 1729 / 100? Of 1729 % 100?
Answer: 17 and 29
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Syntax 4.3 Static Method Call
ClassName.methodName(parameters)
Example:
Math.sqrt(4)
Purpose:
To invoke a static method (a method that does not operate on an
object) and supply its parameters.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Self Check 4.11
Why can't you call x.pow(y) to compute xy?
Answer: x is a number, not an object, and you cannot invoke
methods on numbers.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Self Check 4.12
Is the call System.out.println(4) a static method call?
Answer: No – the println method is called on the object
System.out.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Strings
• A string is a sequence of characters
• Strings are objects of the String class
• String constants:
"Hello, World!"
• String variables:
String message = "Hello, World!";
• String length:
int n = message.length();
• Empty string: ""
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Concatenation
• Use the + operator:
String name = "Dave";
String message = "Hello, " + name; // message is "Hello,
Dave"
• If one of the arguments of the + operator is a string, the other is
converted to a string
String a = "Agent"; int n = 7; String bond = a + n; //
bond is "Agent7"
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Concatenation in Print Statements
• Useful to reduce the number of System.out.print instructions
System.out.print("The total is ");
System.out.println(total);
versus
System.out.println("The total is " + total);
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Substrings
• String greeting = "Hello, World!";
String sub = greeting.substring(0, 5); // sub is "Hello"
• Supply start and “past the end” position
• First position is at 0
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Substrings (cont.)
Substring length is “past the end” - start
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Self Check 4.13
Assuming the String variable s holds the value "Agent", what is
the effect of the assignment s = s + s.length()?
Answer: s is set to the string Agent5
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Reading Input
• System.in has minimal set of features–it can only read one byte
at a time
• In Java 5.0, Scanner class was added to read keyboard input in
a convenient manner
• Scanner in = new Scanner(System.in);
System.out.print("Enter quantity:");
int quantity = in.nextInt();
• nextDouble reads a double
• nextLine reads a line (until user hits Enter)
• nextWord reads a word (until any white space)
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch04/cashregister/CashRegisterSimulator.java
01: import java.util.Scanner;
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03: /**
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This program simulates a transaction in which a user pays for an
item
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and receives change.
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public static void main(String[] args)
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{
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Scanner in = new Scanner(System.in);
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CashRegister register = new CashRegister();
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System.out.print("Enter price: ");
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double price = in.nextDouble();
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register.recordPurchase(price);
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System.out.print("Enter dollars: ");
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int dollars = in.nextInt();
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch04/cashregister/CashRegisterSimulator.java (cont.)
Output:
Enter price: 7.55
Enter dollars: 10
Enter quarters: 2
Enter dimes: 1
Enter nickels: 0
Enter pennies: 0
Your change: is 3.05
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
The if Statement
• The if statement lets a program carry out different
actions depending on a condition
If (amount <= balance)
balance = balance – amount;
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
The if/else Statement
If (amount <= balance)
balance = balance – amount;
else
balance = balance – OVERDRAFT_PENALTY
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Statement Types
• Simple statement
balance = balance - amount;
• Compound statement
if (balance >= amount) balance = balance - amount;
Also
while, for, etc. (loop statements – Chapter 6)
• Block statement
{
double newBalance = balance - amount;
balance = newBalance;
}
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Syntax 5.1 The if Statement
if(condition)
statement
if (condition)
statement1
else
Example:
if (amount
balance
if (amount
balance
else
<= balance)
= balance - amount;
<= balance)
= balance - amount;
Purpose:
To execute a statement when a condition is true or false.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Syntax 5.2 Block Statement
{
statement1
statement2
. . .
}
Example:
{
double newBalance = balance - amount;
balance = newBalance;
}
Purpose:
To group several statements together to form a single
statement.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Self Check 5.2
What is logically wrong with the statement
if (amount <= balance)
newBalance = balance - amount;
balance = newBalance;
and how do you fix it?
Answer: Only the first assignment statement is part of the
if
statement. Use braces to group both assignment
statements
into a block statement.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Comparing Strings
• Don't use == for strings!
if (input == "Y") // WRONG!!!
• Use equals method:
if (input.equals("Y"))
• == tests identity, equals tests equal contents
• Case insensitive test ("Y" or "y")
if (input.equalsIgnoreCase("Y"))
• s.compareTo(t) < 0 means:
s comes before t in the dictionary
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Comparing Strings (cont.)
• "car" comes before "cargo"
• All uppercase letters come before lowercase:
"Hello" comes before "car"
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Nested Branches (cont.)
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Using Boolean Expressions: The Boolean Operators
• &&
and
• ||
or
•!
not
• if (0 < amount && amount < 1000) . . .
• if (input.equals("S") || input.equals("M")) . . .
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
&& and || Operators
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Truth Tables
A
B
A && B
true
true
true
true
false
false
false
Any
false
A
B
A || B
true
Any
true
false
true
true
false
false
false
A
!A
true
false
false
true
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
while Loops
• Executes a block of code repeatedly
• A condition controls how often the loop is executed
while (condition)
statement
• Most commonly, the statement is a block statement (set of
statements delimited by { })
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Calculating the Growth of an Investment
• Invest $10,000, 5% interest, compounded
annually
Year
Balance
0
$10,000
1
$10,500
2
$11,025
3
$11,576.25
4
$12,155.06
5
$12,762.82
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
while Loop Flowchart
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Syntax 6.1 The while Statement
while (condition)
statement
Example:
while (balance < targetBalance)
{
years++;
double interest = balance * rate / 100;
balance = balance + interest;
}
Purpose:
To repeatedly execute a statement as long as a condition is
true.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Common Error: Infinite Loops
• int years = 0;
while (years < 20)
{
double interest = balance * rate / 100;
balance = balance + interest;
}
• int years = 20;
while (years > 0)
{
years++; // Oops, should have been years–
double interest = balance * rate / 100;
balance = balance + interest;
}
• Loops run forever – must kill program
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
do Loops
• Executes loop body at least once:
do
statement
while (condition);
• Example: Validate input
double value;
do
{
System.out.print("Please enter a positive number: ");
value = in.nextDouble();
}
while (value <= 0);
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
do Loops (cont.)
• Alternative:
boolean done = false;
while (!done)
{
System.out.print("Please enter a positive number: ");
value = in.nextDouble();
if (value > 0) done = true;
}
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
do Loop Flowchart
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Spaghetti Code
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
for Loops
• for (initialization; condition; update)
statement
• Example:
for (int i = 1; i <= n; i++)
{
double interest = balance * rate / 100;
balance = balance + interest;
}
• Equivalent to
initialization;
while (condition)
{ statement;
update; }
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
for Loops (cont.)
• Other examples:
for (years = n; years > 0; years--) . . .
for (x = -10; x <= 10; x = x + 0.5) . . .
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
for Loop Flowchart
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Syntax 6.2 The for Statement
for (initialization; condition; update)
statement
Example:
for (int i = 1; i <= n; i++)
{
double interest = balance * rate / 100;
balance = balance + interest;
}
Purpose:
To execute an initialization, then keep executing a statement
and updating an expression while a condition is true.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Processing Sentinel Values
• Sentinel value: Can be used for indicating the end of a data
set
• 0 or -1 make poor sentinels; better use Q
System.out.print("Enter value, Q to quit: ");
String input = in.next();
if (input.equalsIgnoreCase("Q"))
We are done
else
{
double x = Double.parseDouble(input);
. . .
}
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Loop and a half
• Sometimes termination condition of a loop can only be
evaluated in the middle of the loop
• Then, introduce a boolean variable to control the loop:
boolean done = false;
while (!done)
{
Print prompt
String input = read input;
if (end of input indicated)
done = true;
else
{
Process input
}
}
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Random Numbers and Simulations
• In a simulation, you repeatedly generate random numbers
and use them to simulate an activity
• Random number generator
Random generator = new Random(); int n =
generator.nextInt(a); // 0 < = n < a double x =
generator.nextDouble(); // 0 <= x < 1
• Throw die (random number between 1 and 6)
int d = 1 + generator.nextInt(6);
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch06/random1/Die.java
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import java.util.Random;
/**
This class models a die that, when cast, lands on a random
face.
*/
public class Die
{
/**
Constructs a die with a given number of sides.
@param s the number of sides, e.g. 6 for a normal die
*/
public Die(int s)
{
sides = s;
generator = new Random();
}
/**
Simulates a throw of the die
@return the face of the die
Continued
*/
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch06/random1/Die.java (cont.)
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public int cast()
{
return 1 + generator.nextInt(sides);
}
private Random generator;
private int sides;
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch06/random1/DieSimulator.java
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/**
This program simulates casting a die ten times.
*/
public class DieSimulator
{
public static void main(String[] args)
{
Die d = new Die(6);
final int TRIES = 10;
for (int i = 1; i <= TRIES; i++)
{
int n = d.cast();
System.out.print(n + " ");
}
System.out.println();
}
}
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch06/random1/DieSimulator.java (cont.)
Output:
6 5 6 3 2 6 3 4 4 1
Second Run:
3 2 2 1 6 5 3 4 1 2
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Self Check 6.9
How do you use a random number generator to simulate the
toss of a coin?
Answer: int n = generator.nextInt(2); // 0 = heads,
1 = tails
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Using a Debugger
• Debugger = program to run your program and analyze its
run-time behavior
• A debugger lets you stop and restart your program, see
contents of variables, and step through it
• The larger your programs, the harder to debug them simply
by inserting print commands
• Debuggers can be part of your IDE (e.g. Eclipse, BlueJ) or
separate programs (e.g. JSwat)
• Three key concepts:
• Breakpoints
• Single-stepping
• Inspecting variables
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Debugging
• Execution is suspended whenever a breakpoint is reached
• In a debugger, a program runs at full speed until it reaches
a breakpoint
• When execution stops you can:
• Inspect variables
• Step through the program a line at a time
• Or, continue running the program at full speed until it reaches the
next breakpoint
• When program terminates, debugger stops as well
• Breakpoints stay active until you remove them
• Two variations of single-step command:
• Step Over: skips method calls
• Step Into: steps inside method calls
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Single-step Example
Current line:
String input = in.next();
Word w = new Word(input);
int syllables = w.countSyllables();
System.out.println("Syllables in " + input + ": " +
syllables);
When you step over method calls, you get to the next line:
String input = in.next();
Word w = new Word(input);
int syllables = w.countSyllables();
System.out.println("Syllables in " + input + ": " +
syllables);
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Single-step Example (cont.)
However, if you step into method calls, you enter the first line
of the countSyllables method
public
{
int
int
. .
}
int countSyllables()
count = 0;
end = text.length() - 1;
.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Self Check 6.11
In the debugger, you are reaching a call to
System.out.println. Should you step into the method or step
over it?
Answer: You should step over it because you are not
interested
in debugging the internals of the println method.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Self Check 6.12
In the debugger, you are reaching the beginning of a long
method with a couple of loops inside. You want to find out
the return value that is computed at the end of the method.
Should you set a breakpoint, or should you step through the
method?
Answer: You should set a breakpoint. Stepping through
loops
can be tedious.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch06/debugger/Word.java
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/**
This class describes words in a document.
*/
public class Word
{
/**
Constructs a word by removing leading and trailing nonletter characters, such as punctuation marks.
@param s the input string
*/
public Word(String s)
{
int i = 0;
while (i < s.length() && !Character.isLetter(s.charAt(i)))
i++;
int j = s.length() - 1;
while (j > i && !Character.isLetter(s.charAt(j)))
j--;
text = s.substring(i, j);
}
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch06/debugger/Word.java (cont.)
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/**
Returns the text of the word, after removal of the
leading and trailing non-letter characters.
@return the text of the word
*/
public String getText()
{
return text;
}
/**
Counts the syllables in the word.
@return the syllable count
*/
public int countSyllables()
{
int count = 0;
int end = text.length() - 1;
if (end < 0) return 0; // The empty string has no syllables
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch06/debugger/Word.java (cont.)
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// An e at the end of the word doesn't count as a vowel
char ch = Character.toLowerCase(text.charAt(end));
if (ch == 'e') end--;
boolean insideVowelGroup = false;
for (int i = 0; i <= end; i++)
{
ch = Character.toLowerCase(text.charAt(i));
String vowels = "aeiouy";
if (vowels.indexOf(ch) >= 0)
{
// ch is a vowel
if (!insideVowelGroup)
{
// Start of new vowel group
count++;
insideVowelGroup = true;
}
}
}
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch06/debugger/Word.java (cont.)
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// Every word has at least one syllable
if (count == 0)
count = 1;
return count;
}
private String text;
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch06/debugger/SyllableCounter.java
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import java.util.Scanner;
/**
This program counts the syllables of all words in a sentence.
*/
public class SyllableCounter
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
System.out.println("Enter a sentence ending in a period.");
String input;
do
{
input = in.next();
Word w = new Word(input);
int syllables = w.countSyllables();
System.out.println("Syllables in " + input + ": "
+ syllables);
Continued
}
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
ch06/debugger/SyllableCounter.java (cont.)
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25: }
while (!input.endsWith("."));
}
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Debug the Program
• Buggy output (for input "hello yellow peach."):
Syllables in hello: 1
Syllables in yellow: 1
Syllables in peach.: 1
• Set breakpoint in first line of countSyllables of Word class
• Start program, supply input. Program stops at breakpoint
• Method checks if final letter is 'e'
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Debug the Program (cont.)
• Check if this works: step to line where check is made and
inspect variable ch
• Should contain final letter but contains 'l'
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
More Problems Found
• end is set to 3, not 4
• text contains "hell", not "hello"
• No wonder countSyllables returns 1
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
More Problems Found (cont.)
• Culprit is elsewhere
• Can't go back in time
• Restart and set breakpoint in Word constructor
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Debugging the Word Constructor
• Supply "hello" input again
• Break past the end of second loop in constructor
• Inspect i and j
• They are 0 and 4 – makes sense since the input consists of
letters
• Why is text set to "hell"?
• Off-by-one error: Second parameter of substring is the first
position not to include
text = substring(i, j);
should be
text = substring(i, j + 1);
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Debugging the Word Constructor
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Another Error
• Fix the error
• Recompile
• Test again:
Syllables in hello: 1
Syllables in yellow: 1
Syllables in peach.: 1
• Oh no, it's still not right
• Start debugger
• Erase all old breakpoints and set a breakpoint in
countSyllables method
• Supply input "hello."
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Debugging countSyllables (again)
Break in the beginning of countSyllables. Then, single-step
through loop
boolean insideVowelGroup = false;
for (int i = 0; i <= end; i++)
{
ch = Character.toLowerCase(text.charAt(i));
if ("aeiouy".indexOf(ch) >= 0)
{
// ch is a vowel
if (!insideVowelGroup)
{
// Start of new vowel group
count++;
insideVowelGroup = true;
}
}
}
Continued
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Debugging countSyllables (again)
• First iteration ('h'): skips test for vowel
• Second iteration ('e'): passes test, increments count
• Third iteration ('l'): skips test
• Fifth iteration ('o'): passes test, but second if is skipped,
and count is not incremented
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Fixing the Bug
• insideVowelGroup was never reset to false
• Fix
if ("aeiouy".indexOf(ch) >= 0)
{
. . .
}
else insideVowelGroup = false;
• Retest: All test cases pass
Syllables in hello: 2
Syllables in yellow: 2
Syllables in peach.: 1
• Is the program now bug-free? The debugger can't answer
that.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Self Check 6.13
What caused the first error that was found in this debugging
session?
Answer: The programmer misunderstood the second
parameter
of the substring method–it is the index of the first character
not
to be included in the substring.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
Self Check 6.14
What caused the second error? How was it detected?
Answer: The second error was caused by failing to reset
insideVowelGroup to false at the end of a vowel group. It was
detected by tracing through the loop and noticing that the
loop
didn't enter the conditional statement that increments the
vowel
count.
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.
The First Bug
Big Java by Cay Horstmann
Copyright © 2008 by John Wiley & Sons. All rights reserved.