Eo gaddis java_chapter_05_5e

Copyright © 2015 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
C HA PT E R 5
Loops and
Files

Topics
The Increment and Decrement Operators
The while Loop
Using the while Loop for Input Validation
The do-while Loop
The for Loop
Running Totals and Sentinel Values
Nested Loops
The break and continue Statements
Deciding Which Loop to Use
Introduction to File Input and Output

The Increment and Decrement
Operators
There are numerous times where a variable
must simply be incremented or decremented.
number = number + 1;
number = number – 1;
Java provide shortened ways to increment
and decrement a variable’s value.
Using the ++ or -- unary operators, this task
can be completed quickly.
number++; or ++number;
number--; or --number;
Example: IncrementDecrement.java

Differences Between Prefix
and Postfix
When an increment or decrement are the
only operations in a statement, there is no
difference between prefix and postfix
notation.
When used in an expression:
prefix notation indicates that the variable will be
incremented or decremented prior to the rest of the
equation being evaluated.
postfix notation indicates that the variable will be
incremented or decremented after the rest of the
equation has been evaluated.
Example: Prefix.java

The while Loop
Java provides three different looping structures.
The while loop has the form:
while(condition)
{
statements;
}
While the condition is true, the statements will
execute repeatedly.
The while loop is a pretest loop, which means that it
will test the value of the condition prior to executing
the loop.

The while Loop
Care must be taken to set the condition to
false somewhere in the loop so the loop will
end.
Loops that do not end are called infinite
loops.
A while loop executes 0 or more times. If the
condition is false, the loop will not execute.
Example: WhileLoop.java

The while loop Flowchart
statement(s)
true
boolean
expression?
false

Infinite Loops
In order for a while loop to end, the
condition must become false. The following
loop will not end:
int x = 20;
while(x > 0)
{
System.out.println("x is greater than 0");
}
The variable x never gets decremented so it
will always be greater than 0.
Adding the x-- above fixes the problem.

Infinite Loops
This version of the loop
decrements x during each
iteration:
int x = 20;
while(x > 0)
{
System.out.println("x is greater than 0");
x--;
}

Block Statements in Loops
Curly braces are required to enclose
block statement while loops. (like block
if statements)
while (condition)
{
statement;
statement;
statement;
}

The while Loop for Input
Validation
Input validation is the process of ensuring that user
input is valid.
System.out.print("Enter a number in the " +
"range of 1 through 100: ");
number = keyboard.nextInt();
// Validate the input.
while (number < 1 || number > 100)
{
System.out.println("That number is invalid.");
System.out.print("Enter a number in the " +
"range of 1 through 100: ");
number = keyboard.nextInt();
}
Example: SoccerTeams.java

The do-while Loop
The do-while loop is a post-test loop, which means
it will execute the loop prior to testing the condition.
The do-while loop (sometimes called a do loop)
takes the form:
do
{
statement(s);
}while (condition);
Example: TestAverage1.java

The do-while Loop Flowchart
statement(s)
true
boolean
expression?
false

The for Loop
The for loop is a pre-test loop.
The for loop allows the programmer to
initialize a control variable, test a condition,
and modify the control variable all in one line
of code.
The for loop takes the form:
for(initialization; test; update)
{
statement(s);
}
See example: Squares.java

The for Loop Flowchart
statement(s)
trueboolean
expression?
false
update

The Sections of The for Loop
The initialization section of the for loop
allows the loop to initialize its own control
variable.
The test section of the for statement acts in
the same manner as the condition section of
a while loop.
The update section of the for loop is the last
thing to execute at the end of each loop.
Example: UserSquares.java

The for Loop Initialization
The initialization section of a for loop is
optional; however, it is usually provided.
Typically, for loops initialize a counter
variable that will be tested by the test section
of the loop and updated by the update
section.
The initialization section can initialize
multiple variables.
Variables declared in this section have scope
only for the for loop.

The Update Expression
The update expression is usually used to
increment or decrement the counter
variable(s) declared in the initialization
section of the for loop.
The update section of the loop executes last
in the loop.
The update section may update multiple
variables.
Each variable updated is executed as if it
were on a line by itself.

Modifying The Control
Variable
You should avoid updating the control
variable of a for loop within the body
of the loop.
The update section should be used to
update the control variable.
Updating the control variable in the for
loop body leads to hard to maintain
code and difficult debugging.

Multiple Initializations and
Updates
The for loop may initialize and update multiple variables.
for(int i = 5, int j = 0; i < 10 || j < 20; i++, j+=2)
{
statement(s);
}
Note that the only parts of a for loop that are mandatory are
the semicolons.
for(;;)
{
statement(s);
} // infinite loop
If left out, the test section defaults to true.

Running Totals
Loops allow the program to keep running
totals while evaluating data.
Imagine needing to keep a running total of
user input.
Example: TotalSales.java

Sentinel Values
Sometimes the end point of input data is not known.
A sentinel value can be used to notify the program to
stop acquiring input.
If it is a user input, the user could be prompted to
input data that is not normally in the input data
range (i.e. –1 where normal input would be positive.)
Programs that get file input typically use the end-of-
file marker to stop acquiring input data.
Example: SoccerPoints.java

Nested Loops
Like if statements, loops can be nested.
If a loop is nested, the inner loop will execute all of
its iterations for each time the outer loop executes
once.
for(int i = 0; i < 10; i++)
for(int j = 0; j < 10; j++)
loop statements;
The loop statements in this example will execute 100
times.
Example: Clock.java

The break Statement
The break statement can be used to
abnormally terminate a loop.
The use of the break statement in
loops bypasses the normal
mechanisms and makes the code hard
to read and maintain.
It is considered bad form to use the
break statement in this manner.

The continue Statement
The continue statement will cause the
currently executing iteration of a loop to
terminate and the next iteration will begin.
The continue statement will cause the
evaluation of the condition in while and for
loops.
Like the break statement, the continue
statement should be avoided because it
makes the code hard to read and debug.

Deciding Which Loops to Use
The while loop:
Pretest loop
Use it where you do not want the statements to
execute if the condition is false in the beginning.
The do-while loop:
Post-test loop
Use it where you want the statements to execute at
least one time.
The for loop:
Pretest loop
Use it where there is some type of counting variable
that can be evaluated.

File Input and Output
Reentering data all the time could get tedious
for the user.
The data can be saved to a file.
Files can be input files or output files.
Files:
Files have to be opened.
Data is then written to the file.
The file must be closed prior to program termination.
In general, there are two types of files:
binary
text

Writing Text To a File
To open a file for text output you create
an instance of the PrintWriter class.
PrintWriter outputFile = new PrintWriter("StudentData.txt");
Pass the name of the file that you
wish to open as an argument to
the PrintWriter constructor.
Warning: if the file already exists, it will be erased and replaced with a new file.

The PrintWriter Class
The PrintWriter class allows you to write data to
a file using the print and println methods, as you
have been using to display data on the screen.
Just as with the System.out object, the println
method of the PrintWriter class will place a
newline character after the written data.
The print method writes data without writing the
newline character.

PrintWriter outputFile = new PrintWriter("Names.txt");
outputFile.println("Chris");
outputFile.println("Kathryn");
outputFile.println("Jean");
outputFile.close();
Open the file.
Write data to the file.
Close the file.

To use the PrintWriter class, put the
following import statement at the top
of the source file:
import java.io.*;
See example: FileWriteDemo.java

Exceptions
When something unexpected happens in a
Java program, an exception is thrown.
The method that is executing when the
exception is thrown must either handle the
exception or pass it up the line.
Handling the exception will be discussed
later.
To pass it up the line, the method needs a
throws clause in the method header.

Exceptions
To insert a throws clause in a method
header, simply add the word throws and the
name of the expected exception.
PrintWriter objects can throw an
IOException, so we write the throws clause
like this:
public static void main(String[] args)
throws IOException

Appending Text to a File
To avoid erasing a file that already exists,
create a FileWriter object in this manner:
FileWriter fw =
new FileWriter("names.txt", true);
Then, create a PrintWriter object in this
manner:
PrintWriter fw = new PrintWriter(fw);

Specifying a File Location
On a Windows computer, paths contain
backslash () characters.
Remember, if the backslash is used in
a string literal, it is the escape
character so you must use two of them:
PrintWriter outFile =
new PrintWriter("A:PriceList.txt");

Specifying a File Location
This is only necessary if the backslash is in a
string literal.
If the backslash is in a String object then it
will be handled properly.
Fortunately, Java allows Unix style filenames
using the forward slash (/) to separate
directories:
PrintWriter outFile = new
PrintWriter("/home/rharrison/names.txt");

Reading Data From a File
You use the File class and the
Scanner class to read data from a file:
File myFile = new File("Customers.txt");
Scanner inputFile = new Scanner(myFile);
Pass the name of the file as
an argument to the File
class constructor.
Pass the File object as an
argument to the Scanner
class constructor.

Scanner keyboard = new Scanner(System.in);
System.out.print("Enter the filename: ");
String filename = keyboard.nextLine();
File file = new File(filename);
Scanner inputFile = new Scanner(file);
The lines above:
Creates an instance of the Scanner class to read from the
keyboard
Prompt the user for a filename
Get the filename from the user
Create an instance of the File class to represent the file
Create an instance of the Scanner class that reads from the file

Once an instance of Scanner is created, data can
be read using the same methods that you have
used to read keyboard input (nextLine, nextInt,
nextDouble, etc).
// Open the file.
File file = new File("Names.txt");
// Read a line from the file.
String str = inputFile.nextLine();
// Close the file.
inputFile.close();

Exceptions
The Scanner class can throw an
IOException when a File object is
passed to its constructor.
So, we put a throws IOException
clause in the header of the method
that instantiates the Scanner class.
See Example: ReadFirstLine.java

Detecting The End of a File
The Scanner class’s hasNext() method will return
true if another item can be read from the file.
// Open the file.
File file = new File(filename);
// Read until the end of the file.
while (inputFile.hasNext())
{
String str = inputFile.nextLine();
System.out.println(str);
}
inputFile.close();// close the file when done.
See example: FileReadDemo.java

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