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Chapter 5

File Objects and Looping Statements

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Chapter 9 Topics

Using Data Files for I/O While Statement Syntax Count-Controlled Loops Event-Controlled Loops Using the End-of-File Condition Using a While Statement for Summing and

Counting Nested While Loops Loop Testing and Debugging

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To Use File I/O, you must

Import package java.io.* Choose valid identifiers and types for

your file variables and declare them Instantiate a file object for each file

variable Use your file identifiers in your I/O

statements (using available methods such as readLine, print, println)

Close the files when through

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What does instantiating a file do?

Associates the Java identifier for your file with the physical (disk) name for the file

Places a file pointer at the very beginning of the file, pointing to the first character in it

If the output file does not exist on disk, an empty file with that name is created

If the output file already exists, it is erased

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Using files for I/O

your variable

(of type BufferedReader)

your variable

(of type PrintWriter)

disk file“myInfile.dat”

disk file“myOutfile.dat”

executingprogram

input data output data

import.java.io.*;

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readLine() method

readLine() method of BufferedReader class uses no parameters and returns an object of class String.

It reads a line of input from the file, including the end-of-line mark, discards the EOL mark, and returns the rest of the line

String line;

line = inFile.readLine();

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How do you read a number?

Read a string and convert it to a number

int numberOfDependents; line = inFile.readLine();

numberOfDependents = Integer.parseInt(line);

double taxRate;

line = inFile.readLine();

taxRate = Double.parseDouble(line);

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print() and println() methods

print() writes its parameters on the file to which the method is applied

println() is just like print, but it appends an end-of-line before returning.

Sound familiar?

print and println in class PrintWriter behave exactly the same as those methods of the same name we have used with System.out

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An exception is an unusual situation detected while a program is running; it halts the normal execution of the method

Java recognizes two types of exceptions, checked and unchecked

Unchecked exceptions can be ignored, but checked exceptions must be explicitly recognized by the program

I/O Methods and Exceptions

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Forwarding an exception

An exception can be forwarded by adding a throws clause to a method heading. The clause specifies the name of the exception that is being forwarded

public static void main(String[ ] args)

throws IOException

The exception is passed to the method’s caller, until an exception handler is found, or passing terminates with the JVM

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BufferedReader and FileWriter classes throw an exception called IOException.

PrintWriter does not throw any exceptions, but the PrintWriter constructor must be passed a FileWriter object that can throw an IOException

More about exceptions in Chapter 9

IOException

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A loop is a repetition control structure

It causes a single statement or block to be executed repeatedly while an expression is true

What is a loop?

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Two types of loops

count-controlled loops

repeat a specified number of times

event-controlled loopssomething happens inside the loop body that causes the repetition to stop

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While Statement

while ( Expression )

{

.

. // loop body

.

}

NOTE: Loop body can be a single statement, a null statement, or a block

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When the expression is tested and found to be false, the loop is exited and control passes to the statement that follows the loop body

WHILE LOOP

false

true

bodystatement

Expression

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An initialization of the loop control variable An expression to test for continuing the loop An update of the loop control variable to be executed within each iteration of the body

Count-controlled loops contain

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int loopCount; // Declare loop variable

loopCount = 1; // Initialize loop variable

while (loopCount <= 10) // Test expression

{

. // Repeated actions

.

.

loopCount = loopCount++; // Update loop variable

}

Count-controlled Pattern

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int count; // Declare loop variable

count = 1; // Initialize loop variable

while (count <= 4) // Test expression

{ // Repeated action

System.out.println(“count is “ + count); count ++; // Update loop variable

}

System.out.println(“Done”);

Count-controlled Example

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Count-controlled loopint count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println(“Done”);

OUTPUT

count

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int count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

OUTPUT

count

1

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int count;

count = 1;

while ( count <= 4 ) { TRUE

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

count

1 OUTPUT

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int count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

count

1

OUTPUT count is 1

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int count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

count

2

OUTPUT count is 1

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int count;

count = 1;

while ( count <= 4 ) { TRUE

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

count

2

OUTPUT count is 1

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int count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

OUTPUT count is 1 count is 2

count

2

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int count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

count

3

OUTPUT count is 1 count is 2

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int count;

count = 1;

while ( count <= 4 ) { TRUE

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

count

3

OUTPUT count is 1 count is 2

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int count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

OUTPUT count is 1 count is 2 count is 3

count

3

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int count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

OUTPUT count is 1 count is 2 count is 3

count

4

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int count;

count = 1;

while ( count <= 4 ) { TRUE

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

count

4

OUTPUT count is 1 count is 2 count is 3

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int count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

OUTPUT count is 1 count is 2 count is 3

count is 4

count

4

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int count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

OUTPUT count is 1 count is 2 count is 3

count is 4

count

5

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int count;

count = 1;

while ( count <= 4 ) { FALSE

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

count

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OUTPUT count is 1 count is 2 count is 3

count is 4

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int count;

count = 1;

while ( count <= 4 ) {

System.out.println

( “count is “ + count );

count++;

}

System.out.println( “Done” );

count

5

OUTPUT count is 1 count is 2 count is 3

count is 4 Done

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• dataFile contains 100 blood pressures, one to a line

• Use a while loop to read the 100 blood pressures and find their total

Count-Controlled Loop Example

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// Count-controlled loop

int thisBP; int total; int count; count = 1; // Initializetotal = 0;

while (count <= 100) // Test expression{ thisBP = Integer.parseInt(dataFile.readLine()); total = total + thisBP; count++; // Update}

System.out.println(“The total = “ + total);

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Event-controlled loops

Sentinel controlled Keep processing data until a special value which is not a possible data value is entered to indicate that processing should stop

End-of-file controlled Keep processing data as long as there is more data in the file

Flag controlled Keep processing data until the value of a flag changes in the loop body

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Examples of kinds of loops

Count controlled loop Read exactly 100 blood pressures from a file.

End-of-file controlledloop

Read all the blood pressures from a file no matter how many are there.

Flag controlledloop

Read blood pressures until a dangerously high BP (200 or more) is read.

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A sentinel-controlled loop

Requires a “priming read”

“Priming read” means you read one data value (or set of data values) before entering the while loop

Process data value(s) and then read next value(s) at end of loop

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// Sentinel is negative blood pressure.int thisBP; int total; int count; count = 1; // Initializetotal = 0; // Priming readthisBP = Integer.parseInt(dataFile.readLine());while (thisBP > 0) // Test expression{ total = total + thisBP; count++; // Update thisBP = Integer.parseInt(dataFile.readLine());}

System.out.println(“The total = “ + total);

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An end-of-file controlled loop

depends on fact that readLine

returns null if there is no more data

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// Read and sum until end of lineint thisBP; int total; int count; count = 1; // Initializetotal = 0; String line;line = dataFile.readLine();while (line != null) // Test expression{ thisBP = Integer.parseInt(line); total = total + thisBP; count++; // Update line = dataFile.readLine();}System.out.println(“The total = “ + total);

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Flag-controlled loops

Use meaningful name for the flag Initialize flag (to true or false) Test the flag in the loop test

expression Change the value of the flag in loop

body when the appropriate condition occurs

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A flag-controlled loop

Count and sum the first 10 odd numbers in a data file

Initialize flag notDone to true

Use while(notDone) for loop test Change flag to false when 10 odd

numbers have been read or if EOF is reached first

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count = 0;

sum = 0;

notDone = true;

while ( notDone ) {

line = dataFile.readLine( ); // Get a line

if (line != null) // Got a line? { number = Integer.parseInt(line);

if (number % 2 == 1) // Is number odd? {

count++;

sum = sum + number;

notDone = ( count < 10 ); } } else // Reached EOF unexpectedly { errorFile.println(“EOF reached before ten odd values.”) notDone = false; // Change flag value } }

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Loops often used to

Count all data values

Count special data values

Sum data values

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// Flag and EOF controlled loop

countGoodReadings = 0; // Initialize

isSafe = true;

while (isSafe && (line = dataFile.readLine() != null)) {

thisBP = Integer.valueOf(line).intValue();

if (thisBP >= 200)isSafe = false; // Change flag

else countGoodReadings++; // Update

}

System.out.println(“There were ” + countGoodReadings + “ safe blood pressure readings.”);

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initialize outer loop

while ( outer loop condition )

{ . . .

initialize inner loop

while ( inner loop condition )

{

inner loop processing and update

}

. . .}

Pattern of a nested loop

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Nested loop design

Begin with outer loop

When you get to where the inner loop appears, make it a separate module and come back to its design later

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Algorithm uses Nested Loops

Get a data line from the data file While more data

obtain starCount from the data line use a count-controlled loop to print

starCount asterisks to the output file print a newline character to the

output file read next data line from the data file

Print “End” to the output file

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// Using nested loopsline = dataFile.readLine(); // Get data linewhile (line != null) // While more data{ starCount = Integer.parseInt(line); loopCount = 1; // Loop to print

asterisks while (loopCount <= starCount) { outFile.print(‘*’); loopCount ++; } outFile.println(); line = dataFile.readLine(); // Get next data line} outFile.println(“End”);

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Information about 20 books in file

bookfile.dat

P 3.98H 7.41P 8.79

.

.

.

Price of book

Hardback orPaperback?

Write a program to find total value of all books

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String line; // Declarations String kind; double price; double total = 0.0; int count = 1;

while (count <= 20) { line = dataFile.readLine() kind = line.substring(0, 1); price = Double.parseDouble( line.substring(1, line.length())); total = total + price; count ++; }

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Trace of Program Variables

count kind price total 0.0

1 “P” 3.98 3.98

2 “H” 7.41 11.39

3 “P” 8.79 20.18

4 etc.

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21 so loop terminates

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Accessing characters in a line

readLine reads a line of text from a file Lines are converted to numbers using

methods in numeric classes How do you access the characters in a line?

line.charAt(index) returns the character at the index position in the line

line.charAt(0) is the first character; line.charAt(line.length()-1) is the last character

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// Counts number of commas on each line of datalineCount = 0; inLine = inFile.readLine(); // Priming readwhile (inLine != null) // Outer loop test for EOF{ lineCount++; commaCount= 0; index = 0; // Initialize loop count while (index < inLine.length()) { if (inLine.charAt(index) == ',') commaCount++; index++; } System.out.println("Found " + commaCount + " commas on line " + lineCount); inLine = inFile.readLine(); //Update outer exit condition}System.out.println("There are " + lineCount + " lines in the file.");

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Loop Definitions

Iteration counter A counter variable that is incremented in each iteration of a loop

Event counter A variable that is incremented each time a particular event occurs

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Object State The object’s state is the current set of values

that it contains

An object is instantiated with an initial state. If any of its methods can subsequently change its state, the object is said to be mutable

Methods that change an object’s state are called transformers

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Class TrafficLight

TrafficLight

knowCurrentColor

knowTimeRemaining .

. .

changeColor

Private data:

currentColor

timeRemaining

decrementTimeRemaining

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Loop Testing and Debugging Test data should test all sections of the program

Beware of infinite loops -- the program doesn’t stop

Check loop termination condition, and watch for an OBOB (off-by-1 bug)

Use algorithm walk-through to verify that appropriate conditions occur in the right places

Trace execution of loop by hand with code walk-through

Use a debugger (if available) to run program in “slow motion” or use debug output statements