Loops and Files - Purdue University Fort Wayne

CHAPTER 4

Loops and Files

4-2

Chapter Topics

Chapter 4 discusses the following main 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

4-3

Chapter Topics

Chapter 4 discusses the following main topics:

– Nested Loops

– The break and continue Statements

– Deciding Which Loop to Use

– Introduction to File Input and Output

– Generating Random Numbers with the Random

class

4-4

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

IncrementDecrement.java

4-5

The Increment and Decrement Operatorsint number = 0;

String strInitNum = "Initial value of number is %d.\n";

System.out.printf(strInitNum, number);

String strNum2Plus = "number++ is %d.\n";

String strNum2Minus = "number-- is %d.\n";

String str2PlusNum = "++number is %d.\n";

String str2MinusNum = "--number is %d.\n";

number++;

System.out.printf(strNum2Plus, number);

number--;

System.out.printf(strNum2Minus, number);

System.out.printf(strNum2Plus, number++);

System.out.printf(strNum2Minus, number--);

System.out.printf("The final value of number is %d.\n", number);

4-6

The Increment and Decrement Operatorsint number = 0;

String strInitNum = "Initial value of number is %d.\n";


String strNum2Plus = "number++ is %d.\n";

String strNum2Minus = "number-- is %d.\n";

String str2PlusNum = "++number is %d.\n";

String str2MinusNum = "--number is %d.\n";

number++;

System.out.printf(strNum2Plus, number);

number--;

System.out.printf(strNum2Minus, number);

System.out.printf(strNum2Plus, number++);

System.out.printf(strNum2Minus, number--);

System.out.printf("The number storage has %d.\n", number);

Initial value of number is

0.

number++ is 1.

number-- is 0.

number++ is 0.

number-- is 1.

The number storage has 0.

4-7

The Increment and Decrement Operators

number = 0;


++number;

System.out.printf(str2PlusNum , number);

--number;

System.out.printf(str2MinusNum, number);

System.out.printf(str2PlusNum , ++number);

System.out.printf(str2MinusNum, --number);

System.out.printf("The number storage has %d.\n", number);

Initial value of number is 0.

++number is 1.

--number is 0.

++number is 1.

--number is 0.

The number storage has 0.

4-8

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.

e.g., num++; ++num; nos--; -- nos;

• 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.

e.g., sum = sum + ++num; sum = ++num + sum;

postfix notation indicates that the variable will be incremented or decremented after the rest of the equation has been evaluated.

e.g., sum = sum + num++; sum = num++ + sum;

• Example: Prefix.java

Prefix.java

int sum = 10; int num = 1;sum = sum + ++num;System.out.println("sum is " + sum + ". num is " + num);

sum = 10; num = 1;sum = ++num + sum;System.out.println("sum is " + sum + ". num is " + num);

sum = 10; num = 1;sum = sum + num++;System.out.println("sum is " + sum + ". num is " + num);

sum = 10; num = 1;sum = num++ + sum;System.out.println("sum is " + sum + ". num is " + num);

sum is 12. num is 2sum is 12. num is 2sum is 11. num is 2sum is 11. num is 2

int sum = 10; int num = 1;sum = sum + --num;System.out.println("sum is " + sum + ". num is " + num);sum = 10; num = 1;sum = --num + sum;System.out.println("sum is " + sum + ". num is " + num);

sum = 10; num = 1;sum = sum + num--;System.out.println("sum is " + sum + ". num is " + num);

sum = 10; num = 1;sum = num-- + sum;System.out.println("sum is " + sum + ". num is " + num);

sum is 10. num is 0sum is 10. num is 0sum is 11. num is 0sum is 11. num is 0

4-11

Differences Between Prefix and Postfix

int sum = number = 0;

sum = sum + ++number; //number=number+1; sum=sum+number;

System.out.printf("Sum is %d.\n", sum);

sum = number = 0;

sum = ++number + sum; //number=number+1; sum=sum+number;


sum = number = 0;

sum = sum + number++; //sum=sum+number; number=number+1;


System.out.printf("The value of number is %d.\n", number);

sum = number = 0;

sum = number++ + sum; //sum = number + sum; number=number+1;


System.out.printf("The value of number is %d.\n", number);

Sum is 1.

Sum is 1.

Sum is 0.

The value of number is 1.

Sum is 0.

The value of number is 1.

4-12

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.

C

S

t

fint x = 0, y = 3;

while( x <= y)

{

x = x + 1;

y = y – 1;

}

System.out.printf(“x is %d and y is %d.\n”, x, y);

Result is: x is 2 and y is 1.

int x = 0, y = 3;while( x <= y){

x = x + 2;y = y - 1;System.out.printf("x is %d and y is %d.\n", x, y);

}System.out.printf("x is %d and y is %d.\n", x, y);

x is 2 and y is 2.x is 4 and y is 1.x is 4 and y is 1.

4-14

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

int x = 0, y = 3;

while( x > y)

{

x = x + 1;

y = y – 1;

System.out.println(“I was here!”);

}

System.out.printf(“x is %d and y is %d.\n”, x, y);

Result is: x is 0 and y is 3.

WhileLoop.java

4-15

The while loop Flowchart

statement(s)

true

boolean

expression?false

while (Boolean expression)

{

statements;

}

4-16

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");

String str = “x = %d is greater than 0.\n”;System.out.printf(str, x)

}

• The variable x never gets decremented so it will always be greater than 0.

• Adding the x-- above fixes the problem.

4-17

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");

String str = “x = %d is greater than 0.\n”;System.out.printf(str, x);x--;

}

4-18

Block Statements in Loops

• Curly braces are required to enclose block statement while loops. (like block if statements)

while (condition)

{

statement;

statement;

…

statement;

}

4-19

Block Statements in Loops

• Curly braces are required to enclose block statement while loops. (like block if statements)

int c = 3;

while (c > 0)

c = c - 1;

System.out.printf("c is %d.\n", c);

c = 3;

while (c > 0) {

c = c - 1;

System.out.printf("c is %d.\n", c);

}

c is 0.

c is 2.

c is 1.

c is 0.

4-20

The while Loop for Input Validation

• Input validation is the process of ensuring that user input is valid.

Scanner keyboard = new Scanner(System.in);

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.println(“A number is “ + number + ".");

• Example: SoccerTeams.java

SoccerTeams.java

4-21

The while Loop for Input Validation

• Input validation is the process of ensuring that user input is valid.





// Validate the input.

while (number < 1 || number > 100)

{

System.out.println("That number is invalid.");




}

System.out.println(“A number is “ + number + ".");

• Example: SoccerTeams.java

Enter a number in the range of 1 through 100: 101

That number is invalid.

Enter a number in the range of 1 through 100:

That number is invalid.

Enter a number in the range of 1 through 100: 51

A number is 51.

SoccerTeams.java

4-22

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 also called a do loop)

takes the form:

do

{

statement(s);

} while (condition);

• Example: TestAverage1.java

TestAverage1.java

4-23

The do-while Loop Flowchart

statement(s)

trueboolean

expression?

false

do

{

statement(s);

} while (boolean expression);

4-24

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

Squares.java

4-25

The for Loop Flowchart

statement(s)

true

boolean

expression?

false

update

initialization

for(initialization; test; update)

{

statement(s);

}

4-26

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 whileloop.

• The update section of the for loop is the last thing to execute at the end of each loop.

• Example: UserSquares.java

for (int i = 0; i < 5; i++) {

System.out.println(i);}//System.out.println(i); //i is undefined here!

int j = 0;do{

System.out.println(j);j++;

}while (j < 5);

int k = 0;

while (k < 5)

{

System.out.println(k);

k++;

}

012340123401234

UserSquares.java

What are the differences among for statement, do-while statement and while statement?

See the next two slides.

package chapter04;import java.util.Scanner;

public class Chapter04 {

public static void main(String[] args) {Scanner kbInput = new Scanner(System.in);System.out.println("Enter an integer as an index: ");int indexReserved = kbInput.nextInt();int index = indexReserved;

for ( ; index >= 5; index--) {System.out.println("for loop has an output: " + index);

} //end of for

index = indexReserved;do {

System.out.println("do while has an output: " + index);index--;

} while (index >= 5);

index = indexReserved;while (index >= 5 ) {

System.out.println("while has an output: " + index);index--;

} //end of while} //end of main

}

Enter an integer as an index: 5for loop has an output: 5do while has an output: 5while has an output: 5

Enter an integer as an index: 0do while has an output: 0

Scope of the variable iindex;

for (int iindex=6 ; iindex >= 5; iindex--){

System.out.println("for loop has an output: " + iindex);}System.out.println("for loop has an output: " + iindex);

Exception in thread "main" java.lang.Error: Unresolved compilation problem: iindex cannot be resolved to a variable

int iindex;for (iindex=6 ; iindex >= 5; iindex--){

System.out.println("for loop has an output: " + iindex);}System.out.println("for loop has an output: " + iindex);

Scope of the variable iindex;

int iindex;for (iindex=6 ; iindex >= 5; iindex--){

System.out.println("for loop has an output: " + iindex);

}System.out.println("for loop has an output: " + iindex);

for loop has an output: 6for loop has an output: 5for loop has an output: 4

4-31

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 whileloop.

• The update section of the for loop is the last thing to execute at the end of each loop.

• Example: UserSquares.java

UserSquares.java

4-32

The for Loop Initialization

• The initialization section of a for loop is optional; however, it is usually provided.

• Typically, for loops initialize a counter variablethat 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. for (int i = 0; i < 5; i++)

{

System.out.println(i);

}

System.out.println(i); //i is undefined here!

4-33

The for Loop Initialization

• The initialization section of a for loop is optional; however, it is usually provided.

• Typically, for loops initialize a counter variablethat 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.

for (int i = 0; i < 5; i++) {

System.out.println(i);}//System.out.println(i); //i is undefined here!

int c = 0;for (; c < 5; c++) {

System.out.println(c);}System.out.println(c); //i is undefined here!

01234012345

4-34

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.

4-35

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.

4-36

Multiple Initializations and Updates

for (int x = 0, y = -1; (x < 3 || y < 3); x++, y++)

{

System.out.printf("x = %d and y = %d.\n", x, y);

}

//System.out.printf("x = %d and y = %d.\n", x, y);

//x and y cannot be resolved to variable

int x, y;

for (x = 0, y = -1; (x < 3 || y < 3); x++, y++)

{


}

System.out.printf("Outside for loop: x = %d and y = %d.\n", x, y);

4-37

Multiple Initializations and Updates

for (int x = 0, y = -1; (x < 3 || y < 3); x++, y++)

{


}

//System.out.printf("x = %d and y = %d.\n", x, y);

//x and y cannot be resolved to a variable

int x, y;

for (x = 0, y = -1; (x < 3 || y < 3); x++, y++)

{


}

System.out.printf("Outside for loop: x = %d and y = %d.\n", x, y);

x = 0 and y = -1.

x = 1 and y = 0.

x = 2 and y = 1.

x = 3 and y = 2.

x = 0 and y = -1.

x = 1 and y = 0.

x = 2 and y = 1.

x = 3 and y = 2.

Outside for

loop: x = 4 and

y = 3.

4-38

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

TotalSales.java

Logic for Calculating a Running Total

4-39

Set accumulator to 0

Is there another

number to read?

Add the number to

the accumulator

Read the next

number

No

(false)

Yes

(true)

Logic for Calculating a Running Total

4-40

4-41

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 an 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

SoccerPoints.java

import javax.swing.JOptionPane;

public class TotalSales {public static void main(String[] args) {

double sumNumbers = 0;final int totalNumbers;double number;String input = JOptionPane.showInputDialog("Enter " + "total numbers to be entered: ");totalNumbers = Integer.parseInt(input);

for(int count=0; count < totalNumbers; count++ ){

input = JOptionPane.showInputDialog("Enter " +"a number: ");

//number = Double.parseDouble(input);sumNumbers = sumNumbers + Double.parseDouble(input);

}JOptionPane.showMessageDialog(null,

String.format("The sum of %d numbers is %.2f.", totalNumbers, sumNumbers));

System.exit(0);}

}

import java.util.Scanner;public class TotalSales {

public static void main(String[] args) {int sumNumbers = 0;int totalNumbers = 0;int number;

Scanner kb = new Scanner(System.in);

System.out.println("Enter a positive number, " +

"otherwise -1 for ending");

number = kb.nextInt();

while(number != -1)

{

totalNumbers++;

sumNumbers += number;

System.out.println("Enter a positive number, " +

"otherwise -1 for ending");

number = kb.nextInt();

}

System.out.printf("Sum of the %d numbers is %d.\n",

totalNumbers, sumNumbers );

Enter a positive number, otherwise -1 for ending-1Sum of the 0 numbers is 0.00.

Enter a positive number, otherwise -1 for ending101.01Enter a positive number, otherwise -1 for ending-1Sum of the 1 numbers is 101.01.

Enter a positive number, otherwise -1 for ending202.02Enter a positive number, otherwise -1 for ending303.0399Enter a positive number, otherwise -1 for ending-1Sum of the 2 numbers is 505.06.

4-45

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

for(int i = 0; i < 10; i++){

for(int j = 0; j < 10; j++)loop statements;

}

Clock.java

final int iLimit = 3;final int jLimit = 5;

for(int i = 0; i < iLimit; i++){

System.out.println("i is " + i);

for(int j = 0; j < jLimit; j++)System.out.println(" j is " + j);

}

i is 0j is 0j is 1j is 2j is 3j is 4



final int iLimit = 3;final int jLimit = 5;

for(int i = 0; i < iLimit; i++){System.out.println("");

for(int j = 0; j < jLimit; j++)System.out.print(j + " ");}System.out.println("\nThis is");

int x = 0, col = 0;for(int row = 0; row < iLimit; row++){System.out.println("");

for(; col < (row+1)*jLimit; col++){System.out.print(col + " ");x = col + 1;}x = col + 1;}}}

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 This is

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

4-48

Nested Loops

• 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;

for(int i = 0; i < 10; i++)

{for(int j = 0; j < 10; j++){

loop statements;}//end inner for_loop

}//end outer for_loop

4-49

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.

char switchExpr;switchExpr = 'a';switch (switchExpr){

case 'A’:case 'a’:

System.out.println("This is one!");break;

case 'B’:case 'b’:

System.out.println("This is two!");break;

default:System.out.println("This is out!");

}


public class ReadWriteFile {public static void main(String[] args) {

char switchExpr;//switchExpr = 'a';String str = JOptionPane.showInputDialog("Enter a character: ");switchExpr = str.charAt(0);

switch (switchExpr){case 'A': case 'a':

System.out.println("This is one!");break;

case 'B':case 'b':

System.out.println("This is two!");break;

default:System.out.println("This is out!");

}System.exit(0);

}}

String str = JOptionPane.showInputDialog("Enter " + "a word:");

str = str.toUpperCase();switch (str){

case "PETER":System.out.printf("The name is %s.\n", str);break;

case "PAUL":System.out.printf("The name is %s.\n", str);break;

default:System.out.println("No Peter and Paul.");

}



String str = JOptionPane.showInputDialog("Enter " + "a word:");//str = str.toUpperCase();if (str.equalsIgnoreCase("PETER"))

System.out.printf("The name is %s.\n", str);

else if(str.equalsIgnoreCase("PAUL"))System.out.printf("The name is %s.\n", str);

elseSystem.out.println("No Peter and Paul.");

System.exit(0);

}}

if (str.equals("PETER"))switchExpr = str.charAt(0);

else if ("Paul".equalsIgnoreCase(str))switchExpr = str.charAt(1);

else switchExpr = str.charAt(0);

switch (switchExpr){

case 'P':case 'p':

System.out.println("This is one, P!");break;

case 'a’:case 'A’:

System.out.println("This is two, Paul!");break;

default:System.out.println("This is out Not p from" +

" Peter or not a from Paul!");}



String str = JOptionPane.showInputDialog("Enter "

+ "a word:");

System.out.println("The str " + str + “ compareTo(\"PETER\") is "+ str.compareTo("PETER") );

System.exit(0);}

}

The str PETER compareTo("PETER") is 0

The str PETERABC compareTo("PETER") is 3The str PETEr compareTo("PETER") is 32

//ASCII code for r is 114 and R is 82 and the difference is 114-82 = 32

4-56

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.

Import javax.swing.JOptionPane;public class TotalSales {

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

//int totalNumbers = 5;

double number;

String task = "Enter total numbers to be entered: ";

String title = "Determine Sum of Numbers";

String input = JOptionPane.showInputDialog(null, task, title, JOptionPane.QUESTION_MESSAGE);

//create a name constant with the modifier final.

final int totalNumbers = Integer.parseInt(input);

for(int count=0; count < totalNumbers; count++ )

{

input = JOptionPane.showInputDialog(null, "Enter a number: ", title, JOptionPane.QUESTION_MESSAGE);

//number = Double.parseDouble(input);

sumNumbers = sumNumbers + Double.parseDouble(input);

int yesNoCancel = JOptionPane.showConfirmDialog(null, "Continue to enter a number?", title + "Selection One!",

JOptionPane.YES_NO_CANCEL_OPTION);

if (yesNoCancel == JOptionPane.NO_OPTION)

break;

else if (yesNoCancel == JOptionPane.CANCEL_OPTION)

{

int yesNoContinue = JOptionPane.showConfirmDialog(null, "Do you continue to enter a number?",

title + "Selection One!", JOptionPane.YES_NO_OPTION);

if (yesNoContinue == JOptionPane.YES_OPTION)

continue;

else

break;

}

} //end for

JOptionPane.showMessageDialog(null, String.format("The sum of %d numbers is %.2f.", totalNumbers, sumNumbers),

title, JOptionPane.INFORMATION_MESSAGE);

System.exit(0);

}}

Import javax.swing.JOptionPane;public class TotalSales {

public static void main(String[] args) {double sumNumbers = 0, number;

int totalCount = 0;

String task = "Enter total numbers to be entered: ";

String title = "Determine Sum of Numbers";

String input = JOptionPane.showInputDialog(null, task, title, JOptionPane.QUESTION_MESSAGE);

//create a name constant with the modifier final.

final int totalNumbers = Integer.parseInt(input);

for(int count=0; count < totalNumbers; count++ )

{

input = JOptionPane.showInputDialog(null, "Enter a number: ", title, JOptionPane.QUESTION_MESSAGE);

sumNumbers = sumNumbers + Double.parseDouble(input);

totalCount = totalCount + 1;

int yesNoCancel = JOptionPane.showConfirmDialog(null, "Continue to enter a number?", title + "Selection One!",

JOptionPane.YES_NO_CANCEL_OPTION);

if (yesNoCancel == JOptionPane.NO_OPTION)

break;

else if (yesNoCancel == JOptionPane.CANCEL_OPTION)

{

int yesNoContinue = JOptionPane.showConfirmDialog(null, "Do you continue to enter a number?",

title + "Selection One!", JOptionPane.YES_NO_OPTION);

if (yesNoContinue == JOptionPane.YES_OPTION)

continue;

else

break;

}

} //end for

if (totalNumbers <= totalCount)

JOptionPane.showMessageDialog(null, String.format("The sum of %d numbers is %.2f.", totalNumbers, sumNumbers),


else JOptionPane.showMessageDialog(null, String.format("The sum of %d numbers is %.2f.", totalCount, sumNumbers),


System.exit(0);

}}

4-59

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.

4-60

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 ….. Plain Text (*.txt)

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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.

4-62

The PrintWriter Class

• The PrintWriter class allows you to write data to a file using the print and printlnmethods, 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.

Use \r (return) instead of \n

for writing data on files.

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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.

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• To use the PrintWriter class, put the following

import statement at the top of the source file:

import java.io.*;

• See example: FileWriteDemo.java

FileWriteDemo.java

4-65

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.

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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{

}

4-67

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);

4-68

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");

C:\\Users\\apeng\\workspace\\outputToFile\\PersonName.txt

public static void main(String[] args) throws IOException {//get a filenameScanner kb = new Scanner(System.in);System.out.println("Enter a filename such as " +

"PersonName.txt : ");String filename = kb.nextLine();//open the file//PrintWriter outputFile = new PrintWriter("Names.txt");//PrintWriter outputFile = new PrintWriter(filename);

//Appending data to the fileFileWriter fwriter = new FileWriter(filename, true);PrintWriter outputFile = new PrintWriter(fwriter);outputFile.println("Chris Janes");outputFile.println("Kathryn Kennedy");outputFile.println("Jean Smith");outputFile.close(); PrintWriter outputFile1 = new PrintWriter(new

FileWriter(filename, true));outputFile1.println(“Peter Pan");outputFile1.close();

}}

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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");

An example:

C:\Users\apeng\workspace\outputToFile\PersonName.txt

4-71

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.

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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

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• 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");Scanner inputFile = new Scanner(file);// Read a line from the file.String str = inputFile.nextLine();// Close the file.inputFile.close();

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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

ReadFirstLine.java

4-75

Checking for a File’s Existence

//make sure the file exists. If file.exists() method return false,

//indicating the file does not exist.

File file = new File(“Numbers.txt”);

if (!file.exists())

{

System.out.println(“The file Numbers.txt is not found”);

System.exit(0);

}

//Open the file for reading.

Scanner inputFile = new Scanner(file);

…

• See Example: ReadFirstLine.java

ReadFirstLine.java

4-76

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);Scanner inputFile = new Scanner(file);

// 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.

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Detecting the End of a File

• See example: FileReadDemo.java

Next four slides is to deal with output

data to the file and read these data from

this file.

FileReadDemo.java

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}else{FileWriter fwriter = new FileWriter(file0, true);PrintWriter outputFile = new PrintWriter(fwriter);outputFile.println("My Comments on the Exponential Penny

Pay Project.");outputFile.println("One more try!");//close the output file. Need this her for outputFile.close(); //for open file to read.

Scanner fileRead = new Scanner(file0);while (fileRead.hasNext()) {String str = fileRead.nextLine();System.out.println("file read: " + str);

}//close the file.fileRead.close();}System.out.println("The program is ended.");

}}

4-79

Output to the file and read from the filepackage fileIOProjectP;import java.util.Scanner;import java.io.*;

public class FileIO {public static void main(String[] args) throws IOException{

String filename; //Filename//Create a Scanner object for keyboard input.Scanner keyboardInput = new Scanner(System.in);

//Get the filename.System.out.print("Enter the filename, " +

"such as wages.txt: ");filename = keyboardInput.nextLine();

//output to the file and read from the fileSystem.out.println("The program is ended.");

}}

4-80

File file0 = new File(filename);//a wrong way to have FileWriter and PrintWriter here.if (!file0.exists()) {

System.out.println("File does not exit.");}else {

FileWriter fwriter = new FileWriter(file0, true);PrintWriter outputFile = new PrintWriter(fwriter);outputFile.println("My Comments on the Exponential Penny Pay

Project.");outputFile.println("One more try!");//close the output file.outputFile.close(); //need this here for open file.

Scanner fileRead = new Scanner(file0);while (fileRead.hasNext()) {

String str = fileRead.nextLine();System.out.println("file read: " + str); }

//close the files.fileRead.close();}System.out.println("The program is ended.");

}

4-81

Generating Random Numbers with the Random Class

• Some applications, such as games and simulations,

require the use of randomly generated numbers.

• The Java API (application programming interfaces) has

a class, Random, for this purpose. To use the Random

class, use the following import statement and create

an instance of the class.

import java.util.Random;

Random randomNumbers = new Random();

//Random class constructor, create a Random object.

4-82

Generating Random Numbers with the Random Class


…

//Declare an int variable.

int number;

//Create a Random object.

Random randomNumbers = new Random();

//get a random integer and assign it to number of 10 digits.

number = randomNumbers.nextInt();

//get a random integer with the range of 1 through 10.

number = randomNumbers.nextInt(10)+1;

//get a random integer with the range of -50 through +49.

number = randomNumbers.nextInt(100)-50;

4-83

Some Methods of the Random Class

Method Description

nextDouble() Returns the next random number as a double. The number

will be within the range of 0.0 and 1.0.

nextFloat() Returns the next random number as a float. The number

will be within the range of 0.0 and 1.0.

nextInt() Returns the next random number as an int. The number

will be within the range of an int, which is –2,147,483,648

to +2,147,483,647.

nextInt(int n)

Should write

int n;

nextInt(n)

nextInt(100)

This method accepts an integer argument, n. It returns a random number as an int. The number will be within the

range of 0 to n - 1.

Can be written as nextInt(100) for range [0, 99];

nextInt(100) – 60 for range [-60, 39];

int n = 10; nextInt(n) – n/2;

See example: RollDice.java

RollDice.java

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public class RandomNumberGenerator_5_1_02 {

public static void main(String[] args) {int intNumber;double doubleNumber;int n = 12;

for(int count = 0; count < 10; count++){

Random rand = new Random();intNumber = rand.nextInt(n)-n/2; //range [-6, 5]System.out.println(count + "\t" +

intNumber);}

}}

0 01 42 -43 -44 -35 -16 17 -48 59 -6

RollDice.java

4-85







Random rand = new Random();//Range [0.0+1.0, 1.0 + 1.0); i.e., [1.0, 2.0).

doubleNumber = rand.nextDouble() + 1.0;

System.out.println(count + "\t" + doubleNumber);

}}

}

0 1.4859759142009537

1 1.2552330132755973

2 1.752354690086619

3 1.1735749253824075

4 1.0658897461299754

5 1.5554388343617656

6 1.7470721933360904

7 1.6203621752503685

8 1.2553601726070869

9 1.2694153737382547

RollDice.java

4-86







Random rand = new Random();//Range [-0.4, 0.5).

doubleNumber = rand.nextDouble() - 0.5;

System.out.println(count + "\t" + doubleNumber);

}}

}

0 0.1576629888267016

1 -0.27643747955419773

2 -0.010713787727349988

3 0.18216706418850848

4 -0.12342106690756738

5 -0.29396478980704566

6 -0.4954877279193056

7 -0.34704168563781734

8 -0.29613855386752197

9 0.4344266241149557

RollDice.java

Loops and Files - Purdue University Fort Wayne

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