Text file I/O and simple GUI dialogues

(1)

Java Actually 11: Text file I/O and simple GUI dialogues 11-1/24

Chapter 11

Text file I/O and simple GUI dialogues

Lecture slides for:

Java Actually: A Comprehensive Primer in Programming

Khalid Azim Mughal, Torill Hamre, Rolf W. Rasmussen Cengage Learning, 2008.

ISBN: 978-1-844480-933-2

http://www.ii.uib.no/~khalid/jac/

Permission is hereby granted to use these lecture slides in conjunction with the book. Modified: 14/11/08

Overview

• Text File I/O

– File handling – Data records – Text files

• Simple GUI dialogue design: javax.swing.JOptionPane

– Message dialogs – presenting information to the user – Input dialogs – reading data from the user

(2)

Input and Output

• Input: Data a program reads.

– Input comes from a source which produces the data. – For example keyboard, a file

• Output: Data a program writes.

– Output is written to a destination that can receive the data. – For example terminal window, a file

• A data file offers permanent storage of data from a program on some external media.

File handling

• A file denotes a specific storage area on some external media, for example hard disk, where information is stored.

– Data in filer is stored as a sequence of bytes. • Binary file: Data is interpreted as a sequence of bytes.

• Text file: Data is interpreted as a sequence of character, where a character can be represented by one or more bytes.

(3)

Default size of primitive data types

Primitive data types Size in bytes boolean 1 char 2 int 4 long 8 float 4 double 8

File path

• A file path identifies a file in the file system:

String datafilename = "employees.dat"; // File path identifies // the file.

String filepath1 = "company\\employees.dat"; // Windows String filepath2 = "company/employees.dat"; // Unix

(4)

Data Records

• Problem: Store information about employees on a file.

String firstName; String lastName; double hourlyRate; Gender gender;

• A record consists of one or more data fields.

• A field usually contains a primitive value, but it can also be a string.

• See Program 10.1: Classes Employee and PersonnelRegister. MALE 30.00 Olsen Ole field₁ record

field₂ field₃ field₄

MALE 40.00

Bailey Bill

field₁ field₂ field₃ field₄

Text Files

• A text file contains lines of text.

• A text line consists of a sequence of characters terminated by a line terminator string. – Methods in Java ensure that the line terminator string is interpreted correctly. • Procedure for handling files:

1. Open the file.

2. Choose appropriate class to convert values between the program and the file. 3. Read from or write to the file.

(5)

Writing to a text file

FileWriter textFileWriter = new FileWriter(textFileName); // Step 1 PrintWriter textWriter = new PrintWriter(textFileWriter); // Step 2

It's now or never Flaming Star Suspious Minds Angel Crying in the Chapel If I can dream Fever CC Rider Little Sister Are you lonesome tonight? Devil in desguise Object of class PrintWriter Object of class FileWriter text file Step 2 Step 1 print(boolean b) print(char c) print(char[] cArray) print(double d) print(float f) print(int i) print(long l) print(Object obj) print(String str) println() println(...) printf(...) ...

values char bytes

Writing to a text file (cont.)

• See Program 10.1 and Program 10.2.

1. Create a FileWriter object to open the file for writing:

FileWriter textFileWriter = new FileWriter(dataFileName); // (5) 2. Create a PrintWriter object that is connected to the FileWriter from step 1:

PrintWriter textWriter = new PrintWriter(textFileWriter); // (6)

3. Write text representations of values using the print methods of the PrintWriter class. textWriter.printf(

"%s" + FIELD_TERMINATOR + "%s" + FIELD_TERMINATOR + "%.2f" + FIELD_TERMINATOR + "%s%n",

employee.firstName, employee.lastName, employee.hourlyRate, employee.gender); 4. Finish by closing the file:

(6)

Basic Exception Handling

• An exception signals that an error or an unexpected situation has occurred during execution.

– Normal execution is suspended and if no actions is taken by the program, execution is aborted.

• Exception handling is based on throw-and-catch principal.

• If a checked exception can be thrown by a method, the compiler reports an error if the method does not explicitly deal with the exception.

– A method must either catch the exception, or throw it again.

• A throws_{clause in the method header specifies checked exceptions that a method can}

throw.

Employee readEmployeeData(BufferedReader textReader)

throws IOException { // (17) ...

String record = textReader.readLine(); // (18) ...

}

Reading from a text file

FileReader textFileReader = new FileReader(textFileName); // Step 1 BufferedReader textReader = new BufferedReader(textFileReader); // Step 2

It's now or never Flaming Star Suspious Minds Angel Crying in the Chapel If I can dream Fever CC Rider Little Sister Are you lonesome tonight? Devil in desguise

Object of class

BufferedReader

Object of class

FileReader

text file Step 1 Step 2

String readLine()

(7)

Reading from a text file

• See Program 10.1 and Program 10.2.

1. Create a FileReader object to open the file for reading:

FileReader textFileReader = new FileReader(dataFileName); // (12) 2. Create a BufferedReader object that is connected to the FileReader from step 1:

BufferedReader textReader = new BufferedReader(textFileReader);// (13)

3. Read one text line at a time using the readLine() method of the BufferedReader_class:

String record = textReader.readLine(); // (18) – Characters in each field can be extracted (see the figure below):

int fieldTerminatorIndex1 = record.indexOf(FIELD_TERMINATOR); // (19) String firstName = record.substring(0, fieldTerminatorIndex1); // (20) – Characters must be converted to primitive values (see the figure below): String doubleStr = record.substring(fieldTerminatorIndex2 + 1,

fieldTerminatorIndex3);

double hourlyRate = Double.parseDouble(doubleStr); // (21) String genderStr = record.substring(fieldTerminatorIndex3 + 1);

Gender gender; if (genderStr.equals(Gender.MALE.toString())) { // (22) gender = Gender.MALE; } else { gender = Gender.FEMALE; }

4. Close the file:

(8)

Reading records and converting fields to appropriate values

O l e , O l s e n , 3 0 . 0 0 , M A L E \n

Text line index

Step 2: Extract substrings

Step 1: Find the index of the field terminator characters

Step 3: Convert substrings to floating-point to a Gender value

0 3 9 15

fieldTerminatorIndex1 fieldTerminatorIndex2 fieldTerminatorIndex3

Gender.MALE "MALE" 30.0 "30.00" "Olsen" "Ole"

field₁ field₂ field₃ field₄

Simple Dialogue Design with the class

JOptionPane

• The class JOptionPane provides dialog boxes that can be used to:

– present information to the user – read input from the user

– confirm information from the user • The class defines 3 static methods:

– showTypeDialog(), where Type can be replaced with Message, Input or Confirm,

depending on the kind of dialog box. • All dialog boxes are modal.

• GUI-based programs must be terminated by the call System.exit(0)_{in the source}

code.

(9)

Presenting data to the user

• Such a dialog box usually displays a message to the user and an OK button that the user can click to dismiss the dialog box after having read the message.

• The showMessageDialog() can be used for this purpose.

Using the method

showMessageDialog()

(Program 10.3)

(a) JOptionPane.showMessageDialog( // (1) null, "How ya'doin!"); (b) JOptionPane.showMessageDialog( // (2) null,

"You have hit the jackpot!", "Important Message",

JOptionPane.WARNING_MESSAGE);

Reading input from the user

• This type of dialog box presents a text field that user can type in, and 2 buttons (OK

and CANCEL) for submitting the data or cancelling the dialog box.

• The showInputDialog() can be used for this purpose.

– The method returns the contents of the text field as a string that must be explicitly converted to types of values, as required by the program.

(10)

Using the method

showInputDialog()

(Program 10.4)

(c)

JOptionPane.showInputDialog( // (4) null, "City:",

"Input data", JOptionPane.PLAIN_MESSAGE);

(a) (b) JOptionPane.showInputDialog( // (1) "Name:" ); JOptionPane.showInputDialog( // (2) null, "Zipcode:" );

Confirming information from the user

• Such a dialog box usually consists of a question about some fact that the user must confirm.

– In addition the dialog box has 2 buttons (YES and NO) to answer the question.

• The showConfirmDialog() can be used for this purpose.

– The return value can be interpreted as shown in Table 10.5.

(11)

Using the method

showConfirmDialog()

(Program 10.5)

(c)

JOptionPane.showConfirmDialog( // (3) null,

"Java is fun, right?", "Confirmation 3", JOptionPane.OK_CANCEL_OPTION, JOptionPane.PLAIN_MESSAGE); (a) (b) JOptionPane.showConfirmDialog( // (1) null,

"Are you getting married?");

JOptionPane.showConfirmDialog( // (2) null,

"We understand each other, right?", "Confirmation 2",

JOptionPane.YES_NO_OPTION );

Support for Simple GUI dialogue

• See the GUIDialog.java (Program E.3).

Static methods in class GUIDialog _Description

int requestInt(Object prompt) _{Read an}int value double requestDouble(Object prompt) _{Read an}double value String requestString(Object prompt) _{Read an non-empty string} void prompt(Object message) _{Write a message}

(12)

Example: Using the class

GUIDialog

// Using GUIDialog class

public class GUIDialogDemo {

public static void main(String[] args) {

GUIDialog.prompt( // (1) "Program finds the largest number in a sequence of numbers.\n" + "A negative number signals end of the sequence.");

int maxValue = 0; while (true) {

int n = GUIDialog.requestInt("Enter an integer: "); // (2) if (n < 0) { break; } if (n > maxValue) { maxValue = n; } }

GUIDialog.prompt("Largest number: " + maxValue); // (3) System.exit(0);

} }

Example: Dialogue Boxes

(a)