Chapter 11

1. Chapter 11 File Input and Output Introduction to Object-Oriented Programming with Java--Wu

2. Chapter 11 Objectives After you have read and studied this chapter, you should be able to • Include a FileDialog object in your program to let the user specify a file. • Write bytes to a file and read them back from the file using FileOutputStream and FileInputStream. • Write values of primitive data types to a file and read them back from the file using DataOutputStream and DataInputStream. • Write text data to a file and read them back from the file using PrintWriter and BufferedReader. • Write objects to a file and read them back from the file using ObjectOutputStream and ObjectInputStream. • Write exception-handling routines using the try–catch block. Introduction to Object-Oriented Programming with Java--Wu

3. File inFile = new File(“sample.dat”); File inFile = new File (“C:/SamplePrograms/test.dat”); File inFile = new File(“C:\\SamplePrograms”, “one.txt”); File Objects • To operate on a file, we must first create a File object (from java.io). Opens the file sample.dat in the current directory. Opens the file one.txt in the directory C:\SamplePrograms. Notice the use of the escape character \. Opens the file test.dat in the directory C:\SamplePrograms using the generic file separator / and providing the full pathname. Introduction to Object-Oriented Programming with Java--Wu

4. if ( inFile.exists( ) ) { … } if ( inFile.isFile( ) ) { … } File folder = new File(“C:/JavaProjects/Ch11”); String filename[ ] = folder.list( ); for (int i=0; i < filename.length; i++ ){ outputBox.printLine( filename[i] ); } Some File Methods To see if inFile is associated to a real file correctly. To see if inFile is associated to a file or a directory. List the name of all files in the directory C:\JavaProjects\Ch11. Introduction to Object-Oriented Programming with Java--Wu

5. FileDialog fileBox = new FileDialog( mainWindow, “Open”, FileDialog.LOAD ); fileBox.setVisible( true ); String filename = fileBox.getFile( ); FileDialog - Open • FileDialog is a standard file dialog for selecting a file. Introduction to Object-Oriented Programming with Java--Wu

6. FileDialog fileBox = new FileDialog( mainWindow, “Save As”, FileDialog.SAVE ); fileBox.setVisible( true ); FileDialog - Save Introduction to Object-Oriented Programming with Java--Wu

7. //set up file and stream File outFile = new File("sample1.data"); FileOutputStream outStream = new FileOutputStream( outFile ); //data to save byte[] byteArray = {10, 20, 30, 40, 50, 60, 70, 80}; //write data to the stream outStream.write( byteArray ); //output done, so close the stream (very important here) outStream.close(); Low-Level File I/O – Output Introduction to Object-Oriented Programming with Java--Wu

8. //set up file and stream File inFile = new File("sample1.data"); FileInputStream inStream = new FileInputStream(inFile); //set up an array to read data in int fileSize = (int)inFile.length(); byte[] byteArray = new byte[fileSize]; //read data in and display them inStream.read(byteArray); for (int i = 0; i < fileSize; i++) { outputBox.printLine(byteArray[i]); } //input done, so close the stream inStream.close(); Low-Level File I/O – Input Introduction to Object-Oriented Programming with Java--Wu

9. File outFile = new File("sample2.data"); FileOutputStream outFileStream = new FileOutputStream(outFile); DataOutputStream outDataStream = new DataOutputStream(outFileStream); writeInt writeFloat writeDouble 1 outDataStream 2 outFileStream 3 outFile High-Level File I/O – Output Primitive data type values are written to outDataStream. Primitive data type values are converted to bytes. Converted bytes are written to the file. Introduction to Object-Oriented Programming with Java--Wu

10. File inFile = new File("sample2.data"); FileInputStream inFileStream = new FileInputStream(inFile); DataInputStream inDataStream = new DataInputStream(inFileStream); readInt readFloat readDouble 3 inDataStream 2 inFileStream 1 outFile High-Level File I/O – Input Primitive data type values are read from inDataStream. Bytes are converted to primitive data type values. Bytes are read from the file. Introduction to Object-Oriented Programming with Java--Wu

11. //set up file and stream File outFile = new File("sample3.data"); FileOutputStream outFileStream = new FileOutputStream(outFile); PrintWriter outStream = new PrintWriter(outFileStream); //write values of primitive data types to the stream outStream.println(987654321); outStream.println(11111111L); //… outStream.println('A'); outStream.println(true); //output done, so close the stream outStream.close(); Textfile I/O – Output Introduction to Object-Oriented Programming with Java--Wu

12. //set up file and stream File inFile = new File("sample3.data"); FileReader fileReader = new FileReader(inFile); BufferedReader bufReader = new BufferedReader(fileReader); String str; //get integer str = bufReader.readLine(); int i = Convert.toInt(str); //read other data in a similar manner //input done, so close the stream bufReader.close(); Textfile I/O – Input Introduction to Object-Oriented Programming with Java--Wu

13. Handling Exceptions • An exception occurs when any of the semantic constraints of the Java language is violated. • An exception is said to be thrown and caught . • When our program calls a statement that can throw an exception, we must write a code to either • propagate the thrown exception or • handle it by the try-block statement. Introduction to Object-Oriented Programming with Java--Wu

14. void computeSum (String fileName ) throws IOException • { • File inFile = new File(fileName); • FileInputStream inFileStream = new FileInputStream(inFile); • DataInputStream inDataStream = new DataInputStream(inFileStream); • //read three integers • int i = inDataStream.readInt(); • int j = inDataStream.readInt(); • int k = inDataStream.readInt(); • sum = i + j + k; • inDataStream.close(); • } Propagation Approach • Add the phrase throws <Exception> to the declaration for a method that includes a call to a statement that can throw exceptions. Replace <Exception> with the actual exception class, such as IOException, EOFException, and so forth. Introduction to Object-Oriented Programming with Java--Wu

15. void computeSum (String fileName ) { success = true; try { File inFile = new File(fileName); FileInputStream inFileStream = new FileInputStream(inFile); DataInputStream inDataStream = new DataInputStream(inFileStream); //read three integers int i = inDataStream.readInt(); int j = inDataStream.readInt(); int k = inDataStream.readInt(); sum = i + j + k; inDataStream.close(); } catch (IOException e) { success = false; } } try-catch Approach • Use the try-catch statement to include the exception-handling routines. Introduction to Object-Oriented Programming with Java--Wu

16. try { ... } catch (FileNotFoundException e) { success = false; System.out.println("File " + fileName + " does not exist."); } catch (EOFException e) { success = false; System.out.println("Error: " + "Cannot read beyond end of file"); } catch (IOException e) { success = false; System.out.println("General I/O exception is thrown"); } Handling Multiple Exceptions • You can have more than one catch clause for each try. Introduction to Object-Oriented Programming with Java--Wu

17. Object File I/O • It is possible to store objects just as easily as you store primitive data values. • We use ObjectOutputStream and ObjectInputStream to save to and load objects from a file. • To save objects from a given class, the class declaration must include the phrase implements Serializable. For example, class Person implements Serializable { . . . } Introduction to Object-Oriented Programming with Java--Wu

18. Person person = new Person("Mr. Espresso", 20, 'M'); outObjectStream.writeObject( person ); File outFile = new File("objects.data"); FileOutputStream outFileStream = new FileOutputStream(outFile); ObjectOutputStream outObjectStream = new ObjectOutputStream(outFileStream); account1 = new Account(); bank1 = new Bank(); outObjectStream.writeObject( account1 ); outObjectStream.writeObject( bank1 ); Saving Objects Could save objects from the different classes. Introduction to Object-Oriented Programming with Java--Wu

19. File inFile = new File("objects.data"); FileInputStream inFileStream = new FileInputStream(inFile); ObjectInputStream inObjectStream = new ObjectInputStream(inFileStream); Person person = (Person) inObjectStream.readObject( ); Account account1 = (Account) inObjectStream.readObject( ); Bank bank1 = (Bank) inObjectStream.readObject( ); Loading Objects Must type cast to the correct object type. Must read in the correct order. Introduction to Object-Oriented Programming with Java--Wu

20. Person[] people = new Person[ N ]; //assume N already has a value //build the people array . . . //save the array outObjectStream.writeObject ( people ); //load the array Person[ ] people = (Person[ ]) inObjectStream.readObject ( ); Saving and Loading Arrays • Instead of processing array elements individually, it is possible to save and load the whole array at once. Introduction to Object-Oriented Programming with Java--Wu

21. Let's try one… • Start a program that asks if you want to create or open a file • If create: write three different variables to an ASCII file using an InputBox • Else Open • Save the file • When opening the file: • Open the file using a dialog • Print out the file in an OutputBox Introduction to Object-Oriented Programming with Java--Wu