© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--WuChapter Chapter 11 File Input and Output

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter 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.

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter File Objects To operate on a file, we must first create a File object (from java.io). File inFile = new File(“sample.dat”); File inFile = new File(“C:\\SamplePrograms”, “one.txt”); File inFile = new File (“C:/SamplePrograms/test.dat”); 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.

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter Some File Methods 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] ); } 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.

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter FileDialog - Open FileDialog is a standard file dialog for selecting a file. FileDialog fileBox = new FileDialog( mainWindow, “Open”, FileDialog.LOAD ); fileBox.setVisible( true ); String filename = fileBox.getFile( );

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter FileDialog - Save FileDialog fileBox = new FileDialog( mainWindow, “Save As”, FileDialog.SAVE ); fileBox.setVisible( true );

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter Low-Level File I/O – Output //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();

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter Low-Level File I/O – Input //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();

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter High-Level File I/O – Output File outFile = new File("sample2.data"); FileOutputStream outFileStream = new FileOutputStream(outFile); DataOutputStream outDataStream = new DataOutputStream(outFileStream); Primitive data type values are written to outDataStream. Primitive data type values are converted to bytes. Converted bytes are written to the file. outDataStream writeFloat writeInt writeDouble 1 outFileStream 2 outFile 3

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter High-Level File I/O – Input File inFile = new File("sample2.data"); FileInputStream inFileStream = new FileInputStream(inFile); DataInputStream inDataStream = new DataInputStream(inFileStream); Primitive data type values are read from inDataStream. Bytes are converted to primitive data type values. Bytes are read from the file. readFloat readInt readDouble 3 inDataStream 2 inFileStream outFile 1

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter Textfile I/O – Output //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( ); outStream.println( L); //… outStream.println('A'); outStream.println(true); //output done, so close the stream outStream.close();

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter Textfile I/O – Input //set up file and stream FileinFile = new File("sample3.data"); FileReaderfileReader = new FileReader(inFile); BufferedReaderbufReader = 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();

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter 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.

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter Propagation Approach Add the phrase throws to the declaration for a method that includes a call to a statement that can throw exceptions. Replace with the actual exception class, such as IOException, EOFException, and so forth. 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(); }

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter try-catch Approach Use the try-catch statement to include the exception- handling routines. 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; }

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter Handling Multiple Exceptions You can have more than one catch clause for each try. 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"); }

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter 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 {... }

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter Saving Objects FileoutFile = new File("objects.data"); FileOutputStream outFileStream = new FileOutputStream(outFile); ObjectOutputStream outObjectStream = new ObjectOutputStream(outFileStream); Person person = new Person("Mr. Espresso", 20, 'M'); outObjectStream.writeObject( person ); account1= new Account(); bank1 = new Bank(); outObjectStream.writeObject( account1 ); outObjectStream.writeObject( bank1 ); Could save objects from the different classes.

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter Loading Objects FileinFile = 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( ); Must read in the correct order. Must type cast to the correct object type.

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter Saving and Loading Arrays Instead of processing array elements individually, it is possible to save and load the whole array at once. 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 ( );

© 2000 McGraw-HillIntroduction to Object-Oriented Programming with Java--WuChapter 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