Bài giảng Chapter 12: File Input and Output

Chapter 12File Input and Output©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Chapter 12 ObjectivesAfter you have read and studied this chapter, you should be able toInclude a JFileChooser 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.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Chapter 12 Objectives, cont.After you have read and studied this chapter, you should be able toWrite 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.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsThe action of saving, or writing, data to a file is called file output.The action of reading data from a file is called file input.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsBefore we can read data from a file, we must create a File object and associate it to the file from which we wish to read.We do this by calling a File constructor:File inFile = newFile(“sample.data”);©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsThis approach assumes that the file is located in the current directory. Otherwise, we must specify the path and file name when we call the constructor:File inFile = new File(“C:/JavaPrograms”,”xyz.data”);The formatting of the path name and file name is different for different operating systems.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 12.1Directory structure used for the examples in this section. We assume the Windows environment.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsWe can check if a File object is associated correctly to an existing file by calling its exists method:if (inFile.exists()){ //inFile is associated correctly to //an existing file} else { //inFile is not associated to any //existing file}©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsWhen a valid association is established, we say a file is opened. A file must be opened before we can do any input and output to the file.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsA File object may also be associated to a directory.File directory = new File (“C:/JavaPrograms/Ch12”);String filename[] = directory.list(); for (int i=0; i<filename.length; i++){ System.out.println(filename[i]);}©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsTo determine if a File object is associated to a file or directory, we call its boolean method isFile.A javax.swing.JFileChooser object allows the user to select a file.JFileChooser chooser = new JFileChooser();...chooser.showOpenDialog(null);©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 12.2A sample JFileChooser object displayed with the showOpenDialog method. The dialog title and the okay button are labeled Open.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsThe getSelectedFile method retrieves the desired file.The getName and getAbsolutePath methods retrieve the name and full path of a selected file.The showSaveDialog method displays a JFileChooser with a Save button.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 12.3A sample JFileChooser object displayed with the showSaveDialog method. ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser Objects/*File: TestJFileChooser.java*/import java.io.*;import javax.swing.*;class Ch12TestJFileChooser { public static void main (String[] args) { JFileChooser chooser; File file, directory; int status; chooser = new JFileChooser( ); status = chooser.showOpenDialog(null); if (status == JFileChooser.APPROVE_OPTION) { file = chooser.getSelectedFile(); directory = chooser.getCurrentDirectory();©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser Objects System.out.println("Directory: " + directory.getName()); System.out.println("File selected to open: " + file.getName()); System.out.println("Full path name: " + file.getAbsolutePath()); } else { JOptionPane.showMessageDialog(null, "Open File dialog canceled"); } System.out.println("\n\n"); status = chooser.showSaveDialog(null); ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser Objects if (status == JFileChooser.APPROVE_OPTION) { file = chooser.getSelectedFile(); directory = chooser.getCurrentDirectory(); System.out.println("Directory: " + directory.getName()); System.out.println("File selected for saving data: " + file.getName()); System.out.println("Full path name: " + file.getAbsolutePath()); } else { JOptionPane.showMessageDialog(null, "Save File dialog canceled"); } }}©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 12.4A sample output from running the Ch12TestJFileChooser program once.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsA file filter may be used to remove unwanted files from the list.Define a subclass of the javax.swing.filechooser.FileFilter class and provide the accept and getDescription methods.public boolean accept(File file)public String getDescription( )©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsThe accept method returns true if the parameter file is a file to be included in the list.The getDescription method returns a text that will be displayed as one of the entries for the “Files of Type:” drop-down list. ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.1 File and JFileChooser ObjectsWhen the filter class is defined, we can set it to a file chooser to restrict the listing to the desired directories and files.JFileChooser chooser = new JFileChooser();chooser.setFileFilter(new JavaFilter(());int status = chooser.showOpenDialog(null);©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OOnce a file is opened by associating a File object to it, we can access the file.To read data from or write data to a file, we must create one of the Java stream objects and attach it to the file.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OA stream is a sequence of data items, usually 8-bit bytes.Java has two types of streams: an input stream and an output stream. An input stream has a source form which the data items come, and an output stream has a destination to which the data items are going. ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OFileOutputStream and FileInputStream are two stream objects that facilitate file access.FileOutputStream allows us to output a sequence of bytes; values of data type byte.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OWe will process the following byte array:byte[] byteArray = {10, 20, 30, 40, 50, 60, 70, 80}; We create a File object: File outFile = new File (“sample1.data”); Associate a new FileOutputStream object to outFile: FileOutputStream outStream = new FileOutputStream(outFile); and write the whole byte array at once to the file: outstream.write(byteArray);©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OAfter the values are written to the file, we must close the stream:outStream.close();If the stream object is not closed, then data may get lost due to data caching.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OData is saved in blocks of bytes to reduce the time it takes to save all of our data. The operation of saving data as a block is called data caching.To carry out data caching, part of memory is reserved as a data buffer or cache, which is used as a temporary holding place.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OData are first written to a buffer. When the buffer becomes full, the data in the buffer are actually written to a file.If there are any remaining data in the buffer and the file is not closed, those data will be lost.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OTo read data into a program, we reverse the steps in the output routine.We use the read method of FileInputStream to read in an array of bytes. ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OFirst we create a FileInputStream object:File inFile = new File(“sample1.data”);FileInputStream inStream = new FileInputStream(inFile);Then we must declare and create byteArray:int filesize = (int) inFile.length();byte[] byteArray = new byte[filesize];©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OWe use the length method of the File class to determine the size of the file. This allows us to create an array of bytes whose size is the size of the file.Finally, we read the data into an array of bytes:inStream.read(byteArray);©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.2 Low-Level File I/OWe can output data other than bytes if we can type cast them into bytes.To read the data back, we use the read method. Depending on the data type we converted the data from, we may need to type cast back into the original data type.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.3 High-Level File I/OUsing DataOutputStream allows us to output Java primitive data type values.A DataOutputStream object will convert the primitive data type values into a sequence of bytes.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.3 High-Level File I/OThe argument to the DataOutputStream constructor is a FileOutputStream object. A DataOutputStream object does not get connected to a file directly. The role of a DataOutputStream object is to provide high-level access to a file by handling the data type conversions.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 12.5A diagram showing how the three objects outFile, outFileStream, and outDataStream are related.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.3 High-Level File I/OTo read data back from the file, we reverse the operation.We use three objects: File, FileInputStream, and DataInputStream.Data must be read in the order in which it was written; otherwise, the results will be unpredictable.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 12.6A diagram showing how the three objects inFile, inFileStream, and inDataStream are related.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 12.7The order of write and read operations must match to read the stored data back correctly.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.3 High-Level File I/OFileOutputStream and DataOutputStream objects produce a binary file in which the contents are stored in the format (binary format) in which they are stored in the main memory.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.3 High-Level File I/OData may be stored in ASCII format instead of binary format. With ASCII format, all data are converted to string data. A file whose contents are stored in ASCII format is called a text file.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.3 High-Level File I/OPrintWriter is an object we use to generate a textfile. PrintWriter supports only two output methods: printprintln (for print line)©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.3 High-Level File I/OAn argument to the methods may be any primitive data type. The methods convert the parameter to string and output this string value.The constructor of PrintWriter, like that of DataOutputStream, requires an output stream as its argument.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.3 High-Level File I/OTo read data from a text file, we use the FileReader and BufferedReader objects. We first associate a BufferedReader object to a file.Then we read data, using the readLine method of BufferedReader.Finally, we convert the String to a primitive data type as necessary.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.3 High-Level File I/OFile inFile = new File(“sample3.data”);FileReader fileReader = new FileReader(inFile);BufferedReader bufReader = new BufferedReader(fileReader)String str = bufReader.readLine();©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.4 Object I/OWe can now store objects just as we can store primitive data values.To write objects to a file, we use ObjectOutputStream. To read objects from a file, we use ObjectInputStream.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.4 Object I/OIn this example, we will write Person objects to a file.The first step is to modify the Person class definition to allow ObjectOutputStream and ObjectInputStream to perform object I/O.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.4 Object I/OWe modify the definition by adding the phrase implements Serializable to it.import java.io.*;class Person implements Serializable {//the rest is the same}There are no methods for us to define in the implementation class. ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.4 Object I/OTo save objects to a file, we first create an ObjectOutputStream object:File outFile = new File(“objects.dat”);FileOutputStream outFileStream = new FileOutputStream(outFile);ObjectOutputStream outObjectStream = new ObjectOutputStream(outFileStream);©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.4 Object I/OTo save a Person object, we executePerson person = new Person(“Mr. Espresso”, 20, ‘M’);outObjectStream.writeObject(person);©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.4 Object I/ODifferent types of objects may be saved to a single file.We can also mix objects and primitive data type values in the same file.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.4 Object I/OTo read objects from a file, we use FileInputStream and ObjectInputStream. We use the method readObject to read an object.Because we can store any types of objects to a single file, we must type cast the object read from the file.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.4 Object I/OThe readObject method can throw a ClassNotFoundException (wrong type casting) in addition to an IOException. Either exception may be caught or propagated.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.12.4 Object I/OIf a file contains objects from different classes, they must be read in the correct order and the matching type casting must be applied.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.