Java căn bản - Chapter12

Chapter 12 File Input and Output

Chapter 12 Objectives After you have read and studied this chapter, you should be able to Include 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. Write text data to a file and read them back from the file, using PrintWriter and BufferedReader Read a text file using Scanner Write objects to a file and read them back from the file, using ObjectOutputStream and ObjectInputStream

The File Class 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 test.dat in the directory C:\SamplePrograms using the generic file separator / and providing the full pathname. File inFile = new File(“sample.dat”); File inFile = new File (“C:/SamplePrograms/test.dat”);

Some File Methods To see if inFile is associated to a real file correctly. To see if inFile is associated to a file or not. If false, it is a directory. List the name of all files in the directory C:\JavaProjects\Ch12 if ( inFile.exists ( ) ) { if ( inFile.isFile () ) { File directory = new File ( "C:/JavaPrograms/Ch12" ) ; String filename [] = directory.list () ; for ( int i = 0; i < filename.length; i++ ) { System.out.println ( filename [ i ]) ; }

The JFileChooser Class A javax.swing.JFileChooser object allows the user to select a file. To start the listing from a specific directory: JFileChooser chooser = new JFileChooser ( ) ; chooser.showOpenDialog ( null ) ; JFileChooser chooser = new JFileChooser ( "D:/JavaPrograms/Ch12" ) ; chooser.showOpenDialog ( null ) ;

Getting Info from JFileChooser int status = chooser.showOpenDialog ( null ) ; if ( status == JFileChooser.APPROVE_OPTION ) { JOptionPane.showMessageDialog ( null , "Open is clicked" ) ; } else { //== JFileChooser.CANCEL_OPTION JOptionPane.showMessageDialog ( null , "Cancel is clicked" ) ; } File selectedFile = chooser.getSelectedFile () ; File currentDirectory = chooser.getCurrentDirectory () ;

Applying a File Filter A file filter may be used to restrict the listing in JFileChooser to only those files/directories that meet the designated filtering criteria. To apply a file, we 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 ( ) See the JavaFilter class that restricts the listing to directories and Java source files.

12.2 Low-Level File I/O 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. A 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.

Streams for Low-Level File I/O FileOutputStream and FileInputStream are two stream objects that facilitate file access. FileOutputStream allows us to output a sequence of bytes; values of data type byte . FileInputStream allows us to read in an array of bytes.

Sample: Low-Level File 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 outStream.close () ;

Sample: Low-Level File 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++ ) { System.out.println ( byteArray [ i ]) ; } //input done, so close the stream inStream.close () ;

Streams for High-Level File I/O FileOutputStream and DataOutputStream are used to output primitive data values FileInputStream and DataInputStream are used to input primitive data values To read the data back correctly, we must know the order of the data stored and their data types

Setting up DataOutputStream A standard sequence to set up a DataOutputStream object:

Sample Output import java.io.*; class Ch12TestDataOutputStream { public static void main ( String [] args ) throws IOException { . . . //set up outDataStream //write values of primitive data types to the stream outDataStream.writeInt ( 987654321 ) ; outDataStream.writeLong ( 11111111L ) ; outDataStream.writeFloat ( 22222222F ) ; outDataStream.writeDouble ( 3333333D ) ; outDataStream.writeChar ( 'A' ) ; outDataStream.writeBoolean ( true ) ; //output done, so close the stream outDataStream.close () ; } }

Setting up DataInputStream A standard sequence to set up a DataInputStream object:

Sample Input import java.io.*; class Ch12TestDataInputStream { public static void main ( String [] args ) throws IOException { . . . //set up inDataStream //read values back from the stream and display them System.out.println ( inDataStream.readInt ()) ; System.out.println ( inDataStream.readLong ()) ; System.out.println ( inDataStream.readFloat ()) ; System.out.println ( inDataStream.readDouble ()) ; System.out.println ( inDataStream.readChar ()) ; System.out.println ( inDataStream.readBoolean ()) ; //input done, so close the stream inDataStream.close () ; } }

Reading Data Back in Right Order The order of write and read operations must match in order to read the stored primitive data back correctly.

Textfile Input and Output Instead of storing primitive data values as binary data in a file, we can convert and store them as a string data. This allows us to view the file content using any text editor To output data as a string to file, we use a PrintWriter object To input data from a textfile, we use FileReader and BufferedReader classes From Java 5.0 (SDK 1.5), we can also use the Scanner class for inputting textfiles

Sample Textfile Output import java.io.*; class Ch12TestPrintWriter { public static void main ( String [] args ) throws IOException { //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 ( "Hello, world." ) ; outStream.println ( true ) ; //output done, so close the stream outStream.close () ; } }

Sample Textfile Input import java.io.*; class Ch12TestBufferedReader { public static void main ( String [] args ) throws IOException { //set up file and stream File inFile = new File ( "sample3.data" ) ; FileReader fileReader = new FileReader ( inFile ) ; BufferedReader bufReader = new BufferedReader ( fileReader ) ; String str; str = bufReader.readLine () ; int i = Integer.parseInt ( str ) ; //similar process for other data types bufReader.close () ; } }

Sample Textfile Input with Scanner import java.io.*; class Ch12TestScanner { public static void main ( String [] args ) throws IOException { //open the Scanner Scanner scanner = new Scanner ( new File ( "sample3.data" )) ; //get integer int i = scanner.nextInt () ; //similar process for other data types scanner.close () ; } }

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 { . . . }

Saving Objects Could save objects from the different classes. File outFile = 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 ) ;

Reading Objects Must read in the correct order. Must type cast to the correct object type. 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 ( ) ;

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 ) ; //read the array Person [ ] people = ( Person []) inObjectStream.readObject ( ) ;

Problem Statement Problem statement: Write a class that manages file I/O of an AddressBook object.

Development Steps We will develop this program in four steps: Implement the constructor and the setFile method. Implement the write method. Implement the read method. Finalize the class.

Step 1 Design We identify the data members and define a constructor to initialize them. Instead of storing individual Person objects, we will deal with a AddressBook object directly using Object I/O techniques.

Step 1 Code Directory: Chapter12/Step1 Source Files: AddressBookStorage.java TestAddressBookStorage.java Program source file is too big to list here. From now on, we ask you to view the source files using your Java IDE.

Step 1 Test We include a temporary output statement inside the setFile method. We run the test main class and verify that the setFile method is called correctly.

Step 2 Design Design and implement the write method The data member filename stores the name of the object file to store the address book. We create an ObjectOutputStream object from the data member filename in the write method. The write method will propagate an IOException when one is thrown.

Step 2 Code Directory: Chapter12/Step2 Source Files: AddressBookStorage.java TestAddressBookWrite.java

Step 2 Test We run the test program several times with different sizes for the address book. We verify that the resulting files indeed have different sizes. At this point, we cannot check whether the data are saved correctly or not. We can do so only after finishing the code to read the data back.

Step 3 Design Design and implement the read method. The method returns an AddressBook object read from a file (if there's no exception) The method will propagate an IOException when one is thrown.

Step 3 Code Directory: Chapter12/Step3 Source Files: AddressBookStorage.java TestAddressBookRead.java

Step 3 Test We will write a test program to verify that the data can be read back correctly from a file. To test the read operation, the file to read the data from must already exist. We will make this test program save the data first by using the TestAddressBookWrite class from .

Step 4: Finalize We perform the critical review of the final program. We run the final test

Java căn bản - Chapter12

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