1. Introduction to OOP with Java 4th Ed, C. Thomas Wu
Chapter 12
File Input
and Output
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2. Intro to OOP with Java, C. Thomas Wu
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
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3. Intro to OOP with Java, C. Thomas Wu
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.
File inFile = new File(“sample.dat”);
File inFile = new File
(“C:/SamplePrograms/test.dat”);
Opens the file test.dat in the directory C:\SamplePrograms using the generic file separator / and providing the full pathname.
When a program that manipulates a large amount of data practical, we must save the data to a file. If we don’t, then the user must reenter the same data every time he or she runs the program because any data used by the program will be erased from the main memory at program termination.
If the data were saved, then the program can read them back from the file and rebuild the information so the user can work on the data without reentering them. In this chapter you will learn how to save data to and read data from a file. We call the action of saving data to a file file output and the action of reading data from a file file input.
Note: The statements
new File( “C:\\SamplePrograms”, “one.txt”);
and
new File(“C:\\SamplePrograms\\one.text”);
will open the same file.
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4. Intro to OOP with Java, C. Thomas Wu
Some File Methods
if ( inFile.exists( ) ) {
To see if inFile is associated to a real file correctly.
if ( inFile.isFile() ) {
To see if inFile is associated to a file or not. If false, it is 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]); }
List the name of all files in the directory C:\JavaProjects\Ch12
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5. The JFileChooser Class
Intro to OOP with Java, C. Thomas Wu
The JFileChooser Class
A javax.swing.JFileChooser object allows the user to select a file.
JFileChooser chooser = new JFileChooser( );
chooser.showOpenDialog(null);
To start the listing from a specific directory:
We can start the listing from a current directory by writing
String current = System.getProperty("user.dir");
JFileChooser chooser = new JFileChooser(current);
or equivalently
JFileChooser chooser = new JFileChooser( );
chooser.setCurrentDirectory(new File(current));
JFileChooser chooser = new JFileChooser(“C:/JavaPrograms/Ch12");
chooser.showOpenDialog(null);
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6. Getting Info from JFileChooser
Intro to OOP with Java, C. Thomas Wu
Getting Info from JFileChooser
int status = chooser.showOpenDialog(null);
if (status == JFileChooser.APPROVE_OPTION) {
System.out.println("Open is clicked");
} else { //== JFileChooser.CANCEL_OPTION
System.out.println("Cancel is clicked"); }
File selectedFile = chooser.getSelectedFile();
File currentDirectory = chooser.getCurrentDirectory();
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7. Intro to OOP with Java, C. Thomas Wu
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.
The 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.
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8. Intro to OOP with Java, C. Thomas Wu
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.
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9. Streams for Low-Level File I/O
Intro to OOP with Java, C. Thomas Wu
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.
Data 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.
Data 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.
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10. Sample: Low-Level File Output
Intro to OOP with Java, C. Thomas Wu
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();
class TestFileOutputStream {
public static void main (String[] args) throws IOException
//set up file and stream
File outFile = new File("sample1.data");
FileOutputStream outStream = new FileOutputStream(outFile);
//data to output
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(); }
The main method throws an exception. Exception handling is described in Section 11.4.
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11. Sample: Low-Level File Input
Intro to OOP with Java, C. Thomas Wu
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();
import javabook.*;
import java.io.*;
class TestFileInputStream {
public static void main (String[] args) throws IOException
MainWindow mainWindow = new MainWindow();
OutputBox outputBox = new OutputBox(mainWindow);
mainWindow.setVisible( true );
outputBox.setVisible( true );
//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();
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12. Streams for High-Level File I/O
Intro to OOP with Java, C. Thomas Wu
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
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13. Setting up DataOutputStream
Intro to OOP with Java, C. Thomas Wu
Setting up DataOutputStream
A standard sequence to set up a DataOutputStream object:
File outFile = new File( "sample2.data" );
FileOutputStream outFileStream = new FileOutputStream(outFile);
DataOutputStream outDataStream = new DataOutputSteam(outFileStream);
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14. Intro to OOP with Java, C. Thomas Wu
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( );
outDataStream.writeLong( L);
outDataStream.writeFloat( F);
outDataStream.writeDouble( D);
outDataStream.writeChar('A');
outDataStream.writeBoolean(true);
//output done, so close the stream
outDataStream.close(); }
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15. Setting up DataInputStream
Intro to OOP with Java, C. Thomas Wu
Setting up DataInputStream
A standard sequence to set up a DataInputStream object:
File inFile = new File( "sample2.data" );
FileOutputStream inFileStream = new FileOutputStream(inFile);
DataOutputStream inDataStream = new DataOutputSteam(inFileStream);
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16. Intro to OOP with Java, C. Thomas Wu
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(); }
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17. Reading Data Back in Right Order
Intro to OOP with Java, C. Thomas Wu
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.
outStream.writeInteger(…);
outStream.writeLong(…);
outStream.writeChar(…);
outStream.writeBoolean(…);
<integer>
<long>
<char>
<boolean>
inStream.readInteger(…);
inStream.readLong(…);
inStream.readChar(…);
inStream.readBoolean(…);
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18. Textfile Input and Output
Intro to OOP with Java, C. Thomas Wu
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
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19. Sample Textfile Output
Intro to OOP with Java, C. Thomas Wu
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( );
outStream.println("Hello, world.");
outStream.println(true);
//output done, so close the stream
outStream.close(); }
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20. Intro to OOP with Java, C. Thomas Wu
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(); }
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21. Sample Textfile Input with Scanner
Intro to OOP with Java, C. Thomas Wu
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(); }
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22. Intro to OOP with Java, C. Thomas Wu
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 {
. . . }
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23. Intro to OOP with Java, C. Thomas Wu
Saving Objects
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 );
You can even mix objects and primitive data type values. For example,
outObjectStream.writeInt ( );
outObjectStream.writeObject( account1 );
outObjectStream.writeChar ( 'X' );
account1 = new Account();
bank1 = new Bank();
outObjectStream.writeObject( account1 );
outObjectStream.writeObject( bank1 );
Could save objects from the different classes.
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24. Intro to OOP with Java, C. Thomas Wu
Reading Objects
File inFile
= new File("objects.data");
FileInputStream inFileStream
= new FileInputStream(inFile);
ObjectInputStream inObjectStream
= new ObjectInputStream(inFileStream);
Person person
= (Person) inObjectStream.readObject( );
Must type cast to the correct object type.
You can even mix objects and primitive data type values. For example,
outObjectStream.writeInt ( );
outObjectStream.writeObject( account1 );
outObjectStream.writeChar ( 'X' );
Account account1
= (Account) inObjectStream.readObject( );
Bank bank1
= (Bank) inObjectStream.readObject( );
Must read in the correct order.
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25. Saving and Loading Arrays
Intro to OOP with Java, C. Thomas Wu
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 ); {
class FindSum
private int sum;
private boolean success;
public int getSum()
return sum; }
public boolean isSuccess()
return success;
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;
//read the array
Person[ ] people = (Person[]) inObjectStream.readObject( );
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26. Intro to OOP with Java, C. Thomas Wu
Problem Statement
Write a class that manages file I/O of an AddressBook object.
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27. Intro to OOP with Java, C. Thomas Wu
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.
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28. Intro to OOP with Java, C. Thomas Wu
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.
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29. Intro to OOP with Java, C. Thomas Wu
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.
Please use your Java IDE to view the source files and run the program.
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30. Intro to OOP with Java, C. Thomas Wu
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.
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31. Intro to OOP with Java, C. Thomas Wu
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.
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32. Intro to OOP with Java, C. Thomas Wu
Step 2 Code
Directory: Chapter12/Step2
Source Files: AddressBookStorage.java
TestAddressBookWrite.java
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33. Intro to OOP with Java, C. Thomas Wu
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.
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34. Intro to OOP with Java, C. Thomas Wu
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.
Here's the pseudocode to locate a person with the designated name. Notice that for this routine to work correctly, the array must be packed with the real pointers in the first half and null pointers in the last half.
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35. Intro to OOP with Java, C. Thomas Wu
Step 3 Code
Directory: Chapter12/Step3
Source Files: AddressBookStorage.java
TestAddressBookRead.java
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36. Intro to OOP with Java, C. Thomas Wu
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 .
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37. Intro to OOP with Java, C. Thomas Wu
Step 4: Finalize
We perform the critical review of the final program.
We run the final test
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