Download Files

Files from Ch4
File Input and Output
 Reentering data all the time could get
tedious for the user.
 The data can be saved to a file.
 Files can be input files or output files.
 Files:
 Files have to be opened.
 Data is then written to the file.
 The file must be closed prior to program
termination.
 In general, there are two types of files:
 binary
 text
Writing Data To a File
 Objects from the following classes are
used to write data to files:
 FileWriter – This class allows basic file
writing functionality.
 PrintWriter – This class allows the
programmer to write files using the same
style that is used to write to the screen
(i.e. print and println).
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The FileWriter Class
 The FileWriter clas provides other
classes with the basic file writing
functionality.
System.out.println(“Enter the filename.”);
filename = Keyboard.readString();
FileWriter fwriter = new FileWriter(filename);
 This will create an object that can
access the file filename.
 Warning: if the file already exists, it
will be erased and replaced with the
new file.
The PrintWriter Class
 The PrintWriter class adds to the
functionality of the FileWriter class.
 The PrintWriter cannot directly access
the file but must work through the
FileWriter class.
 The PrintWriter needs a FileWriter
object in order to work:
FileWriter fwriter = new FileWriter("StudentData.txt");
PrintWriter outputFile = new PrintWriter(fwriter);
The PrintWriter Class
 Once linked to the fwriter object, the
outputFile object can talk to the file.
outputFile.open();
outputFile.println(“Jim”);
outputFile.close();
 Just as with the System.out object,
the println method of the PrintWriter
class will place a newline character
after the written data.
 The print method can be used to avoid
writing the newline character.
Exceptions
 When something unexpected happens in a
Java program, an exception is thrown.
 The method currently executing when the
exception is thrown must either handle the
exception or pass it up the line.
 Handling the exception will be discussed
later.
 To pass it up the line, the method needs a
throws clause in the method header.
Exceptions
 To insert a throws clause in a method
header, simply add the word throws and the
name of the expected exception.
 The class Exception can be used to catch all
exceptions.
public static void main(String[] args) throws IOException{…}
 File I/O is a checked exception (meaning the
exception must be handled or passed up).
 A program with file I/O will generate a
compile-time error if the exception is not
handled or passed up.
 Example:
FileWriteDemo.java
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import java.util.Scanner; // Needed for Scanner class
import java.io.*;
// Needed for file classes
public class FileWriteDemo
{
public static void main(String[] args) throws IOException
{
String filename;
// File name
String friendName; // Friend's name
int numFriends;
// Number of friends
Scanner keyboard = new Scanner(System.in);
System.out.print("How many friends do you have? ");
numFriends = keyboard.nextInt();
keyboard.nextLine();
System.out.print("Enter the filename: ");
filename = keyboard.nextLine();
FileWriteDemo.java
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}
}
FileWriter fwriter = new FileWriter(filename);
PrintWriter outputFile = new PrintWriter(fwriter);
for (int i = 1; i <= numFriends; i++)
{
// Get the name of a friend.
System.out.print("Enter the name of friend " +
"number " + i + ": ");
friendName = keyboard.nextLine();
outputFile.println(friendName);
}
outputFile.close();
System.out.println("Data written to the file.");
Appending Text to a File
 To avoid erasing a file that already
exists:
 Create a FileWriter object using an
optional boolean argument that tells the
object to append data to the file.
FileWriter fwriter = new FileWriter(“filename”,
true);
 Data written to a file created in such
a manner will be appended to the end
of the current file.
Specifying a File Location
Windows’ Crazy Backslash
 Windows evolved from DOS.
 Since DOS was simply a hacked
version of CP/M, it maintained the
backslash (\) as a directory separator.
 Remember, if the backslash is used in
a String literal, it is the escape
character so there must be two of
them.
FileWriter fwriter = new FileWriter("A:\\PriceList.txt");
PrintWriter outputFile = new PrintWriter(fwriter);
Specifying a File Location
 This is only necessary if the backslash
is in a String literal.
 If the backslash is in a String object
then it will be handled properly.
 Fortunately, Java allows Unix style
filenames using the forward slash (/)
to separate directories.
FileWriter fwriter = new
FileWriter("/home/rharrison/names.txt");
PrintWriter outputFile = new PrintWriter(fwriter);
Reading Data From a File
 Java provides several classes to read
data from a file.
 FileReader
 Open an existing file for reading and establish
a connection with it.
 BufferedReader
 Uses a buffer to allow the reading of full lines
of text at a time rather than one byte at a
time.
The FileReader and
BufferedReader Classes
System.out.print("Enter the filename: ");
filename = Keyboard.readString();
FileReader freader = new FileReader(filename);
BufferedReader inputFile = new BufferedReader(freader);
 The lines above:
 Prompt the user for a filename.
 Get the filename from the user.
 Create an instance of the FileReader class
that is associated with the filename.
 Create an instance of the BufferedReader
class that buffers the instance of the
FileReader class.
The FileReader and
BufferedReader Classes
 Once an instance of BufferedReader is
created, lines of text can be read in.
customerName = inputFile.readLine();
 A file pointer is created when the file
is first opened.
 As the file is read, the pointer moves
to indicate the text that is to be read
next.
Exceptions
 The FileReader and BufferedReader
classes can throw exceptions.
 A throws IOException clause needs to
be placed on the method header of
the method that instantiates a
FileReader or BufferedReader object.
 Also, any method that uses a
FileReader or BufferedReader needs a
throws IOException clause.
Detecting The End of a File
 The readLine() method of the BufferedReader
class will return null if the end of the file has
been reached.
FileReader freader = new FileReader(filename);
BufferedReader inputFile = new BufferedReader(freader);
// Read the first item.
String str = inputFile.readLine();
// If an item was read, display it
// and read the remaining items.
while (str != null)
{
System.out.println(str);
str = inputFile.readLine();
}
inputFile.close();// close the file when done.
Reading a File Flowchart
Open the file
Read the first item
(priming read)
Did
readLine()
return null?
Close the file
Process the item
Read next item
FileReadDemo.java
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import java.util.Scanner; // Needed for the Scanner class
import java.io.*;
// Needed for file classes
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public class FileReadDemo
{
public static void main(String[] args) throws IOException
{
String filename; // File name
String friendName; // Friend's name
Scanner keyboard = new Scanner(System.in);
System.out.print("Enter the filename: ");
filename = keyboard.nextLine();
FileReadDemo.java
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FileReader freader = new FileReader(filename);
BufferedReader inputFile = new
BufferedReader(freader);
friendName = inputFile.readLine();
while (friendName != null)
{
System.out.println(friendName);
friendName = inputFile.readLine();
}
inputFile.close();
}
}