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).

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 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 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 LocationWindows’ 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

Read the first item(priming read)

Process the item

Close the file

Read next item

Open the file

DidreadLine()return null?

FileReadDemo.java

import java.util.Scanner; // Needed for the Scanner class import java.io.*; // Needed for file classes

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 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(); } }