Internet Programming with Java Course

1.2 Java Input/Output Text and Binary Streams

Introduction to Data Streams

Often programs need to bring in information from an external source or send out information to an external destination. The information can be anywhere: in a file, on disk, somewhere on the network, in memory, or in another program. Also, it can be of any type: objects, characters, images, or sounds.

To bring in information, a program opens a stream on an information source (a file, memory, a socket) and reads the information serially, like this:

Similarly, a program can send information to an external destination by opening a stream to a destination and writing the information out serially, like this:

No matter where the information is coming from or going to and no matter what type of data is being read or written, the algorithms for reading and writing data is pretty much always the same.

Reading

Writing

open a stream
while more information
 read information
close the stream
open a stream
while more information
 write information
close the stream

The java.io package contains a collection of stream classes that support these algorithms for reading and writing. These classes are divided into two class hierarchies based on the data type (either characters or bytes) on which they operate.

However, it's often more convenient to group the classes based on their purpose rather than on the data type they read and write. Thus, we can cross-group the streams by whether they read from and write to data "sinks" or process the information as its being read or written.



Overview of I/O Streams

Character Streams

Reader and Writer are the abstract superclasses for character streams in java.io. Reader provides the API and partial implementation for readers--streams that read 16-bit characters--and Writer provides the API and partial implementation for writers--streams that write 16-bit characters.

Subclasses of Reader and Writer implement specialized streams and are divided into two categories: those that read from or write to data sinks (shown in gray in the following figures) and those that perform some sort of processing (shown in white). The figure shows the class hierarchies for the Reader and Writer classes.




Most programs should use readers and writers to read and write information. This is because they both can handle any character in the Unicode character set (while the byte streams are limited to ISO-Latin-1 8-bit bytes).

Byte Streams (Binary Streams)

Programs should use the byte streams, descendants of InputStream and OutputStream, to read and write 8-bit bytes. InputStream and OutputStream provide the API and some implementation for input streams (streams that read 8-bit bytes) and output streams (streams that write 8-bit bytes). These streams are typically used to read and write binary data such as images and sounds.

As with Reader and Writer, subclasses of InputStream and OutputStream provide specialized I/O that falls into two categories: data sink streams and processing streams. Figure 56 shows the class hierarchies for the byte streams.

As mentioned, two of the byte stream classes, ObjectInputStream and ObjectOutputStream, are used for object serialization.

Understanding the I/O Superclasses

Reader and InputStream define similar APIs but for different data types. For example, Reader contains these methods for reading characters and arrays of characters:

int read()

int read(char cbuf[])

int read(char cbuf[], int offset, int length)

InputStream defines the same methods but for reading bytes and arrays of bytes:

int read()

int read(byte cbuf[])

int read(byte cbuf[], int offset, int length)

Also, both Reader and InputStream provide methods for marking a location in the stream, skipping input, and resetting the current position.

Writer and OutputStream are similarly parallel. Writer defines these methods for writing characters and arrays of characters:

int write(int c)

int write(char cbuf[])

int write(char cbuf[], int offset, int length)

And OutputStream defines the same methods but for bytes:

int write(int c)

int write(byte cbuf[])

int write(byte cbuf[], int offset, int length)

 

All of the streams--readers, writers, input streams, and output streams--are automatically opened when created. You can close any stream explicitly by calling its close method. Or the garbage collector can implicitly close it, which occurs when the object is no longer referenced.

How to Use File Streams

File streams are perhaps the easiest streams to understand. Simply put, the file streams - FileReader, FileWriter, FileInputStream, and FileOutputStream - each read or write from a file on the native file system. You can create a file stream from a filename, a File object, or a FileDescriptor object.

The following Copy program uses FileReader and FileWriter to copy the contents of a file named input.txt into a file called output.txt :

import java.io.*;

 

public class Copy {

public static void main(String[] args) throws IOException {

File inputFile = new File("input.txt");

File outputFile = new File("output.txt");

 

FileReader in = new FileReader(inputFile);

FileWriter out = new FileWriter(outputFile);

int c;

 

while ((c = in.read()) != -1)

out.write(c);

 

in.close();

out.close();

}

}

 

This program is very simple. It opens a FileReader on input.txt and opens a FileWriter on output.txt. The program reads characters from the reader as long as there's more input in the input file. When the input runs out, the program closes both the reader and the writer.

 

Notice the code that the Copy program uses to create a FileReader:

 

File inputFile = new File("input.txt");

FileReader in = new FileReader(inputFile);

 

This code creates a File object that represents the named file on the native file system. File is a utility class provided by java.io. This program uses this object only to construct a FileReader on input.txt. However, it could use inputFile to get information about input.txt, such as its full pathname.

 

After you've run the program, you should find an exact copy of input.txt in a file named output.txt in the same directory. 
 

Remember that FileReader and FileWriter read and write 16-bit characters. However, most native file systems are based on 8-bit bytes. These streams encode the characters as they operate according to the default character-encoding scheme. You can find out the default character-encoding by using System.getProperty ("file.encoding"). To specify an encoding other than the default, you should construct an OutputStreamWriter on a FileOutputStream and specify it.

For the curious, here is another version of this program, CopyBytes, which uses FileInputStream and FileOutputStream in place of FileReader and FileWriter :

import java.io.*;

 

public class CopyBytes {

public static void main(String[] args) throws IOException {

File inputFile = new File("input.txt");

File outputFile = new File("output.txt");

 

FileInputStream in = new FileInputStream(inputFile);

FileOutputStream out = new FileOutputStream(outputFile);

int c;

 

while ((c = in.read()) != -1)

out.write(c);

 

in.close();

out.close();

}

}

How to Use LineNumberReader

The LineNumberReader class is a subclass of BufferedReader. Its read() methods contain additional logic to count end-of-line characters and thereby maintain a line number. Since different platforms use different characters to represent the end of a line, LineNumberReader takes a flexible approach and recognizes "\n", "\r", or "\r\n" as the end of a line. Regardless of the end-of-line character it reads, LineNumberReader returns only "\n" from its read() methods.

You can create a LineNumberReader by passing its constructor a Reader. The following example prints out all the lines of a file, with each line prefixed by its number. If you try this example, you'll see that the line numbers begin at 0 by default:

try {

FileReader fileIn = new FileReader("text.txt");

LineNumberReader in = new LineNumberReader(fileIn);

while ((line=in.readLine()) != null) {

System.out.println(in.getLineNumber() + ". " + line);

}

} catch (IOException ioe) {

ioe.printStackTrace();

}

The LineNumberReader class has two methods pertaining to line numbers. The getLineNumber() method returns the current line number. If you want to change the current line number of a LineNumberReader, use setLineNumber(). This method does not affect the stream position; it merely sets the value of the line number.

How to Use PrintWriter

The PrintWriter class is a subclass of Writer that provides a set of methods for printing string representations of every Java data type. A PrintWriter can be wrapped around an underlying Writer object or an underlying OutputStream object. In the case of wrapping an OutputStream, any characters written to the PrintWriter are converted to bytes using the default encoding scheme.[2] Additional constructors allow you to specify if the underlying stream should be flushed after every line-separator character is written.

The PrintWriter class provides a print() and a println() method for every primitive Java data type. As their names imply, the println() methods do the same thing as their print() counterparts, but also append a line separator character.

The following example demonstrates how to wrap a PrintWriter around an OutputStream:

boolean b = true;

char c = '%'

double d = 8.31451

int i = 42;

String s = "R = ";

PrintWriter out = new PrintWriter(System.out, true);

out.print(s);

out.print(d);

out.println();

out.println(b);

out.println(c);

out.println(i);

This example produces the following output:

R = 8.31451
true
%
42

PrintWriter objects are often used to report errors. For this reason, the methods of this class do not throw exceptions. Instead, the methods catch any exceptions thrown by any downstream OutputStream or Writer objects and set an internal flag, so that the object can remember that a problem occurred. You can query the internal flag by calling the checkError() method.

Although you can create a PrintWriter that flushes the underlying stream every time a line-separator character is written, this may not always be exactly what you want. Suppose that you are writing a program that has a character-based user interface, and that you want the program to output a prompt and then allow the user to input a response on the same line. In order to make this work with a PrintWriter, you need to get the PrintWriter to write the characters in its buffer without writing a line separator. You can do this by calling the flush() method.

How to Use DataInputStream and DataOutputStream

This page shows you how to use the java.io DataInputStream and DataOutputStream classes. It features an example, DataIOTest, that reads and writes tabular data (invoices for Java merchandise) :

import java.io.*;

 

public class DataIOTest {

public static void main(String[] args) throws IOException {

 

// write the data out

DataOutputStream out = new DataOutputStream(new

FileOutputStream("invoice1.txt"));

 

double[] prices = { 19.99, 9.99, 15.99, 3.99, 4.99 };

int[] units = { 12, 8, 13, 29, 50 };

String[] descs = { "Java T-shirt",

"Java Mug", "Duke Juggling Dolls",

"Java Pin", "Java Key Chain" };

for (int i = 0; i < prices.length; i ++) {

out.writeDouble(prices[i]);

out.writeChar('\t');

out.writeInt(units[i]);

out.writeChar('\t');

out.writeChars(descs[i]);

out.writeChar('\n');

}

out.close();

 

// read it in again

DataInputStream in = new DataInputStream(new

FileInputStream("invoice1.txt"));

 

double price;

int unit;

String desc;

double total = 0.0;

 

try {

while (true) {

price = in.readDouble();

in.readChar(); // throws out the tab

unit = in.readInt();

in.readChar(); // throws out the tab

desc = in.readLine();

System.out.println("You've ordered " + unit + " units of " +

desc + " at $" + price);

total = total + unit * price;

}

} catch (EOFException e) {

}

System.out.println("For a TOTAL of: $" + total);

in.close();

}

}

The tabular data is formatted in columns, where each column is separated from the next by tabs. The columns contain the sales price, the number of units ordered, and a description of the item, like this:

19.99 12 Java T-shirt
9.99 8 Java Mug

 

DataOutputStream, like other filtered output streams, must be attached to some other OutputStream. In this case, it's attached to a FileOutputStream that's set up to write to a file named invoice1.txt.

 

DataOutputStream dos = new DataOutputStream(new FileOutputStream("invoice1.txt"));

 

Next, DataIOTest uses DataOutputStream's specialized writeXXX methods to write the invoice data (contained within arrays in the program) according to the type of data being written:

 

for (int i = 0; i < prices.length; i ++) {

dos.writeDouble(prices[i]);

dos.writeChar('\t');

dos.writeInt(units[i]);

dos.writeChar('\t');

dos.writeChars(descs[i]);

dos.writeChar('\n');

}

dos.close();

 

Note that this code snippet closes the output stream when it's finished.

Next, DataIOTest opens a DataInputStream on the file just written:

 

DataInputStream dis = new DataInputStream(new FileInputStream("invoice1.txt"));

 

DataInputStream also must be attached to some other InputStream; in this case, a FileInputStream set up to read the file just written - invoice1.txt. DataIOTest then just reads the data back in using DataInputStream's specialized readXXX methods.

 

try {

while (true) {

price = dis.readDouble();

dis.readChar(); // throws out the tab

unit = dis.readInt();

dis.readChar(); // throws out the tab

desc = dis.readLine();

System.out.println("You've ordered " + unit + " units of " + desc

+ " at $" + price);

total = total + unit * price;

}

}

catch (EOFException e) {

}

System.out.println("For a TOTAL of: $" + total);

dis.close();

 

When all of the data has been read, DataIOTest displays a statement summarizing the order and the total amount owed, and closes the stream.

Note the loop that DataIOTest uses to read the data from the DataInputStream. Normally, when reading you see loops like this:

while ((input = dis.readLine()) != null)

{

. . .

}

 

The readLine method returns a value, null, that indicates that the end of the file has been reached. Many of the DataInputStream readXXX methods can't do this because any value that could be returned to indicate end-of-file may also be a legitimate value read from the stream. For example, suppose that you wanted to use -1 to indicate end-of-file? Well, you can't because -1 is a legitimate value that can be read from the input stream using readDouble, readInt, or one of the other read methods that reads numbers. So DataInputStream's readXXX methods throw an EOFException instead. When the EOFException occurs the while (true) terminates.

When you run the DataIOTest program you should see the following output:

You've ordered 12 units of Java T-shirt at $19.99
You've ordered 8 units of Java Mug at $9.99
You've ordered 13 units of Duke Juggling Dolls at $15.99
You've ordered 29 units of Java Pin at $3.99
You've ordered 50 units of Java Key Chain at $4.99
For a TOTAL of: $892.88

File Manipulation

While streams are used to handle most types of I/O in Java, there are some nonstream-oriented classes in java.io that are provided for file manipulation. Namely, the File class represents a file on the local filesystem, while the RandomAccessFile class provides nonsequential access to data in a file. In addition, the FilenameFilter interface can be used to filter a list of filenames.

File

The File class represents a file on the local filesystem. You can use an instance of the File class to identify a file, obtain information about the file, and even change information about the file. The easiest way to create a File is to pass a filename to the File constructor, like this:

new File("readme.txt")

Although the methods that the File class provides for manipulating file information are relatively platform independent, filenames must follow the rules of the local filesystem. The File class does provide some information that can be helpful in interpreting filenames and path specifications. The variable separatorChar specifies the system-specific character used to separate the name of a directory from what follows. In a Windows environment, this is a backslash (\), while in a UNIX or Macintosh environment it is a forward slash (/). File separator can be obtained as System.getProperty('file.separator'), which is how the File class gets it. You can create a File object that refers to a file called readme.txt in a directory called myDir as follows:

new File("myDir" + File.separatorChar + "readme.txt")

The File class also provides some constructors that make this task easier. For example, there is a File constructor that takes two strings as arguments: the first string is the name of a directory and the second string is the name of a file. The following example does the exact same thing as the previous example:

new File("myDir", "readme.txt")

The File class has another constructor that allows you to specify the directory of a file using a File object instead of a String:

File dir = new File("myDir");

File f = new File(dir, "readme.txt");

Sometimes a program needs to process a list of files that have been passed to it in a string. For example, such a list of files is passed to the Java environment by the CLASSPATH environment variable and can be accessed by the expression:

System.getProperty("java.class.path")

This list contains one or more filenames separated by separator characters. In a Windows or Macintosh environment, the separator character is a semicolon (;), while in a UNIX environment, the separator character is a colon (:). The system-specific separator character is specified by the pathSeparatorChar variable. Thus, to turn the value of CLASSPATH into a collection of File objects, we can write:

StringTokenizer s;

Vector v = new Vector();

s = new StringTokenizer(System.getProperty("java.class.path"), File.pathSeparator);

while (s.hasMoreTokens())

v.addElement(new File(s.nextToken()));

You can retrieve the pathname of the file represented by a File object with getPath(), the filename without any path information with getName(), and the directory name with getParent().

The File class also defines methods that return information about the actual file represented by a File object. Use exists() to check whether or not the file exists. isDirectory() and isFile() tell whether the file is a file or a directory. If the file is a directory, you can use list() to get an array of filenames for the files in that directory. The canRead() and canWrite() methods indicate whether or not a program is allowed to read from or write to a file. You can also retrieve the length of a file with length() and its last modified date with lastModified().

A few File methods allow you to change the information about a file. For example, you can rename a file with rename() and delete it with delete(). The mkdir() and mkdirs() methods provide a way to create directories within the filesystem.

Many of these methods can throw a SecurityException if a program does not have permission to access the filesystem, or particular files within it. If a SecurityManager has been installed, the checkRead() and checkWrite() methods of the SecurityManager verify whether or not the program has permission to access the filesystem.

FilenameFilter

The purpose of the FilenameFilter interface is to provide a way for an object to decide which filenames should be included in a list of filenames. A class that implements the FilenameFilter interface must define a method called accept(). This method is passed a File object that identifies a directory and a String that names a file. The accept() method is expected to return true if the specified file should be included in the list, or false if the file should not be included. Here is an example of a simple FilenameFilter class that only allows files with a specified suffix to be in a list:

import java.io.File;

import java.io.FilenameFilter;

public class SuffixFilter implements FilenameFilter {

private String suffix;

public SuffixFilter(String suffix) {

this.suffix = "." + suffix;

}

public boolean accept(File dir, String name) {

return name.endsWith(suffix);

}

}

A FilenameFilter object can be passed as a parameter to the list() method of File to filter the list that it creates. You can also use a FilenameFilter to limit the choices shown in a FileDialog.

RandomAccessFile

The RandomAccessFile class provides a way to read from and write to a file in a nonsequential manner. The RandomAccessFile class has two constructors that both take two arguments. The first argument specifies the file to open, either as a String or a File object. The second argument is a String that must be either "r" or "rw". If the second argument is "r", the file is opened for reading only. If the argument is "rw", however, the file is opened for both reading and writing. The close() method closes the file. Both constructors and all the methods of the RandomAccessFile class can throw an IOException if they encounter an error.

The RandomAccessFile class defines three different read() methods for reading bytes from a file. The RandomAccessFile class also implements the DataInput interface, so it provides additional methods for reading from a file. Most of these additional methods are related to reading Java primitive types in a machine-independent way. Multibyte quantities are read assuming the most significant byte is first and the least significant byte is last. All of these methods handle an attempt to read past the end of file by throwing an EOFException.

The RandomAccessFile class also defines three different write() methods for writing bytes of output. The RandomAccessFile class also implements the DataOutput interface, so it provides additional methods for writing to a file. Most of these additional methods are related to writing Java primitive types in a machine-independent way. Again, multibyte quantities are written with the most significant byte first and the least significant byte last.

The RandomAccessFile class would not live up to its name if it did not provide a way to access a file in a nonsequential manner. The getFilePointer() method returns the current position in the file, while the seek() method provides a way to set the position. Finally, the length() method returns the length of the file in bytes.