Java -
Files and I/O
The java.io
package contains nearly every class you might ever need to perform input and
output (I/O) in Java. All these streams represent an input source and an output
destination. The stream in the java.io package supports many data such as
primitives, object, localized characters, etc.
Stream: A
stream can be defined as a sequence of data. There are two kinds of Streams –
InPutStream − The
InputStream is used to read data from a source.
OutPutStream − The OutputStream is used for writing data to a destination.
Java
provides strong but flexible support for I/O related to files and networks but
this tutorial covers very basic functionality related to streams and I/O. We
will see the most commonly used examples one by one −
Byte Streams
Java byte
streams are used to perform input and output of 8-bit bytes. Though there are
many classes related to byte streams but the most frequently used classes are,
FileInputStream and FileOutputStream. Following is an example which makes use
of these two classes to copy an input file into an output file −
Example
import
java.io.*;
public class
CopyFile {
public static void main(String args[])
throws IOException {
FileInputStream in = null;
FileOutputStream out = null;
try
{
in = new
FileInputStream("input.txt");
out = new
FileOutputStream("output.txt");
int c;
while ((c = in.read()) != -1) {
out.write(c);
}
}finally {
if (in != null) {
in.close();
}
if (out != null) {
out.close();
}
}
}
}
Output
This is test
for copy file.
As a next
step, compile the above program and execute it, which will result in creating
output.txt file with the same content as we have in input.txt. So let's put the
above code in CopyFile.java file and do the following −
$javac
CopyFile.java
$java
CopyFile
Character Streams
Java Byte
streams are used to perform input and output of 8-bit bytes, whereas Java
Character streams are used to perform input and output for 16-bit unicode.
Though there are many classes related to character streams but the most
frequently used classes are, FileReader and FileWriter. Though internally
FileReader uses FileInputStream and FileWriter uses FileOutputStream but here
the major difference is that FileReader reads two bytes at a time and
FileWriter writes two bytes at a time.
We can
re-write the above example, which makes the use of these two classes to copy an
input file (having unicode characters) into an output file −
Example
import
java.io.*;
public class
CopyFile {
public static void main(String args[])
throws IOException {
FileReader in = null;
FileWriter out = null;
try {
in = new
FileReader("input.txt");
out = new
FileWriter("output.txt");
int c;
while ((c = in.read()) != -1) {
out.write(c);
}
}finally {
if (in != null) {
in.close();
}
if (out != null) {
out.close();
}
}
}
}
Now let's
have a file input.txt with the following content −
This is test for copy file.
As a next
step, compile the above program and execute it, which will result in creating
output.txt file with the same content as we have in input.txt. So let's put the
above code in CopyFile.java file and do the following −
$javac
CopyFile.java
$java CopyFile
Standard
Streams
All the
programming languages provide support for standard I/O where the user's program
can take input from a keyboard and then produce an output on the computer
screen. If you are aware of C or C++ programming languages, then you must be
aware of three standard devices STDIN, STDOUT and STDERR. Similarly, Java
provides the following three standard streams
Standard Input − This is
used to feed the data to user's program and usually a keyboard is used as
standard input stream and represented as System.in.
Standard Output − This is used to output the data produced by the user's program and
usually a computer screen is used for standard output stream and represented as
System.out.
Standard Error − This is used to output the error data produced by the user's program and
usually a computer screen is used for standard error stream and represented as
System.err.
Following is a simple program, which creates
InputStreamReader to read standard input stream until the user types a
"q" −
Example
import
java.io.*;
public class
ReadConsole {
public static void main(String args[])
throws IOException {
InputStreamReader cin = null;
try {
cin = new
InputStreamReader(System.in);
System.out.println("Enter characters,
'q' to quit.");
char c;
do {
c =
(char) cin.read();
System.out.print(c);
} while(c != 'q');
}finally {
if (cin != null) {
cin.close();
}
}
}
}
Let's keep
the above code in ReadConsole.java file and try to compile and execute it as
shown in the following program. This program continues to read and output the
same character until we press 'q' −
$javac ReadConsole.java
$java ReadConsole
Enter characters, 'q' to quit.
1
1
e
e
q
q
Reading and Writing Files
As described
earlier, a stream can be defined as a sequence of data. The InputStream is used
to read data from a source and the OutputStream is used for writing data to a
destination.
Here is a
hierarchy of classes to deal with Input and Output streams.
The two
important streams are FileInputStream and FileOutputStream, which would be
discussed in this tutorial.
FileInputStream: This stream is used for reading data from the files. Objects
can be created using the keyword new and there are several types of
constructors available.
Following constructor takes a file name as a string to create
an input stream object to read the file −
InputStream
f = new FileInputStream("C:/java/hello");
Following
constructor takes a file object to create an input stream object to read the
file. First we create a file object using File() method as follows −
File f =
new File("C:/java/hello");
InputStream
f = new FileInputStream(f);
Once you
have InputStream object in hand, then there is a list of helper methods which
can be used to read to stream or to do other operations on the stream.
|
Sr.
|
Method
|
Description
|
|
1
|
public void close() throws IOException{}
|
This method closes the file output stream. Releases any system
resources associated with the file. Throws an IOException.
|
|
2
|
protected
void finalize()throws IOException {}
|
This
method cleans up the connection to the file. Ensures that the close method of
this file output stream is called when there are no more references to this
stream. Throws an IOException.
|
|
3
|
public int read(int r)throws IOException{}
|
This method reads the specified byte of data from the
InputStream. Returns an int. Returns the next byte of data and -1 will be
returned if it's the end of the file.
|
|
4
|
public
int read(byte[] r) throws IOException{}
|
This
method reads r.length bytes from the input stream into an array. Returns the
total number of bytes read. If it is the end of the file, -1 will be
returned.
|
|
5
|
public
int available() throws IOException{}
|
Gives
the number of bytes that can be read from this file input stream. Returns an
int.
|
FileOutputStream is used
to create a file and write data into it. The stream would create a file, if it
doesn't already exist, before opening it for output.
Here are two
constructors which can be used to create a FileOutputStream object.
Following
constructor takes a file name as a string to create an input stream object to
write the file −
OutputStream
f = new FileOutputStream("C:/java/hello")
Following
constructor takes a file object to create an output stream object to write the
file. First, we create a file object using File() method as follows −
File f = new
File("C:/java/hello");
OutputStream
f = new FileOutputStream(f);
Once you
have OutputStream object in hand, then there is a list of helper methods, which
can be used to write to stream or to do other operations on the stream.
|
Sr
|
Method
|
Description
|
|
1
|
public void close() throws IOException{}
|
This method closes the file output stream. Releases any system
resources associated with the file. Throws an IOException.
|
|
2
|
protected
void finalize()throws IOException {}
|
This
method cleans up the connection to the file. Ensures that the close method of
this file output stream is called when there are no more references to this
stream. Throws an IOException.
|
|
3
|
public void write(int w)throws IOException{}
|
This
methods writes the specified byte to the output stream.
|
|
4
|
Public
void write(byte[]w)
|
Write w.length byte from
the mentioned byte array to the OutputStream
|
Example
Following is
the example to demonstrate InputStream and OutputStream −
import
java.io.*;
public class
fileStreamTest {
public static void main(String args[]) {
try {
byte bWrite [] = {11,21,3,40,5};
OutputStream os = new
FileOutputStream("test.txt");
for(int x = 0; x < bWrite.length ;
x++) {
os.write( bWrite[x] ); // writes the bytes
}
os.close();
InputStream is = new
FileInputStream("test.txt");
int size = is.available();
for(int i = 0; i < size; i++) {
System.out.print((char)is.read() +
" ");
}
is.close();
} catch (IOException e) {
System.out.print("Exception");
}
}
}
The above
code would create file test.txt and would write given numbers in binary format.
Same would be the output on the stdout screen.
File Navigation and I/O: There are several other classes that we would be going
through to get to know the basics of File Navigation and I/O.
File
Class
FileReader
Class
FileWriter
Class
Directories in Java: A directory is a File which can contain a list of other files
and directories. You use File object to create directories, to list down files
available in a directory. For complete detail, check a list of all the methods
which you can call on File object and what are related to directories.
Creating Directories
There are
two useful File utility methods, which can be used to create directories −
The mkdir( ) method
creates a directory, returning true on success and false on failure. Failure
indicates that the path specified in the File object already exists, or that
the directory cannot be created because the entire path does not exist yet.
The mkdirs() method
creates both a directory and all the parents of the directory.
Following
example creates "/tmp/user/java/bin" directory −
Example
import
java.io.File;
public class
CreateDir {
public static void main(String args[]) {
String dirname =
"/tmp/user/java/bin";
File d = new File(dirname);
// Create directory now.
d.mkdirs();
}
}
Compile and
execute the above code to create "/tmp/user/java/bin".
Note − Java automatically takes care of path separators on UNIX
and Windows as per conventions. If you use a forward slash (/) on a Windows
version of Java, the path will still resolve correctly.
Listing Directories: You can use list( ) method provided by File object to list
down all the files and directories available in a directory as follows −
Example
import
java.io.File;
public class
ReadDir {
public static void main(String[] args) {
File file = null;
String[] paths;
try {
// create new file object
file = new File("/tmp");
// array of files and directory
paths = file.list();
// for each name in the path array
for(String path:paths) {
// prints filename and directory
name
System.out.println(path);
}
} catch (Exception e) {
// if any error occurs
e.printStackTrace();
}
}
}
This will
produce the following result based on the directories and files available in
your /tmp directory −
Output
test1.txt
test2.txt
ReadDir.java
ReadDir.class
Comments
Post a Comment