Java - Streams, 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.
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.
Java does provide strong, flexible
support for I/O as it relates to files and networks but this tutorial covers
very basic functionality related to streams and I/O. We would see most commonly
used example one by one:
Reading
Console Input:
Java
input console is accomplished by reading from System.in.
To obtain a character-based stream that is attached to the console, you wrap System.in in a BufferedReader object, to create a character stream.
Here is most common syntax to obtain BufferedReader:
BufferedReader br = new BufferedReader(new
InputStreamReader(System.in));
Once BufferedReader is
obtained, we can use read( ) method to reach a character or readLine( ) method
to read a string from the console.
Reading
Characters from Console:
To read a character from a BufferedReader,
we would read( ) method whose sytax is as follows:
int read( ) throws IOException
Each time that read( ) is
called, it reads a character from the input stream and returns it as an integer
value. It returns .1 when the end of the stream is encountered. As you can see,
it can throw an IOException.
The following program
demonstrates read( ) by reading characters from the console until the user
types a "q":
// Use a BufferedReader to read characters from the console.
import java.io.*;
public class BRRead {
public static void main(String args[]) throws IOException
{
char c;
// Create a BufferedReader using System.in
BufferedReader br = new BufferedReader(new
InputStreamReader(System.in));
System.out.println("Enter characters, 'q' to quit.");
// read characters
do {
c = (char) br.read();
System.out.println(c);
} while(c != 'q');
}
}
Here is a sample run:
Enter characters, 'q' to quit.
123abcq
1
2
3
a
b
c
q
Reading
Strings from Console:
To read a string from the
keyboard, use the version of readLine( ) that is a member of the BufferedReader
class. Its general form is shown here:
String readLine( ) throws IOException
The following program
demonstrates BufferedReader and the readLine( ) method. The program reads and
displays lines of text until you enter the word "end":
// Read a string from console using a BufferedReader.
import java.io.*;
public class BRReadLines {
public static void main(String args[]) throws IOException
{
// Create a BufferedReader using System.in
BufferedReader br = new BufferedReader(new
InputStreamReader(System.in));
String str;
System.out.println("Enter lines of text.");
System.out.println("Enter 'end' to quit.");
do {
str = br.readLine();
System.out.println(str);
} while(!str.equals("end"));
}
}
Here is a sample run:
Enter lines of text.
Enter 'end' to quit.
This is line one
This is line one
This is line two
This is line two
end
end
Writing
Console Output:
Console
output is most easily accomplished with print(
) and println( ), described earlier.
These methods are defined by the class PrintStream which is the type of the object
referenced bySystem.out. Even though System.out is a byte stream, using
it for simple program output is still acceptable.
Because PrintStream is an
output stream derived from OutputStream, it also implements the low-level
method write( ). Thus, write( ) can be used to write to the console. The
simplest form of write( ) defined by PrintStream is shown here:
void write(int byteval)
This method writes to the
stream the byte specified by byteval. Although byteval is declared as an
integer, only the low-order eight bits are written.
Example:
Here is a short example that
uses write( ) to output the character "A" followed by a newline to
the screen:
import java.io.*;
// Demonstrate System.out.write().
public class WriteDemo {
public static void main(String args[]) {
int b;
b = 'A';
System.out.write(b);
System.out.write('\n');
}
}
This would produce simply 'A'
character on the output screen.
A
Note: You will not often use write( ) to
perform console output because print( ) and println( ) are substantially easier
to use.
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.
|
SN
|
Methods with 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 end of 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 end of 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. |
There are other important
input streams available, for more detail you can refer to the following links:
FileOutputStream:
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.
|
SN
|
Methods with 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)
Writes w.length bytes from the mentioned byte array to the OutputStream. |
There are other important
output streams available, for more detail you can refer to the following links:
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
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.
Directories
in Java:
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:
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 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.
Reading
Directories:
A directory is a File that
contains a list of other files and directories. When you create a File object
and it is a directory, the isDirectory( ) method will return true.
You can call list( ) on that
object to extract the list of other files and directories inside. The program
shown here illustrates how to use list( ) to examine the contents of a
directory:
import java.io.File;
public class DirList {
public static void main(String args[]) {
String dirname = "/tmp";
File f1 = new File(dirname);
if (f1.isDirectory()) {
System.out.println( "Directory of " + dirname);
String s[] = f1.list();
for (int i=0; i < s.length; i++) {
File f = new File(dirname + "/" + s[i]);
if (f.isDirectory()) {
System.out.println(s[i] + " is a directory");
} else {
System.out.println(s[i] + " is a file");
}
}
} else {
System.out.println(dirname + " is not a directory");
}
}
}
This would produce the
following result:
Directory of /tmp
bin is a directory
lib is a directory
demo is a directory
test.txt is a file
README is a file
index.html is a file
include is a directory
No comments:
Post a Comment