Streams And Files
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 blog 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();
}
}
}
}
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
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.
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
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);
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);
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