Java - FileInputStream

FileInputStream class is a subclass of InputStream abstract class. FileInputStream is used to create an input stream, which is used to read byte/bytes from a file.

Constructors of FileInputStream :

FileInputStream(File file)
This constructor creates a FileInputStream object to read a file specified by the File object, which is passed to this constructor as a parameter. Example - `File file= new File("D:\\Textbook.txt"); FileInputStream fis= new FileInputStream(file);`

FileInputStream(String path)
This constructor creates a FileInputStream to read a file which is accessed by the path mentioned in the parameters of this constructor. Example - `FileInputStream fis= new FileInputStream("D:\\TextBook.txt");`

Both the examples of constructors have created a FileInputStream object to create an input stream to read a file called TextBook.txt which is located in the D: drive.

Some methods for reading FileInputStream

Methods	Description
available()	This method gives the remaining number of bytes available to read from this input stream.
read()	This method reads one byte from this input stream.
read(byte[] b)	This method reads a whole byte array from this input stream.
close()	This method closes this input stream and also frees any resources connected with this input stream.

Reading one byte at a time from a file by using read() method of FileInputStream

We are reading an existing file called "TextBook.txt" using its path "D:\\TextBook.txt". Let's say the contents of this file "TextBook.txt" are-

TextBook.txt

 World peace is extremely important.

//Java - Program to read one byte at a time through an existing file using FileInputStream.

import java.io.*;

class A
{
public static void main(String...ar)
{
try
{
	FileInputStream fin= new FileInputStream("D:\\TextBook.txt");
	int c;
	
	
	//Reading one byte out of a file
	while( (c=fin.read()) !=-1) 
	{
		//casting byte to char for displaying on the screen
		System.out.print((char)c); 
	}

	fin.close();
}

catch(IOException e)
{
	System.out.println(e);
}

}
}

Output is -:

World peace is extremely important

Program Analysis

The method read() is called within a while loop to read each byte(one-by-one), until the EOF(end of file) is reached and -1 is returned by this method, while reading from the stream. This byte is assigned to an int variable, c.

As the variable c was declared as an int, so it's casted to char, to convert the byte contained in it to a character and this character is printed on the screen.

Finally, the input stream is closed by calling the close() method on FileInputStream's object, to free the resources still connected to this stream.

Reading a whole byte array from a file by using read(byte[] b) method.

In the next example, we are reading the contents of the same file TextBook.txt, but this time we are reading a whole byte array at once for faster reading.

//Java - Program to read a byte array out of an existing file using FileInputStream.
 
import java.io.*;

class A
{
public static void main(String...ar)
{
try
{
	FileInputStream fin= new FileInputStream("D:\\TextBook.txt");
	int num= fin.available();	
	System.out.println("Number of bytes available to read =" + num);
	
	byte b[]= new byte[num];
	
	int c;
	
	//read() method stops reading from file when EOF is reached i.e. -1	
	if((c=fin.read(b))!=-1) 
		System.out.println("Number of bytes read = " +c);

	String s= new String(b);
	System.out.println(s);
	fin.close();

}
catch(IOException e)
{
	System.out.println(e);
}

}
}

Output is -:

Number of bytes available to read =34
Number of bytes read = 34
World peace is extremely important

Program Analysis

The available() method returns the number of bytes available to read i.e.34 bytes to read out of the FileInputStream. The number of bytes are assigned to the variable num.