Streams for file I/O

"We'll start with streams for file input/output. But first things first."

There are two classes for reading and writing files: FileInputStream and FileOutputStream. As you probably already guessed, FileInputStream can sequentially read bytes from a file, and FileOutputStream can sequentially write bytes to a file. Here are the methods that these classes have:

FileInputStream(String fileName);

int read();

int available();

void close();

Just for the fun of it, let's calculate the sum of all the bytes in a file. Here's what the code looks like:

public static void main(String[] args) throws Exception
{
 //Create a FileInputStream object bound to «c:/data.txt».
 FileInputStream inputStream = new FileInputStream("c:/data.txt");
 long sum = 0;

 while (inputStream.available() > 0) //as long as there are unread bytes
 {
  int data = inputStream.read(); //Read the next byte
  sum +=  data; //Add it to the running total
 }
 inputStream.close(); // Close the stream

 System.out.println(sum); // Display the sum on the screen.
}

"We've already looked into something like this. How is FileOutputStream organized?"

"OK. Look at this:"

FileOutputStream (String fileName);

void write(int data);

void flush();

void close();

You can't write data to the stream any longer, and flush is called automatically.

"That's it?"

"Yep, there's actually only one method for writing: write(). It writes only one byte at a time. But it lets you write as much information as you want to the file."

Programming is a process of dividing one large and complex task into many small ones. Essentially the same process is happening here: reading and writing large blocks of data is broken into reading and writing in bite-sized pieces—one byte at a time.

Here's how you can use these classes to copy a file on disk:

public static void main(String[] args) throws Exception
{
 //Create a stream to read bytes from a file
 FileInputStream inputStream = new FileInputStream("c:/data.txt");
 //Create a stream to write bytes to a file
 FileOutputStream outputStream = new FileOutputStream("c:/result.txt");

 while (inputStream.available() > 0) //as long as there are unread bytes
 {
  int data = inputStream.read(); // Read the next byte into the data variable
  outputStream.write(data); // and write it to the second stream
 }

 inputStream.close(); //Close both streams. We don't need them any more.
 outputStream.close();
}

"Thanks, Rishi. I finally understand how this code actually works."