Transcript import java.io.

Streams &
File Input/Output (I/O)
I/O Streams

A java stream is an object consisting of a
sequence of bytes that flow from a source to a
destination
Stream
Source
Destination


Source: A program, a file
Destination: Can be a file, the console (i.e. output)
window…..
We read information from an input stream and
write information to an output stream
I/O Streams

Streams are a bit of an elusive concept in Java -- it can
take a while to get used to them

You sort of need to mentally try to picture them over and over again,
and the idea of them begins to become clear

The java.io.* (input output or I/O for short) package
contains many classes that allow us to define various
kinds of streams, each with specific characteristics

A program can manage multiple streams at a time

The I/O package is quite complex and a detailed
analysis of all the classes in this package requires
practice and experience
I/O Streams categories





There are several categories of I/O streams
Today we will limit ourselves to two I/O streams.
The first kind of stream deals with reading
information from text files.
The second deals with writing information to text
files.
Here are the classes that we will use for each:
 FileReader:
To read lines of text from files
 PrintWriter: To write lines of text to files
Standard I/O

We have already dealt with 3 standard I/O
streams
standard input stream– defined by System.in
 A standard output stream – defined by System.out
 A standard error stream– defined by System.err
A

Yet we never had to create these streams. This
is because these streams are very common and
are therefore automatically created for us as
soon as we start executing a program
Streams are objects:

System.out


out is a field of the System class.
The data type of ‘out’ is a class called
PrintStream



You could also say that ‘out’ is an object of type
PrintStream
println is a method of the PrintStream
class.
System.in
in is an object of type InputStream
 read is a method that read only raw byte data

Scanner to the rescue

Instead of having to deal with the System.in
“rawness” we were able to use a Scanner object
Scanner console = new Scanner (System.in);


And the handy methods like next(),
nextInt(), nextDouble() etc. to read
from the console
Today we will see how to use Scanner to read
from a text file
Reading from files
Instead of using System.in (i.e. the
terminal keyboard) as our input source, we
need to create an input source linked to a
text file
 To do this, we need a special stream
class, called FileReader


The FileReader class has methods that
will connect the Scanner class to a specific
text file which can then be read
Reading from files




The Scanner class has a very convenient method
called nextLine() that reads in a line from a text
file.
So, once we have linked the Scanner class to an
input file, we can use the nextLine() method to
read in each line of the file
The nextLine() method returns a String
containing the first line of the file.
The method also positions the cursor at the
beginning of the next line
import java.util.*;
import java.io.*;
public static void main(String[] args)
throws FileNotFoundException
{
FileReader dataFile = new FileReader("data.txt");
);
Scanner fileIn = new Scanner (dataFile);
while (fileIn.hasNextLine())
{
String aLine = fileIn.nextLine();
System.out.println(aLine);
}
fileIn.close();
}
The four steps of file reading
1.
2.
Import the necessary packages (java.io.*)
Create an input stream to the input
source:

FileReader object to the actual file
 Scanner associated to the FileReader
3.
4.
Use the appropriate methods to read the
data: nextLine() , nextInt(), etc.
Close the stream: fileIn.close()
File I/O Can Generate Exceptions




If the program doesn’t find the file or if the file is
somehow protected and denies access, the
program will throw an exception and abort (crash)
Every time you use FileReader you need to either
handle or throw the exception or it will not compile
We can handle by enclosing the I/O code inside a
try/catch sequence.
Or, we can simply throw (delegate) with:
throws FileNotFoundException
import java.util.*;
import java.io.*;
public static void main(String[] args)
throws FileNotFoundException
{
FileReader dataFile = new FileReader(“data.txt”);
Scanner fileIn = new Scanner ( dataFile);
while (fileIn.hasNextLine())
{
String aLine = fileIn.nextLine();
System.out.println(aLine);
}
fileIn.close();
}
The four+ steps of file reading
Import the necessary packages (java.io.*)
 Create an input stream to the input source:

 FileReader
object to the actual file
 Scanner associated to the FileReader
Use the appropriate methods to read the
data: nextLine() , nextInt(), etc.
 Close the stream: fileIn.close()
 Handle or throw the
FileNotFoundException

Writing to a file
We need to create an output stream that is
connected to a file
 For this, we do not use FileReader and
Scanner
 We make use of the PrintWriter stream
class and its methods

import java.util.*;
import java.io.*;
public static void main(String[] args)
throws FileNotFoundException
{
PrintWriter fileOut = new PrintWriter("data.txt");
String aLine = "A line of text";
fileOut.println(aLine);
fileOut.close();
}
The four+ steps of file writing
Import the necessary packages (java.io.*)
 Create an output stream to the output
source:

 PrintWriter
object to the actual file
Use the appropriate methods to write the
data: println() , print(), etc.
 Close the stream: fileOut.close()

Important:

What happens when a program attempts to
open a file that does not yet exist depends
on whether it is for input or output

For input: If the file does not exist, an exception
will be thrown and the program will stop
 For output: If the file does not exist PrintWriter
will create the file and not throw an exception

If the output file already exists, then the PrintWriter
object will overwrite the file that was previously there!
Printing to file
When PrintWriter writes to an existing file,
it overrides whatever is already there, e.g.
it does not append
 So if you do not want to lose what you
already have you should:

1.
2.
store the existing data in memory, then
write the old and the new using PrintWriter