1. Streams & File Input/Output (I/O)

2. I/O Streams
A java stream is an object consisting of a sequence of bytes that flow from a source to a destination
Stream
Destination
Source
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

3. 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

4. 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

5. Standard I/O We have already dealt with 3 standard I/O streams
A standard input stream– defined by System.in
A standard output stream – defined by System.out
A standard error stream– defined by System.err
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

6. Streams are objects: System.out System.in
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

7. 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

8. 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

9. 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

10. 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();

11. 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()

12. 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

13. 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();

14. 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

15. 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

16. 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();

17. 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()

18. 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!

19. 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:
store the existing data in memory, then
write the old and the new using PrintWriter