Download Brief Introduction to Java I/O

Very Brief Introduction to
Java I/O with
Buffered Reader and
Buffered Writer
Introductory Points

Java I/O is primarily “stream-oriented.”
 Data
flows into the program and out of the program,
like a ‘stream.’
 We have ‘input’ streams and ‘output’ streams.

“Standard” I/O streams:
 System.in – defaults to the keyboard
 System.out – defaults to the monitor
 System.err – defaults to where System.out

is: monitor.
These are all public and static and found in the
System class of the Java API.
Standard I/O

We’ve used standard I/O in creating Scanner objects to
process data normally associated with standard input
(keyboard) and standard output (monitor).

“I/O exceptions” are handled for us by Scanner.



Many ‘opportunity for problems in doing any kind of input / output:
Examples include files not defined; file not found; formats not what
were expected, EOF early, etc. and other exceptions.
I/O in Java can be very complicated.
We oftentimes need to be able to read text data or pure
binary data. These can come from external files, memory, or
from strings.

Here, we will emphasize input/output as pertains to external files.
The java I/O package

The java.io package provides us classes that let us define desired
input and output streams.

Because we use the classes themselves (as in class.method) the
methods we use are public and static.





Recall: static methods don’t need to be part of an object to use them, as in the
Math class – recall: Math.sqrt()
(Math is the class; sqrt() is method and we simply say, Math.sqrt()…)
We did not have to instantiate an object of type Math in order to access the
method! (note capitalized class name followed by member name)
Recall: in die1.roll(), die1 was an object; hence object.method() call….
Some classes provide for buffering (manipulate data in stream itself.)

Like, we can change format and more (ahead)

What we do is to combine a number of these classes to lock in to
exactly what we want to do our input/output.

The general topic of java I/O is huge and we will cover only what we
need here.
The java.io package

To restate: so many I/O operations can cause
problems and `throw’ an “I/O Exception” when
we are trying to do I/O operations such as read()
or write() operations.

We must either
Use a try…catch combination:

 ‘catch’ the exception in a ‘try’ block
 (use a try() … catch() sequence) or

and process it
Recognize that any method that may catch a
problem must have a ‘throws’ clause in the
header of the method where the I/O is attempted.


For now, we will elect this approach.
At this time, it is simpler…
Very Simplified I/O – for now
We will just use: throws IOException
clause for File I/O.
 For much more, see Chapter 10 slides.

Code – for Input
import java.io.*;
// You need this to access the classes cited below.
public static void main (String[ ] args) throws IOException. //you need the “throws.” Why?? You must also
// 1. download (right click, Save Target As) your input file (from my web page), and
// 2. put it (drag it) into your project folder so the program can find it and read lines from it.
FileInputStream fis1 = new FileInputStream(“Countries.txt");
// much more in Chapter 10
BufferedReader br1 = new BufferedReader(new InputStreamReader(fis1)); // for an input file to be read//
//
//
//
or
FileReader fr = new FileReader(“Countries.txt”);
BufferedReader br = new BufferedReader (fr);
String inputString;
String countryName
String countryWhatever…. // additional attributes here…
inputString = br1.readLine(); //reads one complete line (record) from the input file into inputString.
// Now you need to get to the parts of the inputString. We call this ‘parsing’ the input. Consider:
while (inputString != null)
// looks at entire string you just read into inputString
{
countryName = inputString.substring(0, 15).trim()
//read country name
countryWhatever = inputString.substring(15, 30),
//read country capital
countryPopulatiion = Integer.parseInt(inputString.substring(55, 60)); //read country code #
//
Note: these are not the precise character positions in ‘your’ file.
//
You will need to check appropriate attribute field lengths.
//
Look in your book about substring method. Echo print everything to verify!!!
<other code – like creating an object from these attributes.>
inputString = br1.readLine(); // read next input line.
} //end while loop
br1.close(); //Close input file being read
//
With a few more comments:
FileReader fr = new FileReader(“Countries.Small.txt”);
BufferedReader br = new BufferedReader (fr);
// Sample names made up to show how the parsing works:
String inputString;
String someName, someCapital; // but declare one attribute per line.
Int somePop;
// Note: this has nothing to do with creating objects or the array.
// Only ensures you are able to access the individual attributes from each input line.
// Once you verify all this, you can comment out the print lines.
inputString = br1.readLine(); //reads one complete line (record) from the input file into inputString.
// Now you need to get to the parts of the inputString. We call this ‘parsing’ the input. Consider:
while (inputString != null)
// looks at entire string you just read into inputString
{
someName = inputString.substring(0, 15).trim()
//read country name
// echo print to verify you ‘read in and parse what you think you did.
System.out.println (“ some Name read in was: “ + someName);
someCapital = inputString.substring(15, 30),
//read country capital
System.out.println (“some Capital read in was: “ + someCapital);
somePop = Integer.parseInt(inputString.substring(55, 62)); //read country code #
// Statements like these are used to extract specific positions from the input record.
// Note: these are not the precise character positions in ‘your’ file.
// Look in your book about substring method. Echo print everything to verify!!!
…. Other code (later: create object and move into array….Later!)
//
<other code – like creating an object from these attributes.>
inputString = br1.readLine(); // read next input line.
} //end while loop
Code – for Output

Must include up top:
import java.io.*;

// this lets us use the classes below.
Alter your method header as follows:
//much more in Chapter 10….
public static void main (String[ ] args) throws IOException. // you need this!

Sample code: (from book)
String file = “test.dat”;
// name your output file
FileWriter fw = new FileWriter (file); // creates object fw of type FileWriter.
BufferedWriter bw = new BufferedWriter (fw); // creates object bw of type ...
PrintWriter outFile = new PrintWriter (bw); // creates object outFile of type ..






…. <other code>
outFile.print (value + “
outFile.println ();
… <other code>
outFile.close();
This writes to an output file.
“); // writes this as a stream. Try it!
// prints a blank line in the file
// at end, close the file
More Later
In using a standard IDE such as
NetBeans, your input file needs to be in
your project package so your program can
find it.
 Be certain to move it (drag it) into your
package and test accessing the file from
your package before you zip up your
project and send it to me.
