File Input / Output

FilesPersistent storage

So your application can ‘remember’

Similar methods to reading / displaying information as before, but a different ‘destination’

Scanner ClassBefore, we read data from ‘standard input’

Scanner myScanner = new Scanner (System.in);

System.in will read from the keyboard

We can pass Scanner other Objects to read from other sources…

Scanner ClassNow we use the FileReader class

Don’t forget your importsjava.utiljava.io

Scanner myScanner = new Scanner (new FileReader(“c:\\myfolder\\myFile.txt”);

FileReaderScanner myScanner = new Scanner (new FileReader(“c:\\myfolder\\myFile.txt”);

Some Things to notice:

We create a FileReader Object “on the fly” It doesn’t even have a name

The double slashesSlash is the escape character

PathsRelative vs. AbsoluteDrive letters

FileReaderNow we can use the same scanner

methodsnextInt( )nextDouble( )hasNext( )

ExampleScanner myScanner = new Scanner (new FileReader(“c:\\myFile.txt”));

String firstName, lastName;

double hoursWorked, payRate, wages;

firstName = myScanner.next( );

lastName = myScanner.next( );

hoursWorked = myScanner.nextDouble( );

payRate = myScanner.nextDouble( );

wages = hoursWorked * payRate;

myScanner.close( );

Writing to Files

Lots of different “Writer and Reader” classes.

PrintWriter is a good, high level class for reading general data from a file

Writing to Files

PrintWriter myPW = new PrintWriter(“c:\\myFile.out”);

Can use the familiar output methods on the fileprint( )println( )

myPW.println(“The pay is: $” + pay);This stores the text in the file “myFile.out”

Closing Files

Don’t forget to close your files!

Saves memory and…

“Flushes the buffer”

Files

What happens if something goes wrong?

Trying to read a file that isn’t there…

Trying to write to a location that doesn’t exist…Like the ‘Z:’ drive

Trying to write to a file that is already there…By default, it overwrites the file.This may not be what you want!

Files

If something goes wrong, the statement that causes a problem throws an exception

One of two things must happen in a method that causes an exception

You deal with it

Or… pass the problem up to the calling method

ExceptionsFor now, we will ‘pass the buck’ and let the

system deal with the potential error

public static void main (String [] args) throws FileNotFoundException {

. . .

// no try – catch block in here

}

Files

In the real-world, files are useful, but many real world data tasks are handled via a database

mySQL

Websites use a database to holdProductsCommentsReviews