File Input / Output

Files Persistent storage So your application can ‘remember’ Similar methods to reading / displaying information as before, but a different ‘destination’

Scanner Class Before, 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 Class Now we use the FileReader class Don’t forget your imports java.util java.io Scanner myScanner = new Scanner (new FileReader(“c:\\myfolder\\myFile.txt”);

FileReader Scanner 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 slashes Slash is the escape character Paths Relative vs. Absolute Drive letters

FileReader Now we can use the same scanner methods nextInt( ) nextDouble( ) hasNext( )

Example Scanner 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 file print( ) 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

Exceptions For 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 hold Products Comments Reviews