1. File FileReader / FileWriter Scanner / PrintWriter
File IO in Java
File
FileReader / FileWriter
Scanner / PrintWriter

2. What We Won’t Do FileInputStream FileOutputStream
These classes are for reading streams of raw data
They are most often used for images
Allows you to read bytes (in binary)
We will focus on characters

3. Packages
We’ll find the classes we’re using in the io and util packages:
import java.io.*;
import java.util.*;

4. File Class Constructor: Here are some useful methods:
File file = new File("c:\\data\\input.txt");
Use the full or relative path (from current directory)
Here are some useful methods:
boolean fileExists = file.exists();
int length = file.length(); //length in bytes
boolean success = file.renameTo(new File("c:\\data\\new.txt"));
boolean success = file.delete();
File file = new File("c:\\data"); //can also be a directory
boolean isDirectory = file.isDirectory();
File file = new File("c:\\data");
String[ ] fileNames = file.list();
File[ ] files = file.listFiles();

5. try... catch blocks
In Java there is structure known as the try catch block.
Objects can throw an exception when something goes wrong to notify the user
Index out of bounds is one
This prevents programs from an all out crash since the code that would cause the problem is never executed

6. try... catch
Example:
public void mustBeOdd(int number) throws Exception {
if( number % 2 != 0 )//not even!!!
throw Exception;
//at this point the code is only executed if even
statements… }

7. try… catch
Since the method declares the keyword throws anyone that uses the method must catch it in case of an error
So we tell Java we’re going to try to do something, but if an exception is thrown, we will catch it and deal with the error.

8. try… catch Files can have all sorts of things go wrong
They might be read only and you try to write
They might be in use and not allow more than one person to read from them
They may no longer exist (could have been delete) …
Most file I/O requires a try/catch block

9. FileReader
Reading each character in a file one at a time with FileReader:
File test = new File("C:\\test.txt");
try {
FileReader fr = new FileReader(test);
int data = fr.read();
int counter = 1;
while(data != -1)
System.out.println("Character #" + counter + " is " + (char)data);
data = fr.read();
counter++; }
fr.close();//close the file
catch(IOException e)
System.out.println(e);

10. FileReader Reading all characters at once with FileReader:
File test = new File("C:\\test.txt");
try {
FileReader fr = new FileReader(test);
char[] allAtOnce = new char[(int)test.length()];
int charsRead = fr.read(allAtOnce);
System.out.println("This file has " + test.length() + " bytes");
System.out.println("Here they are!");
System.out.println(allAtOnce);
fr.close();//close the file }
catch(IOException e)
System.out.println(e);

11. FileReader
Reading all characters at once into an array is OK for small files
Large files may take up too much memory
This will eat up the computers resources
Can affect performance too!
The read method has two other versions:
read(char[] buffer)
Fills the array as much as possible from the files contents
read(char[] buffer, int startIndex, int length)
Fills the array starting at the position given for the number of characters specified

12. FileReader Reading several characters at once with FileReader:
File test = new File("C:\\Documents and Settings\\Cam\\Desktop\\test.txt");
try {
FileReader fr = new FileReader(test);
int amount = 25;
char[] severalChars = new char[amount];
int charsRead = fr.read(severalChars);
String allChars = new String();
while( charsRead != -1 )
allChars += new String(severalChars);//create a string from the chars
charsRead = fr.read(severalChars); }
System.out.println("File says:");
System.out.println(allChars);
fr.close();//close the file
catch(IOException e)
System.out.println(e);

13. FileWriter Writing to a file is similar
Some things to note when you open a file for writing:
If the file does not exist, it will be created for you
If the file does exist the contents are erased, the content you write becomes the new file
To avoid losing all the old information, you must either read the file first and save its contents to an array or use the append method
To write a new line like you expect, \n is not enough for a text file, you must use a carriage return also (\r): \r\n  See example next slide

14. FileWriter Writing a 10x10 multiplication table to a file:
File test = new File("C:\\test.txt");
try {
FileWriter fw = new FileWriter(test);
for( int n = 1; n <= 10; n++ )
for( int k = 1; k <= 10; k++)
fw.write(n*k + "\t");
fw.write("\r\n"); }
fw.close();
catch(IOException e)
System.out.println(e);

15. FileWriter Appending a 10x10 multiplication table to a file:
File test = new File("C:\\test.txt");
try {
FileReader old = new FileReader(test);
char[] oldData = new char[(int)test.length()];
old.read(oldData);//save old information
old.close();
FileWriter fw = new FileWriter(test);
fw.write(oldData);//put old data back so we can append new data
for( int n = 1; n <= 10; n++ )
for( int k = 1; k <= 10; k++)
fw.write(n*k + "\t");
fw.write("\r\n"); }
fw.close();
catch(IOException e)
System.out.println(e);

16. Not Much Help…
The FileReader and FileWriter class do what they say (read/write) but they don’t really have any additional features.
For example, what if you wanted to read a word at a time or maybe a whole line?
What if you wanted to read the data as an int instead of a char?
What if you wanted to print a line of text? …
Let’s take a look at two classes that will help us
Scanner
PrintWriter

17. Scanner
The Scanner class works the same way as you know of it from reading console input
The stream is the only thing that changes
Instead of reading from the console, now we want the scanner to read from a file

18. Scanner Example
This example reads each “word” (string separated by white space) from a file and prints them one line at a time:
File test = new File("C:\\test.txt");
try {
FileReader read = new FileReader(test);
Scanner in = new Scanner( read );
while( in.hasNext() )//is there something to read?
System.out.println(in.next());//next method gets a word
read.close(); }
catch(IOException e)
System.out.println(e);

19. Scanner Methods
The Scanner class is LOADED with methods for reading a stream:
hasNextInt(): next value is an int
hasNext(): next value is a string
hashNextLine(): there is another line of text to read
There are many to check what’s left in the file
next(): reads a token (separated by white space)
nextLine(): reads a line of text
nextInt(): reads the next token as an int …
All of the next____() methods we’ve used to get data from the console can be used on a file

20. PrintWriter The PrintWriter class behaves more like System.out
You can print just about any data type or object with a toString method into a file
You can also use the println method rather than having to use \r\n
The advantage is you can work with more than just chars and not have to worry about casting/conversions

21. Person Class public class Person {
public Person( String name, int age )
public String toString()
return name + “\t” + age; }

22. PrintWriter Example This example prints an array of objects to a file:
File test = new File("C:\\Documents and Settings\\Cam\\Desktop\\test.txt");
try {
FileWriter fw = new FileWriter(test);
PrintWriter writer = new PrintWriter( fw );
Person[ ] pArr = {new Person("John", 10), new Person("Bob", 12),
new Person("Sue", 11), new Person("Anne", 13) };
for( int n = 0; n < pArr.length; n++ )
writer.println(pArr[n]);//Uses Person's toString method to print
fw.close(); }
catch(IOException e)
System.out.println(e);

23. Exercises - FileUtility
The first class we’ll create will be called FileUtility
The class takes a path to a file in its constructor and performs the following tasks (methods):
public int wordCount()
Returns the number of words in the file
public int lineCount()
Returns the number of lines in the file
public int count(String word)
Returns the number of instances of word in the file
public void replace(String target, String replacement)
Should work for all instances, not just whole words
Ex. replace( “ing”, “abc” ) turns “flying” to “flyabc” in the file
public void append( String data )
Appends data to the end of the file

24. Exercises - FileUtility
The last 4 methods are for encryption and decryption, I suggest you use chars (8 bits = 256 values) for these when you read/write
Cipher Encryption works like this:
Take a numeric key and add that to each value in the alphabet
For example a key of 2 makes ac, bd, etc.
The word “hello” with key 2 becomes: “jgnnq”
What happens to “z”?
Wraps around back to a, then b (z  b)
Use mod to get this effect on integers
Create the method:
public void cipherEncrypt( int key )

25. Exercises - FileUtility
Cipher Decryption works like this:
Take a numeric key and subtract value from each in the alphabet
For example a key of 2 makes ay, bz, etc.
The word “eqorwvgt” with key 2 becomes: “computer”
Create the method:
public void cipherDecrypt( int key )
You can test your code by calling
cipherEncrypt( key )
cipherDecrypt( key )
The result is the file is restored if done correctly

26. Exercises - FileUtility
Password Encryption is a little different:
Take in a password and use it to fill the beginning of an array which contains the alphabet
Fill the array with values which have not been used yet
Suppose the password was “Secret”
The alphabet:
{a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z}
becomes:
{s,e,c,r,t,a,b,d,f,g,h,i,j,k,l,m,n,o,p,q,u,v,w,x,y,z}
So the letter ‘a’ is now an ‘s’,
The letter ‘b’ is now an ‘e’, …
The word “testing” becomes “qtpqfkb”
Create the methods
public void passwordEncrypt( String word )
public void passwordDecrypt( String word )
*Your alphabet consists of all 256 chars, not just 26

27. Exercise – CSV files
CSV or comma separated values is a common/simple file type for displaying data in a table.
Columns are separated by commas and rows by new lines
Testing,1,2,3
Testing,A,B,C
Testing 1 2 3 A B C

28. Exercise – CSV files
Make a class that takes the path to a csv file in its constructor and loads the table of data into a 2D array (or ArrayList).
Add the following methods to the class:
public int rows()
Returns the number of rows in the CSV file
public int columns()
Returns the number of columns in the CSV file
public String getRow( String key )
Searches the data and if key is found, returns the row of data where it was discovered

29. Exercise – CSV files HTML tables have the following syntax:
<tr><td></td><td></td></tr> …
</table>
Where tr marks the start of a row and td the start of a column
In the example:
Testing,1,2,3
Testing,A,B,C
The html would be:
<tr><td>Testing</td><td>1</td><td>2</td><td>3</td></tr>
<tr><td>Testing</td><td>A</td><td>B</td><td>C</td></tr>

30. Exercise – CSV files A google search can turn up more info on tables
The tags table, tr, and td also have options for line thickness/color, font, background color etc.
Create the method:
public void exportTo(String path, *options*)
Which creates a new .html file and exports the table there. The user must have at least 4 options included when they export
This could be the colors, font, lines etc.

31. Exercises - Greenfoot
Update your board game so that you can save and load it to/from a file.
When you resume a game it should continue from the same state as before
Score
Player’s turn
Pieces on Board …
You may use any format that’s convenient
If you’d like to use the CSVFile class (or some of it’s code) you can also do that.