Miscellany

1
Miscellany

• We wrap up with a few miscellaneous topics that
  might be useful in constructing your final
  project
• We start with more forms of I/O
• So far, we have only done I/O either using
  JOptionPane or using a textbox in our GUI
• We can also input directly from the console
  window as follows
• import java.io.
• Declare and instantiate a variable to represent
  your keyboard input
• BufferedReader keyboard new BufferedReader(new
  InputStreamReader(System.in))
• Pass your BufferedReader object the message
  readLine( )
• Note that this returns a String, so any numbers
  have to be converted first by parsing them

2
I/O Exceptions

• An Exception is a situation that arises during a
  Java program that is of importance for
  instance, run-time errors are types of Exceptions
• We handle Exceptions by using Exception handler
  code
• If you are going to use any of the io library,
  these classes expect Exception handlers to be
  implemented so that the programs can run safely
  without causing run-time errors
• How do you handle IO Exceptions?
• Two ways
• First, you can pass the buck by letting someone
  else handle them
• this is done by adding throws IOException to the
  method header for the method doing I/O, and to
  any method that calls this method
• Second, you can place the code into a try block
• a try block tries something out and if it fails,
  it throws an exception
• to handle the problem, you implement a catch
  block after your try block

3
Examples
try BufferedReader keyboard new
BufferedReader(new InputStreamReader(System.in)
) temp Integer.parseInt(keyboard.readLine())
catch(IOException e) System.out.println("oops
")
In this approach, the try/catch blocks handle the
Exception if one occurs Here, we throw the
Exception to whatever method called this
one, that method must either use a try/catch
block or throw the Exception as well by adding
throws IOException to
that methods header
public static int getInput2 ( )
IOException int temp 0 BufferedReader
keyboard new BufferedReader(new
InputStreamReader(System.in)) temp
Integer.parseInt(keyboard.readLine()) return
temp
4
Inputting from Files

• Now that you have seen how to do basic input from
  keyboard, we can go over inputting/outputting to
  files because it is very similar
• To input from file
• Replace the previous BufferedReader line with
• BufferedReader infile new BufferedReader(new
  FileReader(filename))
• where filename is the String name of the file
  (including directories if the file is not in the
  current directory) the file should be a text
  file only
• Now input an item using read( ) or readLine( )
• int x Integer.parseInt(infile.readLine( ))
• read inputs an item and leaves the cursor where
  it is in the file for the next input, readLine( )
  reads the next item and returns the cursor to the
  next line for the next input
• use read only if you have set up your file to
  have multiple items on the same line

5
Outputting and Closing

• Outputting to a file is similar to inputting
  except that we use a PrintWriter and FileWriter
  in place of BufferedReader and FileReader
• PrintWriter outfile new PrintWriter(new
  FileWriter(filename))
• again filename is the name of the file in this
  case, the file you want to create or overwrite
• We use print or println for output
• outfile.print() outputs and leaves the
  cursor on the same line
• outfile.println() outputs and returns the
  cursor in the file to start a new line
• When we are done with our file after inputting or
  outputting, we must close the file
• infile.close( )
• outfile.close( )

6
Complete Example
import java.io. public class FileTester
public static void main(String args)
int i, n, item String filename1"test.txt",
filename2"out.txt" try BufferedReader
infile new BufferedReader(new
FileReader(filename1)) PrintWriter
outfile new PrintWriter(new FileWriter(file
name2)) n Integer.parseInt(infile.readLine()
) for(i0iltni) item
Integer.parseInt(infile.readLine()) outfile.p
rintln("Value " (i 1) " is "
item) infile.close() outfile.close()
catch(IOException e) System.out.prin
tln("Error in file access!")
7
Why Use File I/O?

• Save your paint program image
• You cant actually save the Graphics JPanel as a
  bitmap, but you can save where each item was
  painted
• Save the high scores of a game
• Load from the high score file the scores, storing
  them in an array
• If the player earns a score higher than any in
  the list, add this to the list in sorted order
• At the end of the game, save the list back to
  file
• For the write a story program, you could have
  all of the topic areas stored in files and input
  all of the array elements from the files

for(i0iltnumi) outfile.println(shapesi.x
1) outfile.println(shapesi.y1) outfile.pri
ntln(shapesi.x2) outfile.println(shapesi.y
2) outfile.println(shapesi.type)
outfile.println(shapesi.color)
8
More on Files

• Notice in the previous example that we did
  outfile.println(shapesi.color)
• this will not work if the color is of type Color
• So instead, we have to convert the Color into
  something that can be output how about the RGB
  values?
• outfile.println(shapesi.color.getRed( ))
• outfile.println(shapesi.color.getGreen( ))
• outfile.println(shapesi.color.getBlue( ))
• If we want to input all of the Shape info from
  the file, how do we know how many times to read?
• two approaches save the number of Shape
  elements as an int value at the beginning of the
  file, or just list each Shape one at a time and
  iterate while the value is not null
• the first approach is safer, and the approach
  taken in the example 2 slides back, but the
  latter approach is easier

9
Filenames

• In our previous example, we used hard-coded
  file names
• This will be fine if we want to replace the same
  file every time we save our game
• but if we want to be able to save multiple games,
  we will need to input the file name
• How?
• ask the user for the filename using a JOptionPane
  input dialog box
• ask the user for the filename in the console
  window and use a BufferedReader to input it
• add a JTextField to our GUI and have the user
  type in the filename there however, this would
  require that the user input the filename before
  they click on the Load or Save button, or we
  would have to have yet another JButton to click
  on once they typed in the filename this could
  get messy

10
Applets

• An applet is a Java program that runs inside a
  web browser
• This allows you to place your compiled programs
  on your web site while letting anyone who has a
  web browser run them
• this is not possible in most other languages
  because a program written in say C and compiled
  on your computer will not run correctly on other
  computers that are of a different platform or
  Operating System
• changing your program into an applet is not a
  huge amount of work but is not necessarily
  straightforward
• therefore you will only want to do this if you
  want to make your program available on the
  Internet

11
Steps To Convert to JApplet

• JApplet (the newer version of an Applet) is part
  of the javax.swing package, so import that
  package
• Change your class header from
• public class Name extends JFrame to
• public class Name extends JApplet
• Change the name of your main method to init
• public void init( )
• Delete all of the JFrame instructions but leave
  the creation of your JPanel and change the add
  instruction to merely be

lthtmlgt ltheadgt lttitlegtTitle herelt/titlegt lt/headgt ltb
odygt ltapplet codeName.class" height300
width300gt lt/appletgt lt/bodygt lt/htmlgt

• add(jpanelItem)
• as opposed to frame.getContentPane().add(jpanelIte
  m)
• Compile your JApplet
• note that since the JApplet no longer has a main
  method, you cannot directly run it
• instead, create an html file (see to the right),
  open a web browser to that URL and the applet
  will load onto that web page