1. File I/O ICS 111: Introduction to Computer Science I
William Albritton
Information and Computer Sciences Department at the University of Hawai‘i at Mānoa
"The wireless music box has no imaginable commercial value. Who would pay for a message sent to nobody in particular?"
David Sarnoff's associates in response to his urgings for investment in the radio in the 1920s.
12/1/2018
© 2007 William Albritton

2. Java Program Implementation
Implement on UH UNIX using the command line interface
The command line is the line at the prompt on the UNIX shell
Commands
Text editor: emacs Program.java
Compiler: javac Program.java
Interpreter: java Program
A demo will be given in class
12/1/2018
© 2007 William Albritton

3. Command line Arguments
Arguments (strings) entered on the command line can be accessed in your Java program
args is an array of Strings
public static void main(String [] args)
Corresponds to the words listed after the name of the Java program
Below is I/O for program Repeat.java
java Repeat one two three
args[0] is the String "one"
args[1] is the String "two"
args[2] is the String "three"
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© 2007 William Albritton

4. Command line & IDEs See file Repeat.java UHUNIX commands
javac Repeat.java
java Repeat one two three
jGRASP commands
Click Build, Run Arguments
Run Arguments: one two three
Eclipse commands
Click Run, Run..., Arguments, Apply, Run
Program Arguments: one two three
12/1/2018
© 2007 William Albritton

5. Command line & jGRASP
You can also enter command line arguments using jGRASP
Directions
From the top menu, select “Build”
On the drop-down menu, click “Run Arguments”
At the top of the program, you should now have a text box titled “Run Arguments:”
Type your command-line arguments into this text box
12/1/2018
© 2007 William Albritton

6. Meaning of main Meaning of each word of the main method
public static void main(String[] args)
public: other classes can use this method
static: this is a class method, so method call has format ClassName.methodName()
void: this method does not return anything
main: name of this special kind of method which is used to start the program
String[]: an array of Strings
args: the parameter of this method, which contains each word entered on the command line, stored as an array of Strings
12/1/2018
© 2007 William Albritton

7. Class Exercise
Write a Java program that creates a new word by combining the first letter of each word in the command line arguments
The main method’s parameters are an Array of Strings (the command line arguments) & the main method displays a String (the new word)
12/1/2018
© 2007 William Albritton

8. Types of Exceptions
Like other classes, exceptions have a class hierarchy
See Java API & start at class Exception
Can be broken down into two main types
Runtime (unchecked) exceptions
Checked exceptions
12/1/2018
© 2007 William Albritton

9. Types of Exceptions Runtime (unchecked) exceptions
A type of exception that does not require a try-catch block
Usually can be prevented by careful programming
Not considered as a serious error
Subclasses of class java.lang.RuntimeException, such as ArithmeticException, NullPointerException, IndexOutOfBoundsException, etc.
12/1/2018
© 2007 William Albritton

10. Types of Exceptions Checked exceptions
A type of exception that must be placed in a try-catch block
Signify that a “serious problem” has occurred
Subclasses of class java.lang.Exception, such as IOException & FileNotFoundException
12/1/2018
© 2007 William Albritton

11. Files What is a file? Have two main types of files
A collection of data with a name
Allows for long-term storage
Have two main types of files
Binary and text files
12/1/2018
© 2007 William Albritton

12. Two Types of Files Text file A file that contains characters
Words, numbers, symbols, etc.
For example, the source files (*.java) of your programs are text files, as are *.html files
A *.txt file is a text file
Usually, 8-bits are used to store each character
Each character corresponds to a particular numeric code called an “ASCII” code (American Standard Code for Information Interchange)
Modern character standard now uses Unicode, which is used to encode all the languages in the world
12/1/2018
© 2007 William Albritton

13. Two Types of Files Binary file
A file that may include text, but also includes some symbols that cannot be represented as text
Usually must be interpreted by another program
For example, compiled Java class files (*.class) are binary files
Photographs (*.jpg or *.gif) are stored as binary files
Microsoft Word files (*.doc) are stored as binary files, as are PowerPoint files (*.ppt)
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© 2007 William Albritton

14. Writing to a File Need import statements at top of program
import java.util.Scanner; import java.io.PrintWriter; import java.io.FileNotFoundException;
PrintWriter constructor needs a String
The name of the file, if in current directory
"myFile.txt"
Or full name of path to the file
"C:\ics111\progs\myFile.txt"
Note that we are using the 1st command-line argument for the file name (args[0])
12/1/2018
© 2007 William Albritton

15. Writing to a File
PrintWriter constructor either creates a new file if it does not exist, or overwrites an existing file
Must have a try-catch block, as will throw a checked exception if path name cannot be found
String fileName = "myFile.txt"; PrintWriter writer = null; try { writer = new PrintWriter(fileName); } catch(FileNotFoundException e){ System.out.println(e); }
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© 2007 William Albritton

16. Writing to a File Example program See WriteToFile.java
Writes to a file, which has the same name as the 1st command-line argument (args[0])
If no arguments are entered, then the program ends before writing to a file
Note that out of System.out is of class PrintStream, which has many methods which are similar to PrintWriter, such as print() and println()
For file names, use fileName.txt, so it is easy to view the files
12/1/2018
© 2007 William Albritton

17. Reading from a File Need import statements at top of program
import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException;
File constructor needs a string that is either
The name of the file if in current directory
"myFile.txt"
Or full name of path to the file
"C:\ics111\prog\myFile.txt"
This is does not create a file on disk
It only connects our program to a file
12/1/2018
© 2007 William Albritton

18. Reading from a File Use File object in Scanner’s constructor
Must have a try-catch block, as will throw a checked exception if file cannot be found
String fileName = "myFile.txt"; File file = new File(fileName); Scanner scanner = null; try{ scanner = new Scanner(file); } catch(FileNotFoundException e){ System.out.println(e); }
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© 2007 William Albritton

19. Reading from a File Example code See ReadFromFile.java
Reads from a file, which has the same name as the 1st command-line argument (args[0])
If file does not exists, with throw FileNotFoundException and end program
Scanner method hasNextLine() will return “true” if more lines are in the file, otherwise “false”
12/1/2018
© 2007 William Albritton