CS110 Lab 19:

Exception Handling and File Input/Output - The Basics

1. An exception is an unexpected error condition that prevents the program from continuing normally.
   For Example:
   • Reading from a file that does not exist.
   • Reading from a file that has corrupt data.
   • Writing to a file, but the disk/drive is full.
   • Reading input from the user and the user enters an invalid data type.
   • Unexpected division by zero.
   • Accessing an array with a subscript (index) that is out of bounds.
2. Applications must be flexible when handling problems, so as not to terminate abruptly.
   Exception handling provides an adaptable mechanism for handling such problems.
3. Java includes the following common exceptions:
   
   • IOException
   • RuntimeException
   • ArithmeticException
   • IndexOutOfBoundsException
   • ArrayIndexOutOfBoundException
   • NoSuchElementException
   • InputMismatchException
4. File IO exceptions are usually "Checked", meaning they are checked at compile time, such as FileNotFoundException. Code that uses a checked exception will not compile if the catch is not handled.
5. The following is a practical example of exception handling:
   

1. Writes a sentence to a file. The example below shows a value that is hardcoded, however, it can be input via the keyboard.
2. Read from the file and count the number of words in the sentence.
   Display the count to the console.

    import java.io.File;
    import java.io.FileNotFoundException;
    import java.io.IOException;
    import java.io.PrintWriter;
    import java.util.Scanner;

    public class MyApp {

    	public static void main(String[] args) {
        
    		// _____________________________________________________________
    		// TASK 1: WRITE A PARAGRAPH TO A FILE
    		// _____________________________________________________________
    		// Define a file to be written to, data.txt
    		File file = new File("data.txt");

    		// try:
    		//    (a) Create a PrintWriter object to print to a text-output stream.
    		//    (b) Use the PrintWriterWriter object to write a paragraph to the file.
    		//    (c) Close the PrintWriter object when done.
    		// catch:
    		//    (a) Catch an input/output exception.
    		//    (b) Display an error message for this exception
    		try {
    			PrintWriter printWriter = new PrintWriter(file);
    			printWriter.println("These french fries are cold and delicious!");
    			printWriter.close();
    		}
    		catch (IOException e) {
    			System.out.println("This file could not be created");
    		}

    		// _____________________________________________________________
    		// TASK 2: READ A PARAGRAPH FROM A FILE
    		// _____________________________________________________________
    		// Define a Scanner object.
    		Scanner fileScan = null;

    		// try:
    		//    (a) Instantiate the Scanner to read the contents of a file.
    		//    (b) Calls a method to count the words in the file.
    		// catch:
    		//    (a) Catch an exception if the specified file is not found.
    		//    (b) Display an error message .
    		// finally:
    		//    (a)Close the Scanner object, if it was properly instantiated.
    		try {
    			fileScan = new Scanner(new File("data.txt"));
    			System.out.print("There are " + count(fileScan) + " words.");
    		}
    		catch (FileNotFoundException e) {
    			System.out.println("Error - This file could not be found.");
    		}
    		finally {
    			if (fileScan != null)
    				fileScan.close();
    		}
    	}


    	//Method: Reads and counts the words in a file.
    	public static int count(Scanner fileScan) {
    		int countItems = 0;
    		while (fileScan.hasNext()) {
    			fileScan.next();
    			countItems++;
    		}
    		return countItems;
    	}
    }

  

    int [][] myArray =
			{{1,22,5,7,9},
			 {6,8,23,4,3,6},
			 {7,8,9,9,9,10,24,3,4},
			 {3,5,4,7,6,25},
			 {26,3,4,2,3,2,4}
			};