1. When the main method of this class executes, this output is created:

         java.io.FileNotFoundException: demo.txt (The system cannot find the file specified)

          at java.io.FileInputStream.open(Native Method)

          at java.io.FileInputStream.<init>(Unknown Source)

          at java.io.FileInputStream.<init>(Unknown Source)

          at fileio.main(Demo.java:4).

    because there is no catch() block to trap the FileNotFoundException, which then propagates up to the JVM, which finally traps it and displays the stack trace.

    Solution:

public class Demo {
      public static void main(String[] args) {
            try {
                FileInputStream iStream = new FileInputStream("demo.txt");
            } catch (NullPointerException e){
                System.out.println(“Can’t open file for input”)
            } catch (FileNotFoundException e){ // an extra catch block is needed to catch FileNotFoundExceotion
                System.out.println(“Can’t open file for input”);
            }
      }
}
  1. In the space below, complete the method that:
    bulletOpens the text file whose name is supplied as the argument. The text file contains an arbitrary number of integer values (one per line), but may also contain invalid values (non-numeric).
    bulletCreates an ArrayList that can contain integers.
    bulletReads each line of the text file, and for valid integer values, adds the value to the ArrayList. When invalid values are encountered because of NumberFormatExceptions, they are simply ignored.
    bulletThe method should propagate no exceptions, and close the file before returning.

    Solution: 
    public List readValues( String filename ) {
	ArrayList values = new ArrayList();
	Scanner reader = null;
	try { 
		FileInputStream fs = new FileInputStream(filename);
		reader = new Scanner(fs);
		reader.useDelimiter("[;,\\s]+");
		while( reader.hasNext() ) { // repeat while there is something to read...
			try {
				int x = reader.nextInt();
				System.out.println("Value read: " + x );
				values.add(x);
			} catch( Exception e ) { // nextInt() failed because of non-integer data on the line
				// do nothing; ignore
				String junk = reader.next(); // consume the non-int token
				System.out.println("Ignoring " + junk );
			}
		}
	} catch( Exception e ) { // FileInputStream() threw an exception
		System.out.println("Could not open file!");
	} finally { // close the file in any case
		System.out.println("Closing file!");
		reader.close();
	}
	return values;  // return the filled ArrayList
}

    Reading the following file:

    1
2
3 abc
4
 def
6
76
8
9

    produces the following output:

    Value read: 1
Value read: 2
Value read: 3
Ignoring abc
Value read: 4
Ignoring def
Value read: 6
Value read: 76
Value read: 8
Value read: 9
Closing file!