This section describes how to use the three exception handler components — the try, catch, and finally blocks — to write an exception handler. The last part of this section walks through an example and analyzes what occurs during various scenarios.

The following example defines and implements a class named ListOfNumbers. When constructed, ListOfNumbers creates a Vector that contains 10 Integer elements with sequential values 0 through 9. The ListOfNumbers class also defines a method named writeList, which writes the list of numbers into a text file called OutFile.txt. This example uses output classes defined in java.io, which are covered in Basic I/O.

//Note: This class won't compile by design!
import java.io.*;
import java.util.Vector;

public class ListOfNumbers {

    private Vector vector;
    private static final int SIZE = 10;

    public ListOfNumbers () {
        vector = new Vector(SIZE);
        for (int i = 0; i < SIZE; i++) {
            vector.addElement(new Integer(i));
        }
    }

    public void writeList() {
        PrintWriter out = new PrintWriter(
                            new FileWriter("OutFile.txt"));

        for (int i = 0; i < SIZE; i++) {
            out.println("Value at: " + i + " = " +
                         vector.elementAt(i));
        }

        out.close();
    }
}

The first line in boldface is a call to a constructor. The constructor initializes an output stream on a file. If the file cannot be opened, the constructor throws an IOException. The second boldface line is a call to the Vector class's elementAt method, which throws an ArrayIndexOutOfBoundsException if the value of its argument is too small (less than 0) or too large (more than the number of elements currently contained by the Vector).

If you try to compile the ListOfNumbers class, the compiler prints an error message about the exception thrown by the FileWriter constructor. However, it does not display an error message about the exception thrown by elementAt. The reason is that the exception thrown by the constructor, IOException, is a checked exception, and the one thrown by the elementAt method, ArrayIndexOutOfBoundsException, is an unchecked exception.

Now that you're familiar with the ListOfNumbers class and where the exceptions can be thrown within it, you're ready to write exception handlers to catch and handle those exceptions.