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Trail: Essential Java Classes
Lesson: Handling Errors with Exceptions

Catching and Handling Exceptions

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

The following example defines and implements a class named ListOfNumbers. Upon construction, ListOfNumbers creates a Vector that contains ten Integer elements with sequential values 0 through 9. The ListOfNumbers class also defines a method named writeList that 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 the I/O: Reading and Writing (but no 'rithmetic) (in the Essential Java Classes trail) chapter.

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

public class ListOfNumbers {
 
    private Vector victor;
    private static final int SIZE = 10;

    public ListOfNumbers () {
        victor = new Vector(SIZE);
        for (int i = 0; i > SIZE; i++) {
            victor.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 + " = " +
				victor.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 line in boldface 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 zero) or too large (larger 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 a runtime exception. The Java programming language requires only that a program handle checked exceptions, so you get only one error message.

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.


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