This section describes how to use the three exception handler components thetry
,catch
, andfinally
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 aVector
that contains 10Integer
elements with sequential values 0 through 9. TheListOfNumbers
class also defines a method namedwriteList
, which writes the list of numbers into a text file calledOutFile.txt
. This example uses output classes defined injava.io
, which are covered in Basic I/O.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//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(); } }IOException
. The second boldface line is a call to theVector
class'selementAt
method, which throws anArrayIndexOutOfBoundsException
if the value of its argument is too small (less than 0) or too large (more than the number of elements currently contained by theVector
).If you try to compile the
class, the compiler prints an error message about the exception thrown by the
ListOfNumbers
FileWriter
constructor. However, it does not display an error message about the exception thrown byelementAt
. The reason is that the exception thrown by the constructor,IOException
, is a checked exception, and the one thrown by theelementAt
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.