Handling Errors with Exceptions |
ListOfNumbers
Example
The following example defines and implements a class namedListOfNumbers
. TheListOfNumbers
class calls two methods from classes in the Java packages that can throw exceptions.Upon construction,// Note: This class won't compile by design! // See ListOfNumbersDeclared.java or ListOfNumbers.java // for a version of this class that will compile. 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(); } }ListOfNumbers
creates aVector
that contains tenInteger
elements with sequential values 0 through 9. TheListOfNumbers
class also defines a method namedwriteList
that writes the list of numbers into a text file calledOutFile.txt
.The
writeList
method calls two methods that can throw exceptions. First, the following line invokes the constructor for FileWriter, which throws an IOException if the file cannot be opened for any reason:out = new PrintWriter(new FileWriter("OutFile.txt"));Second, the
Vector
class'selementAt
method throws anArrayIndexOutOfBoundsExceptio
n if you pass in an index whose value is too small (a negative number) or too large (larger than the number of elements currently contained by theVector
). Here's howListOfNumbers
invokeselementAt
:out.println("Value at: " + i + " = " + victor.elementAt(i));If you try to compile the
ListOfNumbers
class, the compiler prints an error message about the exception thrown by theFileWriter
constructor, but does not display an error message about the exception thrown byelementAt
. This is because the exception thrown by theFileWriter
constructor,IOException
, is a checked exception and the exception thrown by theelementAt
method,ArrayIndexOutOfBoundsException
, is a runtime exception. Java requires that you catch or specify only checked exceptions. For more information, refer to Java's Catch or Specify Requirement.The next section, Catching and Handling Exceptions, will show you how to write an exception handler for the
ListOfNumbers
'writeList
method.Following that, a section named Specifying the Exceptions Thrown By a Method will show you how to specify that the
ListOfNumbers
'writeList
method throws the exceptions instead of catching them.
Handling Errors with Exceptions |