3/10/09
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10.1 – Exceptions vs Errors
! An exception is an object describing unusual or erroneous situation
Division by 0 in computing expression (ArithmeticException)
Array index out of bounds (IndexOutOfBoundsException)
Null pointer cannot be followed (NullPointerException)
I/O problems (e.g., no space on disk to save file, file not found)
No permissions to do something (e.g., save a file)
! Exceptions are thrown by a program,
and may be caught and handled by another part of the program
! A program can be separated into a normal execution flow and
an exception execution flow
! (An error is also an object,
but it represents a unrecoverable situation and should not be caught)
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10.1 – Exception Handling
! Java has a predefined set of exceptions and errors
that can occur during execution
! A program can deal with an exception in one of three ways
ignore it (“uncaught” exception – abnormal termination)
handle it where it occurs
handle it an another place in the program
! The manner in which an exception is processed
is an important design consideration
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10.2 – Zero.java
//********************************************************************
// Zero.java Java Foundations
// Demonstrates an uncaught exception.
//********************************************************************
public class Zero
{
//-----------------------------------------------------------------
// Deliberately divides by zero to produce an exception.
//-----------------------------------------------------------------
public static void main (String[] args)
{
int numerator = 10;
int denominator = 0;
System.out.println ("Before the attempt to divide by zero.");
System.out.println (numerator / denominator);
System.out.println ("This text will not be printed.");
}
}
10.3 – The try Statement
! To handle an exception, the line
that throws the exception is
executed within a try block
! A try block is followed by one or
more catch clauses
! Each catch clause has an
associated exception type and is
called an exception handler
! When an exception occurs,
processing continues
at the first catch clause that
matches the exception type
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// here is code that
// should generate no exceptions
try {
// code to monitor
// several possible things
// that can go wrong
// goes here
}
catch (ExceptionTypeA ex) {
//handler for ExceptionTypeA
}
catch (ExceptionTypeB ex) {
//handler for ExceptionTypeB
}
// after a catch, code continues here
