Transcript Exceptions
Exceptions
Don’t Frustrate Your User – Handle Errors
KR – CS 1401 Spring 2005
Picture – sysprog.net
Exceptions
Writing Reliable Code
We want to write applications that are reliable.
Reliability depends upon correctness and robustness:
• Correctness - the program produces correct results for
valid input.
• Robustness – the program can handle error conditions
without crashing too easily.
Robustness can be increased by planning for and handling
unusual circumstances, called exceptions.
KR – CS 1401 Spring 2005
Exceptions
Possible sources of Error
An exception is an unusual error condition that occurs
while the program is running.
What are some sources of these errors ?
1. System or network-related I/O problems:
- A file may become corrupted or removed.
- Network congestion or an overloaded server may prevent
access to a file.
- The server may be down.
2. User input error, such as entering a string that cannot
be converted into a number as expected by application.
KR – CS 1401 Spring 2005
Exceptions
When an exception occurs, the system is alerted and an
exception is said to be thrown.
When an exception is thrown, the normal sequence of code
execution is halted, and the exception is caught by
appropriate exception-handling routines.
KR – CS 1401 Spring 2005
Exceptions
Classes of Exception Objects
Exceptions in Java are defined as objects of various
exception classes.
Exception classes form an inheritance hierarchy.
All exceptions are subclasses of the class Throwable.
You may throw any of the predefined exceptions to handle
an error condition, or you may define and throw your own
exception subclass to handle errors in a specific way
appropriate for your application.
KR – CS 1401 Spring 2005
Exceptions
Inheritance Diagram of Exception Classes
KR – CS 1401 Spring 2005
Diagram – Sun Microsystems
Exceptions
How is an exception handled?
If an exception is thrown within a method, the JVM looks
for code within the local method to catch (handle) it.
If there is none, the method returns and the JVM looks in
the calling method for code to catch the exception.
This can cause a chain of returns all the way back to the
JVM. At that point, the program terminates and a stack
trace is displayed.
Up to now we have thrown exceptions to the JVM to
catch:
public static void main (String [] args) throws IOException {…}
KR – CS 1401 Spring 2005
Exceptions
Exception handling
KR – CS 1401 Spring 2005
Diagrams – Sun Microsystems
Exceptions
Writing our own code
We can write our own code to throw and catch exceptions.
The structure to use is a try-catch-finally statement:
•
•
•
Block of code that may result in an error and thrown
exception (try).
One or more blocks of code to handle exception(s)
thrown (catch).
Optional block of code performing “clean-up” or other
statements to be executed regardless of whether there
is an error or not (finally). Ex: closing a file.
KR – CS 1401 Spring 2005
Exceptions
Syntax of try-catch-finally statement
try {
statement
statement …
}
catch ( ExceptionClass exceptionObject ) {
statement
statement …
}
finally {
statement
statement …
}
KR – CS 1401 Spring 2005
Exceptions
Example: One Catch Block
int number;
try {
String userInput = inFile.readLine( );
number = Integer.parseInt ( userInput );
}
catch ( NumberFormatException excep ) {
System.out.println ( “Cannot convert input. Please enter integer.” );
}
finally {
System.out.println ( “End of try-catch-finally statement.” );
}
KR – CS 1401 Spring 2005
Exceptions
Multiple Catch Blocks May Be Defined
try {
statement
statement …
}
catch ( ExceptionClass exceptionObject ) {
statement
statement …
}
catch ( ExceptionClass exceptionObject ) {
statement
statement …
}
finally { … }
KR – CS 1401 Spring 2005
Exceptions
Example: Multiple Catch Blocks
int number;
try {
String userInput = inFile.readLine( );
number = Integer.parseInt ( userInput );
}
catch ( IOException excep ) {
System.out.println ( “Problem reading file.” );
}
catch ( NumberFormatException excep ) {
System.out.println ( “Cannot convert input. Please enter integer.” );
}
finally { … }
KR – CS 1401 Spring 2005
Exceptions
Throwing Our Own Exceptions
int number;
// Ask user to enter a number > = 100
try {
String userInput = inFile.readLine( );
number = Integer.parseInt ( userInput );
if ( number < 100 ) {
throw new Exception (userInput + “must be > = 100” );
}
}
catch ( NumberFormatException excep ) {
System.out.println ( “Cannot convert input. Please enter integer.” );
}
catch ( Exception excep ) {
System.out.println ( “Error: ” + excep.getMessage ( ) );
}
finally { … }
KR – CS 1401 Spring 2005
Exceptions
Order of Multiple Catch Blocks
List multiple catch blocks in the order of more specialized
exception classes to more general exceptions:
catch ( NumberFormatException excep ) {
System.out.println ( “Cannot convert input. Please enter integer.”
);
}
catch ( Exception excep ) {
System.out.println ( “Error: ” + excep.getMessage ( ) );
}
Only one catch block will be executed; the rest will be
skipped.
KR – CS 1401 Spring 2005
Exceptions
Defining Our Own Exception Subclass
int number;
// Ask user to enter a number > = 100
try {
String userInput = inFile.readLine( );
number = Integer.parseInt ( userInput );
if ( number < 100 ) {
throw new ourInputException (userInput + “must be > = 100” );
}
}
catch ( NumberFormatException excep ) {
System.out.println ( “Cannot convert input. Please enter integer.” );
}
catch ( ourInputException excep ) {
System.out.println ( “Error: ” + excep.getMessage ( ) );
}
finally { … }
KR – CS 1401 Spring 2005
Exceptions
What is ourInputException ?
It is the subclass of Exception that we defined:
public class ourInputException extends Exception {
public ourInputException ( ) {
// Call constructor of Exception Class with no argument
super ( ) ;
}
public ourInputException ( String message ) {
// Call constructor of Exception Class and pass message that catch
// clause can use for error handling.
super ( message );
}
KR – CS 1401 Spring 2005