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Exception Handling
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Geoff Holmes
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Week 5
problems
Handling
Throwing
Catching
Multiple
exceptions
Finally clause
Examples
Week 5 Problems
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Write a program to read a file of filenames
representing images (*.jpg). Store the filenames in
an array and iteratively display the contents of
each file (see Section 18.4).
Exercise 16.1 (Exception handling)
Exercise 14.2 – read a file and count the number
of lines, words and characters (covers Utility
classes)
Exercise 19.3 – collection classes, will start to do
either Tuesday or Friday.
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Handling Errors
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In exceptional circumstances (eg bad data input)
we use Java’s exception handling system. Where
do errors come from?
User input errors
 Device errors (printer turned off)
 Physical limits (disk full, out of memory)
 Code errors – array indexes, pop an empty stack.
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Normal error handling is to return (-1) but this
doesn’t always work. Also, it isn’t OOP to return a
number to mark an error state.
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Motivation
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Exceptions are events that disrupt normal program
flow; e.g. processing a file:
 Ask the user for filename
 Open file
 Read contents
 Do something with contents
 Close file
Lots of potential problems (file not found, not
readable, …)
Java supports strong distinction between normal
and abnormal program flow
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Exception hierarchy
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Throwable
 Error
 LinkageError
 VirtualMachineError
 …
 Exception
 IOException
 RuntimeException
• ClassCastException
• …
 …
Every exception is an instance of a subclass of Throwable
Programmers are not forced to deal with Error and RuntimeException
cases (too common)
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Throwing your own
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Suppose you have a problem with premature EOF when
reading data. You know it is an IOException and find
EOFException:
String readData(BufferedReader in) throws EOFException
{
While (…) {
If (n < len) throw new EOFException( );
…
}
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Can give EOFException a string
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String problem = “Should be “ + len + “ but is “ + n;
throw new EOFException(problem);
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So for exisiting exceptions we:
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Find the appropriate class
 Make an object of that class
 Throw it
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No exception fits?
class FileFormatException extends IOException {
public FileFormatException( ) { }
public FileFormatException(String problem) {
super(problem);
}
}
String readData(BufferedReader in) throws
FileFormatException {
While (…) {
If (n < len) throw new FileFormatException( );
…
}
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Catching Exceptions
Use a try/catch block to catch exceptions:
try
{ code
more code
more code
}
catch (ExceptionType e)
{ handler for this type }
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What happens?
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If any code in the try block throws an exception of
the class specified in the catch block then the
program skips the remaining code and executes
the handler code.
If none of the code throws an exception then the
catch block is skipped.
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Example 1 – ExceptionTest
Try to pop an empty stack!
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Rules for passing or treating
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Catch those exceptions that you know how to
handle
Propagate those that you do not!
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Catching any exception –
ExceptionMethods
catch (Exception e) {
System.out.println(“caught an exception”);
}
To get better info on the error we can use:
1.
e.getMessage( )
2.
e.toString( )
3.
e.printStackTrace( )
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Multiple Exceptions
Try { code that might contain many exceptions
}
catch (MalformedURLException e1) {
code to handle bad URLs
}
catch (UnknownHostException e2) {
code to handle unknown hosts
}
catch (IOEXception e3) {
code to handle all other IO problems
}
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Book example
try {
File fd1 = new File(name);
File fd2 = new File(name);
processFile(fd1,fd2);
fd1.close();
fd2.close();
} catch (FileNotFoundException e) {
System.err.println(“Cannot find file “ + e);
} catch (FileOpenFailed e) {
System.err.println(“Cannot open file “ + e);
}
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Finally clause
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Optional last clause in a try/catch block
Will always be executed, no matter what exceptions have
been thrown and handled:
try {
throw new EOFException();
} catch (EOFException e) {
e.printStackTrace();
throw new ClassCastException();
} finally {
System.out.println(“Finally”);
}
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Will even execute when “return” is used!
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Final note
Graphics g = image.getGraphics( );
try { code that might throw exceptions }
catch (IOException e)
{ done = true; }
finally
{ g.dispose( ); }
1.
Suppose no exceptions thrown
2.
Exception thrown of type IOException
3.
Exception thrown not of type IOException
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