Why do we need exceptions?
In C, return variables must be used to indicate errors:
if((fd = fopen(path,...)) == -1){
if(errno==a){ ...}
else if(errno==b){...}
// other error code
}
// do something
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Why do we need exceptions?
• What are the problems with this?
– This complicates the code by adding extra
clutter to check the value of the return
arguments.
– If you have a function f() that calls g() that calls
h(), and h() needs to return an error, both g()
and f() have to contain special code to handle
the exception
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Why do we need exceptions?
How does the code change when we add exceptions?
try{
fd = new FileReader(path,...);
}
catch(IOException e){
// error code
}
catch(FileNotFoundException e){
// error code
}
// do something
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When should exceptions be used?
• Exceptions provide a way to signal errors without
having to use return variables
• They should generally be used when an
unexpected error occurs
• For example, Java uses an exception to signal
when a file you are trying to open could not be
found.
• However, when searching for an item in an array,
Java returns a -1 if it cannot find the item.
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Checked Exceptions
• You can either use one of the existing exceptions,
or create your own
• Most exceptions extend Exception, but a
function can throw any class that extends
Throwable
• Such exceptions are known as checked exceptions
– Functions that throw a checked exception must
advertise this by using the throws keyword
– Every function that calls a function that can throw an
exception must either use a try…catch block or
advertise that it throws the Exception with a throws
keyword
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Unchecked Exceptions
• Unchecked exceptions do not need to be caught by
your program.
• There are two types:
– Exceptions that extend Error are serious virtual
machine errors, like OutOfMemoryError
– Exceptions that extend RuntimeException can be
caught, but do not need to be. These are problems like
NullPointerException and
IndexOutOfBoundsException
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Using Exceptions
In order to signal an error has occurred, your code must throw an
exception:
throw new MyException(“Error Message”)
Also, all functions that throw exceptions must specifically state that
they throw exceptions. A function that throws MyException and
MyOtherException would be defined as follows:
int myFunction() throws MyException, MyOtherException
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Using Exceptions
• A function that calls a function that can
throw an exception must deal with it in one
of two ways:
– Advertise that it throws the exception
– Use a try…catch block
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Using Exceptions
try{
fd = new FileReader(path,...);
}
catch(IOException e){
// error code
}
catch(Exception e){
// error code
}
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Exceptions and Inheritance
• When creating a subclass, do not change the
exceptions thrown by a method you
override, unless you are changing them to a
subclass of the previous exception.
• However, you can remove one or more of
the exceptions from the throws keyword.
– If a method throws MyException and
MyOtherException, a method that
overrides it can throw only MyException
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Finally…
• When working with resources such as files
or network connections, special handling is
required for exceptions.
• Resources often need to be properly closed
to ensure that normal behavior continues,
regardless of whether an exception will be
thrown or not.
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Why Finally?
import java.io.*;
class ProcessFile{
public static void main(String[] args){
if(args.length > 0){
FileReader f;
try{
// Open a file:
f = new FileReader(args[0]);
SomeOtherClass.process(f);
f.close();
}
catch (IOException x){
System.out.println(x);
if (f != null)
f.close();
}}}}
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Why Finally?
import java.io.*;
class ProcessFile{
public static void main(String[] args){
if(args.length > 0){
FileReader f;
try{
// Open a file:
f = new FileReader(args[0]);
SomeOtherClass.process(f);
}
catch(IOException x){
System.out.println(x);
}
finally{
if(f != null) f.close();
}}}
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SaM and Exceptions
• Why do you need to know this?
• In Part 2, your compiler will not have to
throw exceptions.
• However, you will be required to use
SaMTokenizer
• SamTokenizer can throw an exception
when you request a token
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SamTokenizer and Exceptions
public int getInt() throws TokenizerException {
if(next() == StreamTokenizer.TT_NUMBER)
return (int)tokens.nval;
else
throw new TokenizerException("Attempt to
read non-numerical value as an integer",
lineNo());
}
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Exceptions in your Project
• All methods that use SamTokenizer, and any
methods that call those methods, must advertise
that they throw TokenizerException.
• This will allow you to avoid handling the
exceptions.
• Under normal conditions, you should never get
have a TokenizerException thrown, since
you can check the type of the next token using
peekAtKind()
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