Download Input Using Buffered Reader part 2-Data Transfer Programming-Lecture Slides and more Slides Computer Engineering and Programming in PDF only on Docsity!
Propagating Errors Up the Call Stack ^ Exceptions enable you to propagate error reporting up the call stack ofmethods
method
{ errorCodeType
error;
error = call method2;if^ (error)
doErrorProcessing;
else
proceed; } errorCodeType
method
{
errorCodeType
error;
error = call method3;if^ (error)return
error; else
proceed; } errorCodeType
method
{
errorCodeType
error;
error
=^ call
readFile;
if^ (error)
return
error;
else
proceed; }
Call Stack / Call Stack for
Exception
Grouping Error Types and Error
Differentiation
^ Exceptions fall intocategories orgroups.
Throwing Exceptions:
throw
clause
^ Before you can catch an exception, someJava code somewhere must throw one ^ Regardless of who (or what) throws theexception, it's always thrown with theJava
throw
statement
throw
someThrowableObject
;
^ In Java, throwable objects are instances ofany subclass of the
Throwable
public Object pop()
throws EmptyStackException
{
Object obj;if (size == 0)
throw new EmptyStackException();
obj = objectAt(size - 1);setObjectAt(size - 1, null); size--;return obj;}
Exception Specification
^ Catch
- A method can catch an exception by providingan exception handler for that type of exception
^ Specify
- If a method chooses not to catch an exception,the method must specify that it can throw thatexception
public
class Coin { public void read(BufferedReader in)throws IOException {value = Double.parseDouble(in.readLine());name
=in.readLine(); } }
- Better to declare exception than to handle itincompetently
Exception Specification
^ Checked Exceptions
- Java exceptions are of different typesincluding I/O Exceptions, runtime exceptions,and exceptions of your own creation•^ Runtime exceptions
occur within the JRE, this
includes: arithmetic exceptions, pointerexceptions, and
indexing exceptions
- The compiler does not require that you catchor specify runtime exceptions•^
Checked exceptions
are not runtime
exceptions and are checked by the compiler• Better to declare exception than to handle itincompetently
Designing Your Own Exceptions
if (amount > balance)
throw new InsufficientFundsException(); ^ Make it an unchecked exception--programmercould have avoided it by calling
getBalance()
first Extend RuntimeException and supply twoconstructors public class InsufficientFundsException extends RuntimeException {public InsufficientFundsException(){} public InsufficientFundsException(
String reason){super(reason); } }
Catching Exceptions
try
{ BufferedReader in = new BufferedReader(
new InputStreamReader(System.in)); System.out.print("How old are you?");String inputLine = in.readLine();int age = Integer.parseInt(inputLine);age++;^ System.out.println(
"Next year,you'll be " + age);
}^
catch
(IOException exception) { System.out.println(
"Input/output error " + exception);
}^ catch
(NumberFormatException exception){ System.out.println(
"Input was not a number");
}^ }}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
The
finally
Clause
^ Executed when try block comes to normalendExecuted if a statement in try blockthrows an exception, before exception isthrown out of try block ^ Can also be combined with catch clauses
BufferedReader in = null; try
{ in = new BufferedReader(
new FileReader(filename)); purse.read(in); }^ finally
if (in !=null) in.close(); }
Example
^ Program
- reads coin descriptions from file• adds coins to purse• prints total
^ What can go wrong?
- File might not exist• File might have data in wrong format
^ Who can detect the faults?
PurseTest
interacts with user
- main method can report errors• Other methods pass exceptions to caller
The
read
method of the
Purse
class
^ Unconcerned with exceptions ^ Just passes them to caller
public void read(BufferedReader in)
throws IOException {boolean done = false;while (! done) {
Coin c = new Coin();if (c.read(in)) {
add(c); } else {
done =true; } } }
The
readFile
method of the
Purse
class
^ finally clause closes files if exceptionhappens
public void readFile(String filename)
throws IOException {BufferedReader in = null;try {
in = new BufferedReader(
new FileReader(filename)); read(in); } finally {
if (in != null)
in.close();
} }
Scenario
^ PurseTest.main
calls
Purse.readFile
^ Purse.readFile
calls
Purse.read
^ Purse.read
calls
Coin.read
^ Coin.read
throws an
EOFException
^ Coin.read
has no handler for the exception and
terminates immediately. Purse.read
has no handler for the exception and
terminates immediately Purse.read
File has no handler for the exception
and terminates immediately after executing the finally
clause and closing the file.
^ PurseTest.main
has a handler for an
IOException
, a superclass of
EOFException
That handler prints a message to the user.
