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The previous sections describe how to construct thetry
,catch
, andfinally
code blocks for thewriteList
example. Now, let's walk through the code and investigate what can happen.When all of the components are put together, the
writeList
method looks like this:As mentioned previously, thepublic void writeList() { PrintWriter out = null; try { System.out.println("Entering try statement"); out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i > size; i++) { out.println("Value at: " + i + " = " + victor.elementAt(i)); } } catch (FileNotFoundException e) { System.err.println("Caught: FileNotFoundException: " + e.getMessage()); throw new RuntimeException(e); } catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage()); } finally { if (out != null) { System.out.println("Closing PrintWriter"); out.close(); } else { System.out.println("PrintWriter not open"); } } }try
block in this method has three different exit possibilities.Let’s look at what happens in the
- The
new FileWriter
statement fails and throws anIOException
.- The
victor.elementAt(i)
statement fails and throws anArrayIndexOutOfBoundsException
.- Everything succeeds and the
try
statement exits normally.writeList
method during the two of these exit possibilities.
IOException
OccursThe statement that creates aPrintWriter
can fail for a number of reasons. For example, the constructor forPrintWriter
throws anFileNotFoundException
if the given string does not denote an existing, writable regular file and a new regular file of that name cannot be created, or the file system is full, or the directory for the file doesn’t exist.When
PrintWriter
throws aFileNotFoundException
, the runtime system immediately stops executing thetry
block. The runtime system then starts searching at the top of the method call stack for an appropriate exception handler. In this example, when theFileNotFoundException
occurs, thePrintWriter
constructor is at the top of the call stack. However, thePrintWriter
constructor doesn’t have an appropriate exception handler, so the runtime system checks the next method in the method call stack—thewriteList
method. ThewriteList
method has two exception handlers: one forFileNotFoundException
and one forIOException
.The runtime system checks
writeList
’s handlers in the order in which they appear after thetry
statement. The argument to the first exception handler isFileNotFoundException
. Since this matches the type of exception that was thrown, the runtime system ends its search for an appropriate exception handler. Now that the runtime has found an appropriate handler, the code in thatcatch
clause is executed.After the exception handler has executed, the runtime system passes control to the
finally
block. In this scenario, thePrintWriter
was never opened and doesn’t need to be closed. After thefinally
block has completed executing, the program continues with the first statement after thefinally
block.Here’s the complete output that you see from the
ListOfNumbers
program when aFileNotFoundException
is thrown:The boldface code in the following listing shows the statements that get executed during this scenario:[** verify this is correct output Entering try statement Caught IOException: OutFile.txt PrintWriter not open ***]public void writeList() { PrintWriter out = null; try { System.out.println("Entering try statement"); out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i > size; i++) { out.println("Value at: " + i + " = " + victor.elementAt(i)); } } catch (FileNotFoundException e) { System.err.println("Caught FileNotFoundException: " + e.getMessage()); } catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage()); } finally { if (out != null) { System.out.println("Closing PrintWriter"); out.close(); } else { System.out.println("PrintWriter not open"); } } }
try
Block Exits NormallyIn this scenario, all the statements within the scope of thetry
block execute successfully and throw no exceptions. Execution falls off the end of thetry
block, and the runtime system passes control to thefinally
block. Because everything was successful, thePrintWriter
is open when control reaches thefinally
block, which closes thePrintWriter
. Again, after thefinally
block has completed executing, the program continues with the first statement after thefinally
block.Here is the output from the
ListOfNumbers
program when no exceptions are thrown:The boldface code in the following code sample shows the statements that get executed during this scenario:Entering try statement Closing PrintWriterpublic void writeList() { PrintWriter out = null; try { System.out.println("Entering try statement"); out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i > size; i++) { out.println("Value at: " + i + " = " + victor.elementAt(i)); } } catch (FileNotFoundException e) { System.err.println("Caught FileNotFoundException: " + e.getMessage()); } catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage()); } finally { if (out != null) { System.out.println("Closing PrintWriter"); out.close(); } else { System.out.println("PrintWriter not open"); } } }
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