The previous sections described how to construct the try
, catch
, and finally
code blocks for the writeList
method in the ListOfNumbers
class. Now, let's walk through the code and investigate what can happen.
When all the components are put together, the writeList
method looks like the following.
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 + " = " + vector.elementAt(i)); } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught " + "ArrayIndexOutOfBoundsException: " + 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"); } } }
As mentioned previously, this method's try
block has three different exit possibilities; here are two of them.
try
statement fails and throws an exception. This could be an IOException
caused by the new FileWriter
statement or an ArrayIndexOutOfBoundsException
caused by a wrong index value in the for
loop.try
statement exits normally.Let's look at what happens in the writeList
method during these two exit possibilities.
The statement that creates a FileWriter
can fail for a number of reasons. For example, the constructor for the FileWriter
throws an IOException
if the program cannot create or write to the file indicated.
When FileWriter
throws an IOException
, the runtime system immediately stops executing the try
block; method calls being executed are not completed. The runtime system then starts searching at the top of the method call stack for an appropriate exception handler. In this example, when the IOException
occurs, the FileWriter
constructor is at the top of the call stack. However, the FileWriter
constructor doesn't have an appropriate exception handler, so the runtime system checks the next method the writeList
method in the method call stack. The writeList
method has two exception handlers: one for IOException
and one for ArrayIndexOutOfBoundsException
.
The runtime system checks writeList
's handlers in the order in which they appear after the try
statement. The argument to the first exception handler is ArrayIndexOutOfBoundsException
. This does not match the type of exception thrown, so the runtime system checks the next exception handler IOException
. This matches the type of exception that was thrown, so the runtime system ends its search for an appropriate exception handler. Now that the runtime has found an appropriate handler, the code in that catch
block is executed.
After the exception handler executes, the runtime system passes control to the finally
block. Code in the finally
block executes regardless of the exception caught above it. In this scenario, the FileWriter
was never opened and doesn't need to be closed. After the finally
block finishes executing, the program continues with the first statement after the finally
block.
Here's the complete output from the ListOfNumbers
program that appears when an IOException
is thrown.
Entering try statement Caught IOException: OutFile.txt PrintWriter not open
The boldface code in the following listing shows the statements that get executed during this scenario:
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 + " = " + vector.elementAt(i)); } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught " + "ArrayIndexOutOfBoundsException: " + 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"); } } }
In this scenario, all the statements within the scope of the try
block execute successfully and throw no exceptions. Execution falls off the end of the try
block, and the runtime system passes control to the finally
block. Because everything was successful, the PrintWriter
is open when control reaches the finally
block, which closes the PrintWriter
. Again, after the finally
block finishes executing, the program continues with the first statement after the finally
block.
Here is the output from the ListOfNumbers
program when no exceptions are thrown.
Entering try statement Closing PrintWriter
The boldface code in the following sample shows the statements that get executed during this scenario.
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 + " = " + vector.elementAt(i)); } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught " + "ArrayIndexOutOfBoundsException: " + 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"); } } }