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Trail: Essential Java Classes
Lesson: Handling Errors with Exceptions

Putting It All Together

The previous sections describe how to construct the try, catch, and finally code blocks for the writeList method in the ListOfNumbers (in a .java source file) class. 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:

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 (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, the try block in this method has three different exit possibilities.
  1. Code in the try statement fails and throws an exception. This could be an IOException caused by the new FileWriter statement, an ArrayIndexOutOfBoundsException caused by a wrong index value in the for loop, or a RuntimeException caused by an error in the Java runtime system.
  2. Everything succeeds and the try statement exits normally.
Letís look at what happens in the writeList method during the two of these exit possibilities.

Scenario 1: An Exception Occurs

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 can not create or write to the file indicated.

When FileWriter throws a IOException, the runtime system immediately stops executing the try block. The 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 doesnít have an appropriate exception handler, so the runtime system checks the next method in the method call stack — the writeList method. The writeList method has two exception handlers: one for IOException and one for ArrayIndexOutOfBoundsException.

The runtime system checks writeListís 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 has executed, 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 doesnít need to be closed. After the finally block has completed executing, the program continues with the first statement after the finally block.

Hereís the complete output that you see from the ListOfNumbers (in a .java source file) program 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 + 
		               " = " + victor.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");
            }
        }
    }

Scenario 2: The try Block Exits Normally

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 has completed executing, the program continues with the first statement after the finally block.

Here is the output from the ListOfNumbers (in a .java source file) program when no exceptions are thrown:

Entering try statement
Closing PrintWriter
The boldface code in the following code 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 + " = " 
		  +  victor.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");
            }
        }
    }

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