Trail: Essential Classes
Lesson: Exceptions
Section: Catching and Handling Exceptions
Putting It All Together
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Putting It All Together

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.

  1. Code in the 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.
  2. Everything succeeds and the try statement exits normally.

Let's look at what happens in the writeList method during these two 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 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");
        }
    }
}

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 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");
        }
    }
}

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