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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;

    int list[] = {1, 2, 3, 4};

    try {
      System.out.println("Entering" + " try statement");

      out = new PrintWriter(new FileWriter("OutFile.txt"));
      //Will case an IndexOutOfBoundsException exception
      for (int i = 0; i <= list.length; i++) {
        out.println("Value at: " + i + " = " + list[i]);
      }
    } catch (IndexOutOfBoundsException e) {
      System.err.println("Caught IndexOutOfBoundsException: "
          + 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 IndexOutOfBoundsException 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 code below will cause an IndexOutOfBoundException since i==4 is not leagal.

But since we define final block so this program will execute final block first then catch the error.

Entering try statement
Closing PrintWriter
Caught IndexOutOfBoundsException: 4 

The boldface code in the following listing shows the statements that get executed during this scenario:

public void writeList() {
    PrintWriter out = null;

    int list[] = {1, 2, 3, 4};

    try {
      System.out.println("Entering" + " try statement");

      out = new PrintWriter(new FileWriter("OutFile.txt"));
      //Will case an IndexOutOfBoundsException exception
      for (int i = 0; i <= list.length; i++) {
        out.println("Value at: " + i + " = " + list[i]);
      }
    } catch (IndexOutOfBoundsException e) {
      System.err.println("Caught IndexOutOfBoundsException: "
          + 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;

    int list[] = {1, 2, 3, 4};

    try {
      System.out.println("Entering" + " try statement");

      out = new PrintWriter(new FileWriter("OutFile.txt"));
      for (int i = 0; i < list.length; i++) {
        out.println("Value at: " + i + " = " + list[i]);
      }
    } catch (IndexOutOfBoundsException e) {
      System.err.println("Caught IndexOutOfBoundsException: "
          + 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");
      }
    }
  }

Source code

Github