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Trail: Essential Java Classes
Lesson: Handling Errors with Exceptions

## Answers to Questions and Exercises: Exceptions

### Questions

> 1. Question: Is the following code legal?

>    >     try {
>    >         ...
>    >     } finally {
>    >         ...
>    >     }

>    Answer: Yes, it's legal. A `try` statement does not have to have a `catch` statement if it has a `finally` statement. If the code in the `try` statement has multiple exit points and no associated `catch` clauses, the code in the `finally` statement is executed no matter how the `try` block is exited.

> 2. Question: What exception types can be caught by the following handler?

>    >     catch (Exception e) {
>    >         ...
>    >     }

>    What is wrong with using this type of exception handler?

>    Answer: This handler catches exceptions of type `Exception`; therefore, it catches any exception. This can be a poor implementation because you are losing valuable information about the type of exception being thrown and making your code less efficient. As a result, the runtime system is forced to determine the type of exception before it can decide on the best recovery strategy.

> 3. Question: What exceptions can be caught by the following handler?

>    >     ...
>    >     } catch (Exception e) {
>    >         ...
>    >     } catch (ArithmeticException a) {
>    >         ...
>    >     }

>    Is there anything wrong with this exception handler as written? Will this code compile?

>    Answer: This first handler catches exceptions of type `Exception`; therefore, it catches any exception, including `ArithmeticException`. The second handler could never be reached. This code will not compile.

> 4. Question: Match each situation in the first column with an item in the second column.

>    `int[] A; A[0] = 0;` The Java VM starts running your program, but the VM can’t find the Java platform classes. (The Java platform classes reside in `classes.zip` or `rt.jar`.) A program is reading a stream and reaches the end of stream marker. Before closing the stream and after reaching the end of stream marker, a program tries to read the stream again. | error checked exception runtime exception no exception

>    Answer:

>    1. 3 (runtime exception).
>    2. 1 (error).
>    3. 4 (no exception). When you read a stream, you expect there to be an end of stream marker. You should use exceptions to catch unexpected behavior in your program.
>    4. 2 (checked exception).

### Exercises

> 1. Exercise: Add a `readList` method to ``ListOfNumbers.java``. This method should read in `int` values from a file, print each value, and append them to the end of the vector. You should catch all appropriate errors. You will also need a text file containing numbers to read in.

>    Answer: See ```ListOfNumbers2.java``.`

> 2. Exercise: Modify the following `cat` method so that it will compile:

>    >     public static void cat(File named) {
>    >         RandomAccessFile input = null;
>    >         String line = null;

>    >         try {
>    >             input = new RandomAccessFile(named, "r");
>    >             while ((line = input.readLine()) != null) {
>    >                 System.out.println(line);
>    >             }
>    >             return;
>    >         } finally {
>    >             if (input != null) {
>    >                 input.close();
>    >             }
>    >         }
>    >     }

>    Answer: The code to catch exceptions is shown in red:

>    >     public static void cat(File named) {
>    >         RandomAccessFile input = null;
>    >         String line = null;

>    >         try {
>    >             input = new RandomAccessFile(named, "r");
>    >             while ((line = input.readLine()) != null) {
>    >                 System.out.println(line);
>    >             }
>    >             return;
>    >         } catch(FileNotFoundException fnf) {
>    >             System.err.println("File: " + named + " not found.");
>    >         } catch(Exception e) {
>    >             System.err.println(e.toString());
>    >         } finally {
>    >             if (input != null) {
>    >                 try {
>    >                     input.close();
>    >                 } catch(IOException io) {
>    >                 }
>    >             }
>    >         }
>    >     }

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