- Question: Is the following code legal?
Answer: Yes, it's legal — and very useful Atry { } finally { }try
statement does not have to have acatch
block if it has afinally
block. If the code in thetry
statement has multiple exit points and no associatedcatch
clauses, the code in thefinally
block is executed no matter how thetry
block is exited. Thus it makes sense to provide afinally
block whenever there is code that must always be executed. This include resource recovery code, such as the code to close I/O streams.
- Question: What exception types can be caught by the following handler?
What is wrong with using this type of exception handler?catch (Exception e) { }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, your program may be forced to determine the type of exception before it can decide on the best recovery strategy.
- Question: Is there anything wrong with this exception handler as written? Will this code compile?
Answer: This first handler catches exceptions of typetry { } catch (Exception e) { } catch (ArithmeticException a) { }Exception
; therefore, it catches any exception, includingArithmeticException
. The second handler could never be reached. This code will not compile.
- Question: Match each situation in the first list with an item in the second list.
int[] A;
A[0] = 0;- The JVM starts running your program, but the JVM can't find the Java platform classes. (The Java platform classes reside in
classes.zip
orrt.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
- __compile error
- __no exception
Answer:
- 3 (compile error). The array is not initialized and will not compile.
- 1 (error).
- 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.
- 2 (checked exception).
- Exercise: Add a
readList
method to. This method should read in
ListOfNumbers.java
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
- Exercise: Modify the following
cat
method so that it will compile:public static void cat(File file) { RandomAccessFile input = null; String line = null; try { input = new RandomAccessFile(file, "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 file) { RandomAccessFile input = null; String line = null; try { input = new RandomAccessFile(file, "r"); while ((line = input.readLine()) != null) { System.out.println(line); } return; } catch(FileNotFoundException fnf) { System.err.format("File: %s not found%n", file); } catch(IOException e) { System.err.println(e.toString()); } finally { if (input != null) { try { input.close(); } catch(IOException io) { } } } }