Java Exceptions

1. Java Exceptions Dan Fleck CS211

2. What is an Exception • An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions. • Intuition: It’s something you didn’t expect

3. What happens? • If an exception occurs an “Exception” object is created, and then passed up the call stack until someone handles it. main Exception Method 1: I’ll handle it Exception Method 2:I can’t Handle it Exception Catches I had a problem Forwards Call Stack Throws

4. Catch or Specify • Code that can generate certain exceptions must either catch or specify it. • Catch: • try { … multiple statements …} catch (SpecificException specificExc) { … handle exception …} • Specify: • public void myMethod() throws SpecificException { … }

5. Exception Types • Checked exceptions • Must satisfy the catch or specify requirement • Are subclasses of java.lang.Exception • Checked exceptions should be handled and recovered from • Examples: user types in a bad filename, etc…

6. Exception Types (cont.) • Unchecked exceptions • Not subject to the catch or specify requirement • Errors - unforeseen error conditions due to external issues that an application probably cannot recover from. • Example: IOError when reading a file (due to bad sectors on the disk) • RuntimeExceptions - error conditions internal to the program that an application probably cannot recover from. • Generally these are programming errors or logic errors • Example: IllegalArgumentException - someone passed in an invalid argument to your program

7. Why ever use checked? • You should almost always use checked exceptions. These exceptions allow a well-written program to catch and handle the condition. Only use unchecked when a reasonable program cannot deal with the problem.

8. Catching exceptions • try { …. Code …} catch (Exception1 exc) { } catch (Exception2 exc) {} • Exceptions thrown in “code” are caught by the catch blocks. • Catch blocks catch exceptions in order, so put super-classes at the bottom (catch specific exceptions first) • Example:} catch (IOException ioe) { .. Do something specific to an IOException ..} catch (Exception e) { .. Catch all other general exceptions and do something for them}

9. Finally -- the finally block • try { …. } catch (Exception e) { …. } finally { … Code …} • The finally block always executes when the try block completes. This is where you perform any cleanup needed after the try block completes successfully or throws an exception • Having a finally block is optional -- you don’t need to create one if you have no cleanup to do, but if you have one it WILL execute everytime

10. Altogether now.. public void writeFile(String fileName) { PrintWriter out = null; try { out = new PrintWriter( new FileWriter(fileName)); for (int ind=0; ind < MAX_SIZE; ind++) { out.println(“Value at “+ind+” is “+vect.elementAt(ind));} } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught ArrayIndexOutOfBoundsException: “ + e.getMessage()); } catch (IOException ioe) {System.out.println(“Caught IOException:”+ioe.getMessage()); } finally {if (out != null) { out.close();} }

11. Another way: Pass the buck public void writeFile(String fileName) throws ArrayIndexOutOfBoundsException , IOException { PrintWriter out = null; try { out = new PrintWriter( new FileWriter(fileName)); for (int ind=0; ind < MAX_SIZE; ind++) { out.println(“Value at “+ind+” is “+vect.elementAt(ind));} } finally {if (out != null) { out.close();} } NOTE: You can still use the finally block exactly the same! Now code calling this method must handle the IOException. The code may optionally handle ArrayIndexOutOfBoundsException since it is a RuntimeException (unchecked). Which means this method signature is also valid: public void writeFile(String fileName) throws IOException

12. How to throw exceptions • Your code may throw exceptions when needed using the throws keyword. This keyword takes a subclass of Throwable as an argument (Exceptions/Errors are all subclasses of Throwable) • public void addElement(int index, int[] array, int value) {if (index >= array.length) { throw new IllegalArgumentException( “Array length is:”+array.length+ ” and you attempted to add element “+index);}array[index] = value;} • Same rules apply -- if you throw a checked exception it must be caught or specified.

13. Writing your own exceptions • You may also create your own exceptions by subclassing Exception, RuntimeException or Error. • In general you never want to subclass Error and rarely subclass RuntimeException. You usually create checked exceptions by subclassing Exception. • public MyException extends Exception { public MyException(String message) { super(message); }} • Frequently this all you want to do. Occasionally you may add more features or information to your Exceptions.

14. Handling Exceptions • Most ways to handle exceptions depend on the exception type. However, some general debugging tips are: • Use the printStackTrace() method of exception to see where the exception occurred • Use getMessage() to show a message to the user • The best exception is one never thrown -- if you can prevent a user error that would cause an exception before it happens, do that! (For example, rather than letting the user type letters into a numeric field, watch the field and prevent letters from ever going into the field.)