Exception Handling

Intro
         Users have high expectations for the code we produce. Users will use our programs in unexpected ways. Due to design errors or coding errors, our programs may fail in unexpected ways during execution. It is our responsibility to produce quality code that does not fail unexpectedly. Consequently, we must design error handling into our programs.It is our responsibility to produce quality code that does not fail unexpectedly. Consequently, we must design error handling into our programs.

Errors and Error Handling:
         An Error is any unexpected result obtained from a program during execution. Unhandled errors may manifest themselves as incorrect results or behavior, or as abnormal program termination. Errors should be handled by the programmer, to prevent them from reaching the user.

Some typical causes of errors:

• Memory errors (i.e. memory incorrectly allocated, memory leaks, “null pointer”)

• File system errors (i.e. disk is full, disk has been removed)

• Network errors (i.e. network is down, URL does not exist)

• Calculation errors (i.e. divide by 0)

More typical causes of errors:

• Array errors (i.e. accessing element –1)

• Conversion errors (i.e. convert ‘q’ to a number)

• Can you think of some others?

Traditional Error Handling

          Every method returns a value (flag) indicating either success, failure, or some error condition.  The calling method checks the return flag and takes appropriate action.
Downside: programmer must remember to always check the return value and take appropriate action.  This requires much code (methods are harder to read) and something may get overlooked.
Where used: traditional programming languages (i.e. C) use this method for almost all library functions (i.e. fopen() returns a valid file or else null)
 Create a global error handling routine, and use some form of “jump” instruction to call this routine when an error occurs.
Downside: “jump” instruction (GoTo) are considered “bad programming practice” and are discouraged. Once you jump to the error routine, you cannot return to the point of origin and so must (probably) exit the program.
Where used: many older programming texts (C, FORTRAN) recommended this method to programmers. Those who use this method will frequently adapt it to new languages (C++, Java).

Exceptions – a better error handling

Exceptions are a mechanism that provides the best of both worlds.
Exceptions act similar to method return flags in that any method may raise and exception should it encounter an error.
Exceptions act like global error methods in that the exception mechanism is built into Java; exceptions are handled at many levels in a program, locally and/or globally.

What are Exceptions?

         An exception is a representation of an error condition or a situation that is not the expected result of a method.
Exceptions are built into the Java language and are available to all program code.
Exceptions isolate the code that deals with the error condition from regular program logic.

Exceptions fall into two categories:

• Checked Exceptions

• Unchecked Exceptions

Checked exceptions are inherited from the core Java class Exception. They represent exceptions that are frequently considered “non fatal” to program execution
Checked exceptions must be handled in your code, or passed to parent classes for handling. Unchecked exceptions represent error conditions that are considered “fatal” to program execution. You do not have to do anything with an unchecked exception.  Your program will terminate with an appropriate error message.

Examples:

 Checked exceptions include errors such as “array index out of bounds”, “file not found” and “number format conversion”.
 Unchecked exceptions include errors such as “null pointer”.

How do you handle exceptions?

           Exception handling is accomplished through the “try – catch” mechanism, or by a “throws” clause in the method declaration. For any code that throws a checked exception, you can decide to handle the exception yourself, or pass the exception “up the chain” (to a parent class). To handle the exception, you write a “try-catch” block.  To pass the exception “up the chain”, you declare a throws clause in your method or class declaration. If the method contains code that may cause a checked exception, you MUST handle the exception OR pass the exception to the parent class (remember, every class has Object as the ultimate parent)

Try-Catch Mechanism

         Wherever your code may trigger an exception, the normal code logic is placed inside a block of code starting with the “try” keyword. After the try block, the code to handle the exception should it arise is placed in a block of code starting with the “catch” keyword. You may also write an optional “finally” block. This block contains code that is ALWAYS executed, either after the “try” block code, or after the “catch” block code. Finally blocks can be used for operations that must happen no matter what (i.e. cleanup operations such as closing a file)

Example

                    try {
                            … normal program code
                     }
                    catch(Exception e) {
                             … exception handling code
                     }

Passing the exception

        In any method that might throw an exception, you may declare the method as “throws” that exception, and thus avoid handling the exception yourself

Example:

                    public void myMethod throws IOException {
                     … normal code with some I/O
                     }

Types of Exceptions

All checked exceptions have class “Exception” as the parent class.
You can use the actual exception class or the parent class when referring to an exception.
Where do you find the exception classes?
Reference books such as “Java in a Nutshell” (O’Reilly, 2001), or the Java Documentation.

Examples:

•  public void myMethod throws Exception {

• public void myMethod throws IOException {

• try { … }

          catch (Exception e) { … }

• try  { … }

         catch (IOException ioe) { … }

      1. Example  of an unchecked exception :NullPointerException
      2. Example of checked exceptions:
          Passing a DivideByZeroException
          Handling a DivideByZeroException

Exceptions are a powerful error handling mechanism. Exceptions in Java are built into the language. Exceptions can be handled by the programmer (try-catch), or handled by the Java environment (throws).