Boo Primer: [Part 14] Exceptions

Added by Cameron Kenneth Knight

Definition: Exception - A mechanism designed to handle runtime errors or other problems (exceptions) inside a computer program.

Exceptions are very important, as they are raised whenever an error occurs in the system. (Or at least they should be.)

Catching Exceptions

An exception stops the program if it is not caught.

Division by Zero

print 1 / 0

// Output:
// System.DivideByZeroException: Attempted to divide by zero.
//    at Test.Main(String[] argv)

Which stopped the program.

To handle the situation, exceptions must be caught.

Exceptions are either caught in a try-except statement, a try-ensure statement, or a try-except-ensure statement.

Also, all Exceptions are derived from the simple Exception.

try-except example

import System

try:
    print 1 / 0
except e as DivideByZeroException:
    print "Whoops"
print "Doing more..."

// Output:
// Whoops
// Doing more...

This prevents the code from stopping and lets the program keep running even after it would have normally crashed. There can be multiple except statements, in case the code can cause multiple Exceptions. Try-ensure is handy if you are dealing with open streams that need to be closed in case of an error.

try-ensure example

import System

try:
    s = MyClass()
    s.SomethingBad()
ensure:
    print "This code will be executed, whether there is an error or not."

// Output:
// This code will be executed, whether there is an error or not.
// System.Exception: Something bad happened.
//    at Test.Main(String[] argv)

As you can see, the ensure statement didn't prevent the Exception from bubbling up and causing the program to crash.

A try-except-ensure combines the two.

import System

try:
    s = MyClass()
    s.SomethingBad()
except e as Exception:
    print "Problem! $(e.Message)"
ensure:
    print "This code will be executed, whether there is an error or not."

// Output:
// Problem: Something bad happened.
// This code will be executed, whether there is an error or not.

Recommendation: If you don't solve the problem in your except statement, use the raise command without any parameters to re-raise the original Exception.

Raising Exceptions

There are times that you want to raise Exceptions of your own.

Raising an Exception

import System

def Execute(i as int):
    if i < 10:
        raise Exception("Argument i must be greater than or equal to 10")
    print i

If Execute is called with an improper value of i, then the Exception will be raised.

Recommendation: In production environments, you'll want to create your own Exceptions, even if they are just wrappers around Exception with different names.

Recommendation: Never use ApplicationException.

Exercises

1. Think of an exercise

Go on to Part 15 - Functions as Objects and Multithreading

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