Smalltalk and Object Orientation: an Introduction - Free

• Correct it This allows the user to correct the fault.
5.5.2 Types of run time error
When encountering a new language for the first time, it is quite common to get extremely frustrated
while you attempt to determine why a piece of code is causing a run time error. This often turns out to
be caused by a trivial fault which would have been dealt with immediately if you had been working in a
language with which you were familiar. This sort of problem afflicts us all and is the source of a great
deal of resistance to change. However, the problems can be eased somewhat if you are awa re of the
types of root causes for different failures. The following list attempts to ease this sort of problem. It
presents the types of run time error (other than incorrect message sends) which can occur:
• Sending a message to an object which does not respond to that message.
• Trying to create an instance of Character or Boolean.
• Trying to create instances with inappropriate instance creation messages.
• Evaluating a block (instance of BlockContext) with the wrong number of arguments.
• Numeric errors, such as divide by zero, or square root of a negative number.
• Lots of collection errors, such as trying to remove an element not in a collection, or trying to use
the add: message on a collection class which does not support add: for example Array etc.
• Sending a message to an object where the corresponding method should have been defined in a
subclass. This can be identified by finding the following statement in a method:
self subclassResponsibility
• Control messages to objects which are not Booleans (ifTrue) or Blocks such as []
(whileTrue:).
5.6 Some Smalltalk
Let’s look at a little Smalltalk, just to get you going. You have already seen (and possibly written) some
Smalltalk when you typed in the Smallta lk version of the “Hello World” program in the last chapter.
Now let’s add two numbers together. First we shall do it in a non -object oriented language such as
Pascal. For example, we might write:
int a, b, c;
This says som ething like, “create three variables to hold
a := 1;
b := 2;
integer values (call them a, b and c). Store the value 1 into
c := a + b;
the variable a and 2 into variable b. Add the two numbers
together and save them into the third variable c”.
Now let’s look at how we could write the same thing in Smalltalk.
| a b c |
As you can see this looks basically the same (apart from the
a := 1.
b := 2.
use of a full stop (also known as a period) instead of a semi -
c := a + b.
colon). We also apparently forget to declare the types of the
variables a, b and c (and put some bars around them).
However, although the affect is the same, and the look similar, the meaning is
different. In Smalltalk, this actually says:
dramatically
“Define three temporary variables a, b and c (we don’t care what they will hold). Assign the
object 1 to variable a. Assign the object 2 to the variable b. Take the object in a, which has the
value 1, and send it the message “+”, which includes the argument b, which in turn has the
value 2. Object a, receives the message and performs the action requested. This action is to add
the value of the argument to itself. Create a new object and return this result as the result of
evaluating the message. Then save this object into the variable c.”
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