Smalltalk and Object Orientation: an Introduction - Free

If you are confused by this description of a bag, think of it as a shopping bag. At a supermarket, you
pick objects up from the shelves and place them in your shopping bag. For example, you pick up a pint
of milk, a box of corn flakes, a packet of biscuits, three bags of potato crisps, and a few bananas (see
Figure 10.3).
Corn
Flakes
Salt n'
Vinegar
Milk
Biscuits
Figure 10.3: A shopping bag
Each of the objects in the bag is a different type of thing, with different characteristics etc. There is
no particular order to them, they will have moved about in the bag w hile you were shopping and while
you brought them home. When you reach into the bag at home to remove the objects, the order in which
they come out will not be predictable. If you think of a bag collection in these terms then you will not be
far off the mark.
As with any other class we create an instance of Bag by sending the message new to the class Bag.
For example:
| temp |
temp := Bag new.
The bag object responds to the add: message to add objects as well as the add:withOccurrences:
messages which can be used to add an object n times. For example:
temp add: 'John'.
temp add: 'Hello' withOccurrences: 2.
At this point the bag object will contain three objects, the string ‘John’ and two copies of the string
‘Hello’. We can examine the contents of the bag using the inspect message, for example:
temp inspect.
You will notice that the bag object does not record the objects in the order in which they were
added. Instead, the order is not specified (it is actually determined for efficiency). You will also notice
that there are instance variables numbered 1 - 7. This is because collections are a special type of object
whose instance variables are defined dynamically as and when required. They are termed “variable”
classes.
It is also possible to remove objects from a set using either the remove: ifAbsent: message or
the removeAllOccurrencesOf: ifAbsent: message. The first is used to remove a single
reference to an object, while the second is used to remove all reference s to an object. In both cases a
block must be supplied which is executed if the object is not a member of the bag. For example:
temp remove: 'Paul' ifAbsent: [].
temp remove: 'Paul' ifAbsent: [Transcript show: 'No Record of Paul'].
In the first example, if the string ‘Paul’ is not in the bag then the empty block will be executed, which
will result in nothing happening. In the second example, a message will be printed to the Transcript.
10.6 Set
The class Set is basically the same as the class Bag, with the exception that it does not allow
duplicates. That is, it is only possible to hold a single reference to an object in a set. For example, try the
following out in a Workspace:
90