Note. Usually in TMAs when you are asked to supply code, a comment
is also required.
The value of the comment is of immense importance. It states what
the method does rather how it does it.
Glossary / terminology
This Glossary is quite heavy reading, but it contains many
of the main key words that are essencial for you to understand.
This understanding will take some time to aquire. You should
re-read this Smalltalk tutorial and this glossary from time
to time, as the principles of Smalltalk programming in sink
in.
The main principle of Smalltalk is very simple ...
You send a message to a receiving object, to make that object
do something.
Note: The message causes a method of the same name to be executed.
|
| Term
|
Meaning |
| Answers
|
See Message answer |
| Argument |
An argument contains additional information that
Binary and Keywordmessages
require. Unary messages do NOT have arguments.
The argument is past to the method that corresponds to the message.
A variable in the header of the method will refer to this argument.
Workspace example...
anAccount balance: 100
anAccount is the receiver.
balance: is a keyword message selector.
balance: 100 is the message.
100 is the argument.
The method balance:
balance: anAmount
"Set balance of receiver to anAmount (a number).
Answer the receiver."
balance := anAmount
See method in this Glossary for more information
on this method |
| Binary message |
The selector in a binary message is composed
of one or two non-alphanumeric characters.
Examples: Arithmetic Binary selectors
+ - * / . E.g. 2 + 3
Comparison selectors=<==<=>
>=E.g. ( aNumber < anotherNumber )
The , (comma) character is binary selector
used to concatenate(join together) objects . An example
of the concatenate message with string literal objects is
'Hello', ' World'
Which would answer with the string literal 'Hello World'.
See Keyword and Unary
|
| Class |
A class describes the behaviour of a set
of objects of the same kind |
| Comment |
Commentsare ignored by Smalltalk and are
not executed. Comments are enclosed in double quotes. A comment
can be placed anywhere in the program code but the initial comment
should describe what the method does.
Other comments can be used to explain how code works. |
| Evaluate
|
An expression can be evaluated i.e. compiled and
executed. An answer is returned. |
| Expression
|
See Message expression. |
| Getter |
See getter message and getter method. |
| Keyword message |
A keyword is a selector with one or
more arguments. The keyword always ends with a colon,
there is no space between the keyword and the colon. The colon
is followed by a space, then an argument. E.g. with a single argument
plus: 3
at:put: is an example of a keyword requiring 2 arguments.
E.g. 'ABDDE' at: 3 put: $C. The arguments are 3 and $C.
Note when referring to the selector at:put: there are no
spaces. How in use (see example) the keyword selector, at:put:,
is split up with a space and argument following each colon
'ABDDE' at: 3 put: $C.
See also Binary and Unary
messages. Tutorial
3. at:put:
Top
|
| Message |
- A message is sent to an object to make the
object do something.
- The receiving objects does this by executing one of its
methods
- The method has the same name as the message.
- E.g. If the message left is sent to
an instance of a Frog then the method left
in the protocol of Frog is executed.
- Every message in a classes protocol, has a corresponding
method of the same name
Because messages and methods have the same name,
learn as early as possible to recognise the difference. The difference
is where and how the name is used.
The message printstring
- Refers to its corresponding method. Think of the
message such as printString as selecting
" the method printString, and I want
the code in that method evaluated ."
- At the start of the course you are writting messages in
the Workspace. In the Workspace the name used
is always a message, never a method.
- In some other Smalltalk versions the Workspace
is known as a transcript window.
- Later in the course when you start writting your own methods,
you will incorporate messages within those methods. Messages
that in the workspace are preceded by objects such as frog1,
myaccount, ahoverfrog are now preceded by a psedo variable
self or super.
|
| Getter
and Setter Methods |
Objects have instance variables (Called attributes
in the early weeks of the M206 course). These instance variables
contain information about the object.
| Class |
Instance variable (attribute) |
| Frog |
colour, position |
| Account |
balance, holder, overlimit |
The 5 instance variables in the above table have getter and
setter methods to either obtain (get), or change (set) the
value in the instance variable. These special methods are often
known as assessor pair methods.
The names of getter methods are usually the same as the name
of the instance variable. A colon is added to the end of setter
method name e.g. colour:
|
| Getter Methods |
Getter message or get message, asks for information.
E.g.
frog1 colour
The getter message colour is asking frog1 "what is your
colour"
refer to the corresponding getter method.
|
| Getter
message |
A getter method or get methodis one of the 2
assessor pair methods.
Getter methods
- Provide (get) information
- return the current value of its corresponding attribute
(instance variable).
- E.g.. aHoverFrog height, would answer with
the the value of the attribute height.
The English equivalent is "This is a question to aHoverfrog,
What is your current height ?"
|
| Setter Message |
Setter message or set method, refer to the corresponding
setter method. The setter method always has a colon after
the name, this colon is followed by a space then a parameter
E.g. Frog1 position: 2
The English equivalent is "Frog1 make position 2 you current
position"
|
| Setter
method |
A setter method or set method is the
other of the 2 assessor pair methods.
- Setter methods set the value of an attribute, by
assigning a value to the attribute.
- This causes a change in an object's state.
- They usually return the receiver object.
- E.g. Evaluating aHoverFrog height: 3 message expression
in the WorkSpace, would cause the setting of the attribute
height of aHoverFrog by the method height:,
which after setting the value of the attribute height
to 3, would answer with aHoverFrog. The textual
representation of the answer would appear in the WorkSpace
answer pane.
Top
|
| Message
answer |
When a message is sent to a receiving object, an
answer is always returned. The message answer may or may not be
used. If an answer is not explicitly coded in the method using
the caret ^, then the default answer is the receiving object.
Suitable message answer object may be used subsequently as the
receiver or argument of another message.
|
| Message
expression |
- A message expression consists of a receiver,
and a message. It is often abbreviated to 'expression'.
The message may require an argument
- Messages consist of a message selector, and zero
or more arguments.
- A message expression can be typed in and evaluated in the
Workspace, or can be found in the code of a method.
|
| Message
selector |
Selector is the word given to the name of a message.
- The selector for the expression aFrog left is
left a unary message selector.
- The selector for the expression 'ABDDE' at: 3
put: $C is at:put:
- This is an example of a keyword requiring 2
arguments. E.g. 'ABDDE' at: 3 put: $C. The
arguments are 3 and $C.
- Message selectors refer to a method with the same name
, that will be evaluated, when the message is evaluated. You
can think of the message selectors selecting the method
- The receiver must be able to understand the message, that
means the method must be in the protocol of the receiver.
(Covered later).
|
| Method |
A method contains the code that is
executed when the message with the corresponding name
is sent to an object.
- .
- A method is evaluated when a receiver is sent a message
of the same name.
- Methods can be viewed, created and writen, and some
can be amended in the class
browser. (Tutorial 5)
- A method consists of:-
- The heading, containing the method name
and if required the argument(s).
- The body containing the "comment"
in double quotes
- The code
Workspace example...
See Argument in this Glossary for information
on the following line
anAccount balance: 100
The message balace: 100 has been sent to the receiving
object anAccount.
This message is saying in very plain English,"Make the balance
of anAccount equal 100".
In Smalltalk jargon this would be "Set the value of the attribute
(instance variable) balance, of the object refered to by anAccount,
to 100."
Lets look at the method balance:
balance: anAmount
"Set balance of receiver to anAmount (a number).
Answer the receiver."
balance := anAmount
- balance: is a keyword method, the :
(colon) indicates this.
- Keyword methods always have at least 1 argument,
anAmount, in the above.
- balance: is a setter method, its directly
sets the value of the attribute (instance variable)
balance.
Examination of this method...
| Section
of the method |
Comments
on that section |
balance: anAmount
|
The heading |
balance:
|
Method name |
anAmount
|
A variable to refer
to the argument |
"Set balance of receiver to anAmount (a number).
Answer the receiver."
|
The comment. |
balance := anAmount
|
The code that does
all the work. |
balance
|
An attribute (instance
variable) that is directly assigned the value of the variable
anAmount. |
:=
|
The assignment operator |
anAmount
|
The argument variable,
whos value is 100 in our example |
Top
|
| Object |
Everything in Smalltalk is an object. As long as you
remember this a sentence may make more sense if, in your
mind, you remove the majority of occurances of the word 'object'
from that sentance.
E.g.
A message object is sent to a receiver object, which will
result in an answer object being returned.
The above is easier to read as:
A message is sent to a receiver, which will result in an answer
being returned.
The Open universities course M206 in emphasising the point
that every thing in Smalltalk is an object over uses the word
to an extent that it almost confuses the student.
- Think of an object as almost anything in the world.
- Think of a car as an object, for example.
- A car has certain properties or attributes,
like its make, model, colour, engine size, etc.
- A car has methods for doing things e.g.
it can move forward, reverse, accelerate, and sound a
horn, etc.
- Smalltalk is an object-oriented language and everything
in Smalltalk is an object.
Smalltalk
- Objects are the basic building blocks of the
Smalltalk language.
- Objects have a unique identity (name).
- Objects communicate with other objects by
sending and receiving messages.
- There is a class hierarchy of these object, that
look like the roots of a tree. This hierarchy is depicted
in the Class list box of the class
browser. (Tutorial 5) by the degree of indentation
- The top-level class is called called Object.
- Every Smalltalk class is either a subclass of Object
or may be further down the roots of the tree being a subclass
of a subclass etc.
- Every object has a textual representation associated
with it.
- Textual representations of message answers may
be...
- Uninformative. an Account for Account
class,
- More informative An instance of class Frog (position
1, colour Green) for Frog class
- The textual representation can be a default one
as in the Account class, or can be coded within a special
method for the Frog class.
- During the M206 course you will do exercises that add
additional attributes (instance variables) to a
method, say the age of a frog. The code that changes the
textual representation also requires to be changed
but this is fairly advanced coding and you will not be
asked to change this. Therefore the faulty
textual representation will not display the additional
age attribute. By this time you will have learnt how to
inspect an object, and inspection will provide information
both on the original attributes and the new attribute(s)
you have created in the exercise.
The Open universities course M206 in emphasising the point
that every thing in Smalltalk is an object over uses the word
to an extent that it almost confuses the student.
Tip:
Everything in Smalltalk is an object.
As long as you remember this a sentence may make more sense
if, in your mind, you remove the majority of occurances
of the word 'object' from that sentance.
E.g.
A message object is sent to a receiver object, which will
result in an answer object being returned, emphasising that
everything in Smalltalk is an object.
The above is easier to read as:
A message is sent to a receiver, which will result in an answer
being returned, emphasising that everything in Smalltalk is
an object.
|
| Protocol
|
The set of messages an object understands.
Each object responds to a particular set of messages (its protocol).
|
| Receiver
|
The object which receives a message. Also
known as the receiver object.
anAccount balance: 100
In the above expression anAccount is the receiver of message
balance: 100 |
| Setter |
See set message and set method. |
| Unary message |
Unary messages do NOT have arguments. Examples
of Unary expressions
123 printString
'123' asNumber
See Binary and Keyword
messages |
|
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