Smalltalk and Object Orientation: an Introduction - Free

7.5.3 true, false and nil variables
These variables represent the two boolean states, true and false and a null value (referred to as nil). This
should not be confused with the null pointer in languages such as C. It really means nothing or no value.
The three variables are:
• nil The single instance of class Undefined Object (the non-value).
• true Represents truth. It is the only instance of class True.
• false represents falsehood. Which is the only instance of the class False.
True and False are subclasses of
structures.
Boolean which implements boolean algebra and control
7.6 Messages and message selectors
7.6.1 Message expressions
Message express ions describe messages to receivers. They are composed of a receiving object (the
receiver), the message selector (which indicates which method to execute) and zero or more parameters.
Figure 7.2 illustrates the main components of a message expression.
A message expression
Parameters
anAddressBook addAddress: 'C47, UWA' for: 'John Hunt'.
Receiver
Message selector
Figure 7.2: The components of a message expression
The value of the expression is determined by the method it invokes. For example, in the f ollowing
example, the result returned by the method marries: is saved into the variable newStatus.
7.6.2 Message types
newStatus := thisPerson marries: thatPerson.
There are three different forms which a message can take. From the programmers point of view they are
essentially the same. They are only distinguished by the format they take and by the precedence they
have. The following description explains each of the three types of message and states their precedence.
The only exception to this is that parenthesized expressions take precedence over all message
expressions.
Messages without any arguments are termed Unary Messages, for example:
#($a $b 23 'john') size.
13 odd.
Time now.
Messages with a single argument, where the selector is composed of one or two non -alphanumeric
symbols are termed Binary Messages. They have a higher precedence than keyword message s, but a
lower precedence than unary messages. Examples of binary messages include:
a >= b. 24 * 7. recordCount + 1.
The second character in a binary message selector cannot be a minus sign.
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