Smalltalk and Object Orientation: an Introduction - Free

Just as with the class Date, these times can be compared, added and subtracted. For example:
(currentTime = previousTime) ifTrue:
[Transcript show: 'Time has stood still!'].
differentTime := currentTime subtractTime: previousTime.
However, the usefulness of the subtractTime: message is somewhat limited. For example,
having evaluated this expression I obta ined the result 2:57:18 PM, when what I really wanted to know
was the difference in hours. In the case of time, I can obtain this by converting the time into seconds
and dividing by 3600 (the number of seconds in an hour). For example:
| currentTime previousTime differentTime |
currentTime := Time now.
previousTime := Time readFromString: '1:00:20 am'.
differentTime := currentTime asSeconds - previousTime asSeconds.
differentTime // 3600.
There are two class side messages which may be of general use. The first is
millisecondClockValue. This message returns a value indicating the number of milliseconds
since the system clock was reset. It can be useful as a means for generating an unique name for a
temporary file etc. The uniqueness of such a filename can not be guaranteed as there is a vague chance
that a file with the same name may have been left around from an execution of the system prior to the
resetting of the system clock (although the chances of a temporary file being created with a name
derived from the same millisecond clock reading are remote). For example:
Time millisecondClockValue.
The second class side message is the millisecondsToRun: message. This takes a block as a
parameter and generates a report on the time required to evaluate that blo ck. This message is most often
used when determining the performance of various constructs. For example, in the last chapter it was
used to compare the do: loop with the inject: construct on two collection classes Bag and Array.
For example:
Time millisecondsToRun:
[1 to: 10000 do: [:i | Transcript show: ' ' , i ] ].
On a Pentium 120 PC with 16 megabytes of memory this resulted in a value of
returned.
79056 seconds being
14.5 The class Character
Characters such as letters (e.g. A, b or Z) are objects in the ir own right in Smalltalk. When they are
written alone they are preceded by a dollar sign. For example:
$A, $b, $Z
These objects can be combined together to form strings (which are actually collections of characters).
As the Character class inherits from Magnitude, you can compare characters to determine if
one is greater, less than or equal to another. For example:
Transcript show: ($A < $a) printString.
In addition, Character adds another comparison method called sameAs:. This metho d ignores
case. Thus it is possible to evaluate the following and have true printed in the Transcript:
Transcript show: ($A sameAs: $a) printString.
The Character class also understands the differences between alphabetic characters, numeric
characters, vowels, uppercase and lowercase letters etc. These are provided by methods in the testing
protocol, which include:
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