Navigation: Appendix B: Dolphin Pattern Book > Method Patterns >
Constant Access Method
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Navigation: Appendix B: Dolphin Pattern Book > Method Patterns > Constant Access Method |
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Context
In Smalltalk it is often considered bad practice to hard-code constant values in methods. A hard-coded constant is hard to find, may end up being duplicated, and is difficult to override in subclasses.
Solution
Implement a method whose sole purpose is to answer the constant. Place this method in the constants category. This technique has several advantages over hard-coding the constant in a method where it is used:
| • | the constant is easier to find |
| • | the comment in the constant access method can be used to describes the constant value |
| • | the constant will exist in just one place |
| • | the constant will be easier to override when new subclasses are added |
Example
Consider a class Modem. It uses a constant to define the default connection timeout period.
Modem>>defaultTimeoutSeconds
"Answers the default modem timeout period in seconds."
^30
We might later add a ISDN modem subclass with a much shorter timeout:
ISDNModem>>defaultTimeoutSeconds
"Answers the default timeout period in seconds for ISDN modems."
^5
Known Uses
Class View contains a constant access method which answers the default size for new instances.
View>>defaultExtent
"Private - Answer the default size of the receiver. This is used only to
set the size of a view when it is created. Thereafter preferredExtent is
used when the desired size of a view is requested."
^##(100 @ 100)
The sample class VideoTape contains a class method #defaultTapeLength for specifying the most common duration of video tapes.
defaultTapeLength
"Private - Answer a default length (in minutes) to use for instances of the receiver.
Illustrated Patterns:
Constant Access Method
Private Method
"
^240
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