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A
central role of object oriented design is to group each
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operation
with one class
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Each
operation must have exactly one class that is
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responsible
for carrying it out
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This
means you must be careful to not have one class act
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on
the data of another class.
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For
example, if we had a message class which allowed a
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message
to be left, erased, or played. What would be
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wrong
with adding this message to a mailbox of
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messages?
How could a message add itself to a mailbox?
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A
mailbox, instead could add a message! This would have
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required
a message to know the internal mailbox
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structure
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