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The front-line Smalltalk interpreter gathers all text until a ’!’ character and executes it. So the actual Smalltalk code executed was:
'Hello, world' printNl
This code does two things. First, it creates an object of
type String which contains the characters “Hello, world”.
Second, it sends the message named printNl to the object.
When the object is done processing the message, the code is
done and we get our prompt back.
You’ll notice that we didn’t say anything about printing
ing the string, even though that’s in fact what happened.
This was very much on purpose: the code we typed in doesn’t
know anything about printing strings. It knew how to get a
string object, and it knew how to send a message to that
object. That’s the end of the story for the code we wrote.
But for fun, let’s take a look at what happened when the
string object received the printNl message. The string object
then went to a table 22
which lists the messages which strings can receive, and what code to
execute. It found that there is indeed an entry for
printNl in that table and ran this code. This code then walked through
its characters, printing each of them out to the terminal. 23
The central point is that an object is entirely self-contained; only the object knew how to print itself out. When we want an object to print out, we ask the object itself to do the printing.
Which table? This is determined by the type
of the object. An object has a type, known as the
class to which it belongs. Each class has a table
of methods. For the object we created, it is
known as a member of the String class. So we go
to the table associated with the String class.
Actually, the message printNl was inherited
from Object. It sent a print message, also
inherited by Object, which then sent printOn: to
the object, specifying that it print to the Transcript
object. The String class then prints its characters to the
standard output.