Smalltalk and Object Orientation: an Introduction - Free

external device (including a file) then it essentially acts as an interface to that external device. It thus
allows messages to be sent to and received from an external device object enabling it to accomplish
various activities including input and output. If a stream is connected to a collection (such as a string) it
can act as a means for processing the contents of a collection. In both cases the stream views the source
(or sink) of the data as being able to provide (or receive) a data item on request.
13.3 ASCII files
13.3.1 Working with ASCII files
There are a wide range of file handling facilities available within Smalltalk. Most are provided by one
of the subclasses of the class Filename and Stream . The Filename class is an abstract superclass in
which the basic framework for files is defined. Most of the work of this class is actually performed by
one of its (platform specific) subclasses. It is this which is one of the facilities which allows you to
ensure that your code can run on any platform, even if you are accessing the host platform’s file system!
In the remainder of this section we shall look at some of the most common operations.
13.3.2 Creating, reading and writing a file
To create a file you must first have a filename to work with. A filename is created by sending the
message asFilename to a string. This converts the string into an appropriate form for the host operating
system. You now have a filename, however you need to link this file name with an actual file. To do
this you use an instance of one of the streams classes (see earlier in chapter). This can be done by
sending the message writeStream, readStream or readWriteStream to a filename (it is rarely a good idea
to use the readWriteStream option. It is invariably better to use either the readStream or writeStream
messages). In effect this links the resulting instance directly to the actual file. In the case of creating a
new file, it also creates the file for you when the first character is written to it. You now have something
to which you can send ASCII text.
You may wish to check to see if a file exists before attempting to open it (e.g. to be sure there is
something to read from or to make sure you don’t overwrite an existing file). You can do this with the
exists message. This message returns true if the file is already present and false if it does not exist.
To close the file once it has been opened there is a close message. This message closes the
associated file as well as the data stream to that file.
Try typing the following into the Workspace and evaluating it.
| filename stream |
filename := 'temp.txt' asFilename.
(fileName exists)
ifTrue:
[Dialog warn: 'file ' , fileName asString, ' already exists'.]
ifFalse: [
stream := fileName writeStream.
stream nextPutAll: 'Hello World'.
stream close.].
Transcript show: fileName fileSize printString.
This example creates a file called “temp.txt” in the current working directory, assuming that one
does not already exist. It then puts the string “Hello World” in that file and closes the file. It then prints
the size of the file in the system Transcript. For example, using VisualWorks 2.5 on a Windows -95 PC,
this results in the value 11 being printed in the Transcript. If the code is evaluated a second time, it
results in a dial og box (illustrated in Figure 13.2) being displayed which warns that the temp.txt file
already exists.
Figure 13.2: A file exists dialog
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