Smalltalk and Object Orientation: an Introduction - Free

13.4.2 Reading from BOSS
You can either load the whole contents of a BOSS file, read individual objects from a BOSS file (or if
you know the position of the object you are interested in) access a single object from any position in the
file. This last technique is much faster than reading n - 1 objects to get to the nth object, but it does
assume that you know that the object you are interested in is the nth object.
The procedure to follow is presented below.
1. First create a data stream to be used to read the BOSS file. This is typically a chieved by send a
readStream message to a file name which represents the name of the BOSS file.
2. Now create a BOSS object by sending an onOld: creation message with the newly created data
stream as the parameter. You can optimize this process if you know t hat you are not going to
write to the BOSS file. To do this you can use the onOldNoScan: message (rather than the
onOld: message).
3. Obtain the objects in the BOSS file using the appropriate access message (e.g. contents or
next).
4. Once you have finished reading from the boss file you should close it.
The next message listed in item 3 above will return a single object which can then be saved into an
appropriate collection / data structure etc. The contents message will return an array of objects
which can then be manipulated in the same way as any other array of objects. For example:
| valuesFileStream bosFileStream|
valuesFileStream := bosFileName asFilename readStream.
bosFileStream := BinaryObjectStorage onNew: valuesFileStream.
aBag := Bag new.
[[bosFileStream atEnd]whileFalse: [aBag add: bosFileStream next]]
valueNowOrOnUnwindDo: [valuesFileStream close].
This example illustrates two things, first it illustrates the equivalent of a while not end of file (EOF)
style construct for BOSS files. This cons truct is the atEnd message. This message is used to test
whether the end of the BOSS file has been reached. It is therefore possible to use it in a while loop
which says “read while not end of file”. The other thing this illustrates is the practical use of a
whileFalse: loop. Many other languages would force a construct such as not (fileName EOF) onto
the programmer. The whileFalse: construct is far more elegant.
Finally, the position: message is used to position the BOSS file pointer so that a single objec t
can be read from a specific point in the BOSS file. However, a record must have been maintained of
what the position is (note the third object stored is unlikely to have been stored at position 3). This
message could be used to create a file based versio n of a collection class. We shall look at this in more
detail in the next section and so will leave a detailed discussion of this message until then.
13.4.3 Warnings about BOSS files
You should only use BOSS files to store data objects and never for interface objects. This is because the
objects you BOSS out will lose their reference to onscreen objects. You should also avoid BOSSing out
any objects associated with the execution machinery of VisualWorks. Finally, you should take care
never to BOSS out a circular references as this can confuse the BOSS system.
13.5 Using files with the Financial Manager
This application builds on that carried out in the last chapter. The aim of that FinancialManager
application was to keep a record of deposits an d withdrawals from a current account. Associated with
this, it kept an up to date balance as well as allowing statements to be printed.
A problem with that version of the application was that there was no way of permanently storing the
account information. As soon as the FinancialManager object was destroyed (or reference to it
lost) then all the information associated with it was also lost.
The aim of this exercise is to extend the existing application by adding the following features:
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