OBJECT DESIGN CONSTRUCTS in Java

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that runs unmodified against two or more different classes Just as important, you can replace the implementation of an entire class without rewriting any calling code The ability to plug-and-play class implementations without touching the calling code is extremely important for code maintenance In addition to the obvious advantage of having to rewrite only one side of the code, you also get a built-in test case for the new code implementation The best convention to provide for the reuse of code that operates against a particular interface is to operate against only a single interface from each object in a particular procedure By breaking code into autonomous units, each of which works with a specific interface, you can greatly increase code reuse For example, suppose you have defined an IReport interface and an lOutput interface You have several classes that implement IReport and several that implement lOutput You could write nonreusable code as follows 'clsOrders implements IReport 'clsRecordToFile implements IOutput Sub OutputOrders(Orders As clsOrders, strOutFile As String) Dim FileRecord As clsRecordToFile Dim IReport As IReport Dim IOutput As IOutput 'Initialize output class Set FileRecord = New clsRecordToFile FileRecordFile = strOutFile 'Bring orders up to date OrdersFreeze OrdersReconcile 'Output orders Set IReport = Orders Set IOutput = FileRecord 'Output processing goes here ' Clean up FileRecordClose OrdersUnfreeze End Sub
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Any code in this procedure that runs against the IReport and lOutput interfaces is not directly reusable The only code reuse comes from cut and paste, and that leads to a maintenance nightmare as you paste bugs along with code By breaking this procedure in two, suddenly the code becomes reusable The OutputReport procedure knows about only the IReport and lOutput interfaces, which are retrieved on the parameter pass instead of with a Set statement Note that the input parameters are ByVal to eliminate two additional QI calls when the procedure returns (see "Parameters and IUnknown" in 3) Sub OutputOrders(Orders As clsOrders, strOutFile As String) Dim FileRecord As clsRecordToFile 'Initialize output class Set FileRecord = New clsRecordToFile FileRecordFile = strOutFile 'Bring orders up to date OrdersFreeze OrdersReconcile 'Output orders OutputReport Orders, FileRecord ' Clean up FileRecordClose OrdersUnfreeze End Sub Sub OutputReport(ByVal IRep As IReport, _ ByVal IOut As IOutput) 'Output processing here End Sub
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The Implements keyword is very useful for creating reusable calling code It is also a wonderful design tool because it provides a stable framework for building a complex application in which modular code communicates via abstract interface definitions The major shortcoming of Implements is that it provides absolutely no inherent code implementation, so although the calling code is reusable, the class module code is not Of course, this is also one of the major features of COM
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interfaces you are free to use any implementation you like as long as all the procedures in the interface behave as the interface consumer expects There is only one way to reuse an implementation of a given interface: delegation With delegation, a class contains a reference to another implementation of the interface You then simply forward all the calls to the corresponding functions in the referenced implementation The most obvious, most common, and most work-intensive way to delegate is to use the Implements keyword in a class, put in all the stub functions, and add one line of code to each function The single line of code calls the corresponding function in the interface implementation Private Function IFoo_DoBar _ ByVal BarData As String) As String IFoo_DoBar = m_IFooDoBar(BarData) End Function There are several obvious drawbacks to this technique, and some that aren't so obvious Writing this code in the first place is a hassle: VB doesn't provide any sort of wizard to do it for you Regenerating the code when the interface changes is even worse You also take a significant performance hit from all the parameter passing Although there are no QueryInterface calls because the object types all match exactly, AddRef/Release is present around ByVal object-type calls There is also an extra String copy for ByVal As String and ByVal As Variant parameters Other ByVal parameters need to be evaluated before the inner function call If you're not moving huge amounts of data in Strings and Variants, the extra overhead of passing parameters during delegation isn't large, but it is definitely there The initial coding and maintenance costs are also quite real Unfortunately, this brute force delegation model is the only alternative available in straight VB Although the aggregation techniques discussed later eliminate the code-maintenance costs and minimize the delegation costs, I'll first look at ways to avoid delegation by using some design tweaks in straight VB
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Indirect Interface Implementation The biggest problem with the Implements keyword is that you are forced to provide an implementation for every member function This is little more than a point-and-click hassle when you first implement the interface; annoying, but
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