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4: Structural Patterns: Adapter and Fa ade
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completely independent of the other classes and is oblivious to any naming conventions or signatures that they have. Now, let s consider the roles in the pattern:
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ITarget
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The interface that the Client wants to use
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Adaptee
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An implementation that needs adapting
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Adapter
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The class that implements the ITarget interface in terms of the Adaptee
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Request
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An operation that the Client wants
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SpecificRequest
The implementation of Request s functionality in the Adaptee
The pattern applies to a single computer, which would only have either the PowerPC or the Intel chip. In this case, it has the Intel chip hence the need for the adapter. There is no Target class present, just the ITarget interface.
QUIZ
Match the Adapter Pattern Players with the Mac OS X Migration Illustration
To test whether you understand the Adapter pattern, cover the lefthand column of the table below and see if you can identify its players among the items from the illustrative example (Figure 4-1), as shown in the righthand column. Then check your answers against the lefthand column.
Client ITarget Request Adapter Adaptee DifferentRequest
Mac OS X (or any Mac application) The specification of the AltiVec instruction set A call to an AltiVec instruction The Accelerate framework An Intel processor with an SSE instruction set A Call to an SSE instruction
Implementation
It is best to illustrate the structure of the adapter with a small example, even at the theory code level. Suppose that technical readings are being collected and reported at a high level of precision, but the client can only make use of rough estimates.
Adapter Pattern
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The signatures for the interface would be couched in terms of integers, and for the actual implementation in terms of double-precision numbers. Thus, an adapter is needed, as shown in Example 4-1.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 using System; // Adapter Pattern - Simple Judith Bishop Oct 2007 // Simplest adapter using interfaces and inheritance // Existing way requests are implemented class Adaptee { // Provide full precision public double SpecificRequest (double a, double b) { return a/b; } } // Required standard for requests interface ITarget { // Rough estimate required string Request (int i); } // Implementing the required standard via Adaptee class Adapter : Adaptee, ITarget { public string Request (int i) { return "Rough estimate is " + (int) Math.Round(SpecificRequest (i,3)); } } class Client { static void Main ( ) { // Showing the Adapteee in standalone mode Adaptee first = new Adaptee( ); Console.Write("Before the new standard\nPrecise reading: "); Console.WriteLine(first.SpecificRequest(5,3)); // What the client really wants ITarget second = new Adapter( ); Console.WriteLine("\nMoving to the new standard"); Console.WriteLine(second.Request(5)); } } /* Output Before the new standard Precise reading: 1.66666666666667 Moving to the new standard Rough estimate is 2 */
78 |
4: Structural Patterns: Adapter and Fa ade
The main program in the client shows two scenarios. First, there is an example of how the Adaptee could be called directly (line 33) its output is shown in line 43. However, the client wants to work to a different interface for requests (lines 17 and 38). The Adapter implements the ITarget interface and inherits the Adaptee (line 21). Therefore, it can accept Request calls with a string-int signature and route them to the Adaptee with a double-double-double signature (line 23). The new output is shown on line 46. A feature of adapters is that they can insert additional behavior between the ITarget interface and the Adaptee. In other words, they do not have to be invisible to the Client. In this case, the Adapter adds the words "Rough estimate is" to indicate that the Request has been adapted before it calls the SpecificRequest (line 23). Adapters can put in varying amounts of work to adapt an Adaptee s implementation to the Target s interface. The simplest adaptation is just to reroute a method call to one of a different name, as in the preceding example. However, it may be necessary to support a completely different set of operations. For example, the Accelerate framework mentioned in the Illustration section will need to do considerable work to convert AltiVec instructions to those of the Intel Core Duo processor. To summarize, we have the following options when matching adapter and adaptee interfaces: Adapter interface and adaptee interface have same signature This is the trivial case, with not much work to do.
Many examples of the Adapter pattern operate at this level and are not illustrative or helpful in explaining its real power. Beware of them.
Adapter interface has fewer parameters than adaptee interface The adapter calls the adaptee with some dummy input.
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