Recipe16.3.Implementing Interfaces Using Aspects

Recipe 16.3. Implementing Interfaces Using Aspects

Problem

You want to add an interface to a class.

Solution

Use the declare parents statement to specify a particular class that implements a specific interface or interfaces.

Discussion

Example 16-3 shows how the declare parents statement in AspectJ can apply an interface to a class that may have had no knowledge of the interface prior to the aspect being applied.

Example 16-3. Using aspects to declare a new interface on an existing class

public aspect ImplementInterfaceRecipe {         declare parents : MyClass implements MyInterface; }

Figure 16-3 shows how the static class architecture is affected by the application of the aspect in Example 16-3.

Figure 16-3. The static class structure before and after the aspect introduces a new interface

Having the capability to apply an interface to an existing class allows objects of that class to be referenced by the interface type, as shown in Example 16-4.

Example 16-4. Referring to an object according to an aspect declared interface

// Create an instance of MyClass MyInterface myObject = new MyClass( ); // ... // Work with the interface reference myObject.foo(1, "Russ");

By applying a new interface to a class, the class can be used in ways in which it was not originally designed. This is particularly useful when a new role is assigned to a class.

The class must implement any methods the interface declares. In traditional object orientation, the Adapter design pattern would be a good solution. However, AspectJ does offer an alternative implementation of the Adapter pattern. Using the mechanisms shown in Recipe 12.1, an aspect can add implementations of the interfaces methods to the class without the need for a separate Adapter.

Recipe16.3.Implementing Interfaces Using Aspects