2.2 Ballworld, composition-based  (Page 3/3)

Balls that change their strategies

Let's consider a the notion of a ball that changes its behavior. Since we have modeled a ball as having a strategy, we can simply say that in some manner, it is the ball's strategy that changes. We could say that the ball changes its strategy, but since the ball doesn't know which strategy it has to begin with, it really doesn't know one strategy from another. One could argue that it therefore can't know when or if it should ever change its strategy. Therefore, the ball cannot be coded to change its own strategy! So, whose baliwick is the changing of the strategy?

Since the changing of a strategy is a strategy for updating the ball, it is the strategy that determines the change. The strategy changes the strategy! Let's consider the following strategy: package ballworld; import java.awt.*;public class Change1Strategy implements IUpdateStrategy { private int i = 500; // initial value for ipublic void updateState(Ball context) {if(i==0) context.setStrategy(new CurveStrategy()); // change strategy if i reaches zero else i--; // not yet zero, so decrement i} } This strategy acts just like a StraightStrategy for 500 updates and then it tells the ball (its context ) to switch to a CurveStrategy . Once the CurveStrategy is installed, the ball becomes curving, without the need for any sort of conditionals to decide what it should do. The context ball fundamentally and permanently becomes curving.

A key notion above is that a strategy can contain another strategy. In the above example, the Change1Strategy could have easily pre-instantiated the CurveStrategy and saved it in a field for use when it was needed. But the does it matter exactly which concrete strategy is being used? If not, why not work at a higher abstraction level and let one strategy hold a reference to an abstract strategy? For instance, consider the following code: package ballworld; import java.awt.*;public class SwitcherStrategy implements IUpdateStrategy {private IUpdateStrategy _strategy = new StraightStrategy();public void updateState(Ball context) { _strategy.updateState(context);}public void setStrategy(IUpdateStrategy newStrategy) { _strategy = newStrategy;} } This strategy doesn't look like it does much, but looks are deceiving. All the SwitcherStrategy does is to delegate the updateState method to the _strategy that it holds. This does not seem much in of itself, but consider the fact that the SwitcherStrategy also has a settor method for _strategy . This means that the strategy held can be changed at run time! More importantly, suppose a ball is instantiated with a SwitcherStrategy . The behavior of the ball would be that of whatever strategy is being held by the SwitcherStrategy since the switcher just delegates to the held strategy. If one were to have a reference to that SwitcherStrategy instance from somewhere else, one could then change the internal strategy. The ball is none the wiser because all it has is a reference to the SwitcherStrategy instance, which hasn't changed at all! However, since the held strategy is now different, the ball's behavior has completely changed! This is an example of the Decorator Design Pattern , where the SwitcherStrategy class is formally called the decorator and the held strategy is formally called the decoree . In theoretical terms, the decorator is what is known as an indirection layer , which is like a buffer between two enities that enables them to depend on each other but yet still be free to move and change with respect to each other. A very useful analogy for indirection layers is like the thin layer of oil that will enable two sheets of metal to slide easily past each other.

Balls with multiple, composite behaviors

Now that we can dynamically change a ball's behavior, let's tackle another problem:

Abstraction is the key here. We want to write code that represents that abstraction of multiple, composite strategies. Does what we were doing above depend on the particular concrete strategies that we were using? No? Then we should eliminate the concrete classes, raise the abstraction level and use the abstract superclass (interface) instead. For a combination of two behaviors, we end up with the following: package ballworld; import java.awt.*;public class MultiStrategy implements IUpdateStrategy { private IUpdateStrategy _s1;private IUpdateStrategy _s2; public MultiStrategy(IUpdateStrategy s1, IUpdateStrategy s2) {_s1 = s1; _s2 = s2;}public void updateState(Ball context) { _s1.updateState(context);_s2.updateState(context); }} Notice how we have added a constructor that enables us to initialize the two abstract strategy fields. All we have to do is to construct a MultiStrategy object with the two desired strategies, and we're good to go!

Composite patterns

So what have we wrought here? Let's take a look at the UML diagram of our to most abstract strategies.

Switcherstrategy and multistrategy

The key to the power that lies in the SwitcherStrategy and the MultiStrategy lies in the fact that they hold references to their own superclass, IUpdateStrategy . This is what enables them to be create any behavior they want, including combinations of behaviors and dynamic modifications of behaviors. This self-referential class structure is known as the Composite Design Pattern (The Decorator pattern can be considered to be specialized form of the Composite pattern). The massive power, flexibility and extensiblility that this pattern generates warrants further, more formal study, which is where we're heading next. Stay tuned!