Lecture 17: The Strategy and Decorator Patterns
The strategy and decorator patterns are very similar idioms of using delegation to refactor aspects of a class’s behavior into separate classes, for easier enhancement or modification, and tighter cohesiveness. The strategy pattern is somewhat more intuitive to motivate with real-world examples, so we’ll start there.
1 The strategy pattern
Let’s consider a straightforward game for which the technique for winning is easily, thoroughly mastered: TicTacToe. How might we play this game? Here are some ideas:
Play in an open square
Play to an open corner
Block the opponent from winning
Place a third piece in a row to win
All but the first of these aren’t complete strategies. If the corners are all full, for instance, then the second approach doesn’t have any recommendation to make. So perhaps more accurately, our complete strategies might be:
Play in an open square
Play to an open corner, or else play in an open square
Block the opponent from winning if they’re about to, otherwise play to an open corner, otherwise just play to an open square
Place a third piece in a row to win, or else block the opponent if they’re about to win, or else play in an open corner, or else just play to an open square
It’s probably intuitively clear to you, as an expert human player of TicTacToe,
that the final strategy above is pretty good.1It’s not perfect —
1.1 A strategy is just a function object
Suppose in our program we had a TicTacToeModel containing the board
state, and a Piece enumeration naming X and O. Our program
also contains two Player objects (one for each Piece), which may
call the model’s playMove(Piece player, int row, int col) method to make
a move. How should the Players choose where to move? What information
would be needed to make an informed choice? We’d need to know the state of the
board, and also know which player we’re trying to choose a move for:
class Coord {
int row;
int col
}
interface TicTacToeStrategy {
Coord chooseMove(TicTacToeModel model, Piece forWhom);
}Implementing our first strategy is pretty easy. It doesn’t care who it’s choosing a move for; it just seeks out the first open square:
class AnyOpenSquare implements TicTacToeStrategy {
public Coord chooseMove(TicTacToeModel model, Piece forWhom) {
for (int r = 0; r < 3; r++)
for (int c = 0; c < 3; c++)
if (model.getPieceAt(r, c) == null)
return new Coord(r, c);
return null;
}
}How might we implement the second strategy? We could implement the entirety of the logic (choose corners or else choose random), but part of that logic has been implemented already... Let’s implement just the new part, first:
class AnyOpenCorner implements TicTacToeStrategy {
public Coord chooseMove(TicTacToeModel model, Piece forWhom) {
if (model.getPieceAt(0, 0) == null) return new Coord(0, 0);
else if (model.getPieceAt(0, 2) == null) return new Coord(0, 2);
else if (model.getPieceAt(2, 0) == null) return new Coord(2, 0);
else if (model.getPieceAt(2, 2) == null) return new Coord(2, 2);
else return null;
}
}This strategy looks for an open corner, or else gives up. So how might we combine the pieces we have so far to produce the composite strategy?
1.2 Strategies can be composed
The any-open-square strategy and the any-open-corner strategy are both functions that take in a board state and return a coordinate to move. We can easily, and generally, combine the two of them into a higher-order strategy that first tries one and then, if it fails, tries the other:
class TryTwo implements TicTacToeStrategy {
TicTacToeStrategy first, second;
public Coord chooseMove(TicTacToeModel model, Piece forWhom) {
Coord ans = this.first.chooseMove(model, forWhom);
if (ans != null) return ans; // the first strategy succeeded
return this.second.chooseMove(model, forWhom);
}
}Now, our full second strategy is simply
new TryTwo(new AnyOpenCorner(), new AnyOpenSquare()). We can continue
implementing the other strategy components (blocking an opponent, or going for
an immediate win), and combining them with TryTwo. (These other
components need the forWhom argument —
More broadly, strategies can be composed in lots of different ways, analogous
to mapping, and-map, or-map, or other higher-order combinations. We can
generalize from TryTwo to TryMany (that takes a list of
strategies); we can generalize to randomly selecting among several strategies,
or more sophisticated choices among several strategies, etc.
1.3 Strategies can be dynamically selected
Suppose we wanted to build a TicTacToe game for players of varying ability, and wanted “easy”, “medium”, and “hard” difficulty levels. We could easily mix and match compositions among the four simple strategies above, and dynamically assign some such object at runtime:
User computerUser = ...;
Strategy easy = new TryTwo(new PlayToWin(), new AnyOpenSquare());
Strategy medium = new TryTwo(new PlayToWin(), new TryTwo(new DontLose(), new AnyOpenSquare()));
Strategy hard = new TryTwo(new PlayToWin(),
new TryTwo(new DontLose(),
new TryTwo(new AnyOpenCorner(), new AnyOpenSquare())));
if (difficulty == EASY)
computerUser.playStrategy(easy);
else if (difficulty == MEDIUM)
computerUser.playStrategy(medium);
else
computerUser.playStrategy(hard);Rather than hardcoding the difficulty level of the game, we have refactored it out and made it easily configurable.
We can also define an AskUser strategy...and now we have something the
controller can use to interact with a human player, in the same framework as
these other strategy choices.
1.4 Strategies make for easy testing
If we supply two strategies to our controller and say “play the game with two
players using these respective strategies”, what could happen? If both
strategies are fully automatic, then we’ve built a great test harness to run
through the game automatically! Combining this with the Appendable
output we’ve seen in earlier lectures, and we have a straightforward means to
simulate a playthrough of the game. Moreover, testing different strategies
against each other lets us not only test whether they work properly, but also
how well they do at playing the game.
2 The decorator pattern
The decorator pattern is similar to the strategy pattern in terms of how
objects delegate from one to another, but the purpose is different. The
canonical example of decorators is a UI widget library, such as Swing. We
might have a basic JPanel class that just describes a rectangle of
on-screen content. We might have a JScrollPanel class, which is a
subclass of JPanel, that visually wraps around some other JPanel
and adds scrollbars and the ability to scroll the view in one or more
directions. Or a JSplitPanel, which surrounds two panels and produces a
split-screen effect. Or a JGridPanel, or others... Each of these
classes obeys a fairly sophisticated interface (all the methods of the base
JPanel class), and then does two things: it provides some new
functionality of its own, and delegates the rest of the functionality to inner
panels. Visually, each surrounding panel decorates the inner ones by
adding borders, scrollbars, splitters, etc. Just as with the strategy pattern,
each class is responsible for one fragment of functionality: the complete
functionality comes about by composing several decorators together around some
base object. (Unlike strategies, where there can often be useful strategies
that do not need to delegate at all; decorators are all about the delegation.)
3 Strategies, Decorators and Inheritance
When should you use a strategy pattern or a decorator pattern, instead of just
using inheritance to customize behavior? After all, don’t subclasses
specialize the behavior of their superclasses? For example, could we produce
the same effect by having our User class just have an abstract method
chooseMove, and then creating subclasses that define various
possible implementations for that method?
Yes...but only sometimes. The power of the strategy and decorator patterns comes from the dynamism inherent in ability to delegate from one simple strategy to the next, or from one decorator to its contained content. Rather than being fixed at compile time, we can use higher-order strategies to mix-and-match strategy pieces, or decorate panels into elaborate UIs, without having to hard-code those choices in advance. Additionally, the strategy or decorator classes are appealingly tiny, self-contained, and easy to read: they have very high cohesion, because they’re built to do exactly one thing and nothing else.
1It’s not perfect —