Lecture 17: The Strategy and Decorator Patterns
1 The strategy pattern
1.1 A strategy is just a function object
1.2 Strategies can be composed
1.3 Strategies can be dynamically selected
1.4 Strategies make for easy testing
2 The decorator pattern
3 Strategies, Decorators and Inheritance
7.1

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:

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:

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 — suppose two players were following this same strategy. Would the game end in a win, loss, or tie for the first player? How could you fix any problems you might encounter? How might we implement these strategies?

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 — try implementing them yourself.)

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 — suppose two players were following this same strategy. Would the game end in a win, loss, or tie for the first player? How could you fix any problems you might encounter?