Question

The following code sample is an implementation of the Strategy pattern copied from Wikipedia. My full question follows it…

The Wiki’s main method:

//StrategyExample test application

class StrategyExample {

    public static void main(String[] args) {

        Context context;

        // Three contexts following different strategies
        context = new Context(new ConcreteStrategyAdd());
        int resultA = context.executeStrategy(3,4);

        context = new Context(new ConcreteStrategySubtract());
        int resultB = context.executeStrategy(3,4);

        context = new Context(new ConcreteStrategyMultiply());
        int resultC = context.executeStrategy(3,4);

    }

}

The pattern pieces:

// The classes that implement a concrete strategy should implement this

// The context class uses this to call the concrete strategy
interface Strategy {

    int execute(int a, int b);

}

// Implements the algorithm using the strategy interface
class ConcreteStrategyAdd implements Strategy {

    public int execute(int a, int b) {
        System.out.println("Called ConcreteStrategyA's execute()");
        return a + b;  // Do an addition with a and b
    }

}

class ConcreteStrategySubtract implements Strategy {

    public int execute(int a, int b) {
        System.out.println("Called ConcreteStrategyB's execute()");
        return a - b;  // Do a subtraction with a and b
    }

}

class ConcreteStrategyMultiply implements Strategy {

    public int execute(int a, int b) {
        System.out.println("Called ConcreteStrategyC's execute()");
        return a  * b;   // Do a multiplication with a and b
    }

}

// Configured with a ConcreteStrategy object and maintains a reference to a Strategy object
class Context {

    private Strategy strategy;

    // Constructor
    public Context(Strategy strategy) {
        this.strategy = strategy;
    }

    public int executeStrategy(int a, int b) {
        return strategy.execute(a, b);
    }

}

Considering specifically the above example, is the Context class redundant?

For example, I can come up with the following alternate main implementation by using the existing classes and interface except Context and it will work exactly the same. It is still loosely coupled.

(( Edit: In this simple scenario, when I leave out the Context class, will I be making a future mistake? ))

public static void main(String[] args) {

    IStrategy strategy;

    // Three strategies
    strategy = new ConcreteStrategyAdd();
    int resultA = strategy.executeStrategy(3,4);

    strategy = new ConcreteStrategySubtract();
    int resultB = strategy.executeStrategy(3,4);

    strategy = new ConcreteStrategyMultiply();
    int resultC = strategy.executeStrategy(3,4);

}

---

Summary Update

Listing in point form what was discovered through answers and comments:

• The Context allows for variation in how the composited Strategy is used (e.g. timing of it’s call). Different Contexts might do different internal work before and after calling the given Strategy.
• The context is a high level “black box”. The Context logic can change, also the composited Strategy can change (or a different one used) without breaking the client because the client understands only how to call the context.
• Even though I created an alternate implementation of the Wikipedia sample code by leaving out the Context, and although it worked the same as the original, the entire situation was simplified (in both cases) and my changes actually meant: 1. it’s not a Strategy pattern anymore, 2. I miss the benefits of the spirit of the Strategy pattern that are mentioned here.
• My alternate implementation used the main method like a Context, so I might as well keep the Context if effectively simulating it. By creating an impure Strategy pattern, confusion was created. I didn’t need to reinvent the wheel or try to be smarter (in this case).

If any other points would be useful or if this needs correction leave a comment and I’ll modify the list accordingly.

Answer 1

As the name suggests, the Context is what encapsulates the point at which the strategy is performed. Without that, you just have a naked Strategy, and the calling class now picks up an extra responsibility: knowing when to call the Strategy itself. Your example is perhaps a bit too simple, and in this particular case, I would say that the Context isn’t getting you too much.

An example that perhaps better illustrates the usefulness of a Context is more like the following:

public class LoadingDock {   // Context.
  private LoadStrategy ls;   // Strategy.

  public void setLoadStrategy(LoadStrategy ls) { ... }

  // Clients of LoadingDock use this method to do the relevant work, rather
  // than taking the responsibility of invoking the Strategy themselves.
  public void shipItems(List<ShippingItem> l) {
    // verify each item is properly packaged     \
    // ...                                        |  This code is complex and shouldn't be
    // verify all addresses are correct           |  subsumed into consumers of LoadingDock.
    // ...                                        |  Using a Context here is a win because
    // load containers onto available vehicle     |  now clients don't need to know how a
    Vehicle v = VehiclePool.fetch();        //    |  LoadingDock works or when to use a
    ls.load(v, l);                          //   /   LoadStrategy.
  }
}

Notice how the Strategy will never be called directly from an external client. Only shipItems uses the strategy, and the details of the steps it follows are a black box. This allows the Context to adjust how it uses the strategy without affecting clients. For instance, the steps could be completely reordered or adjusted (or removed entirely) to meet performance objectives or other goals — but for the client, the external interface of shipItems() looks exactly the same.

Notice, also, that our example Context, the LoadingDock, could change its LoadStrategy at any time based on its internal state. For example, if the dock is getting too full perhaps it will switch to a more aggressive scheduling mechanism that gets crates off the dock and into trucks faster, sacrificing some efficiency in doing so (maybe the trucks don’t get loaded up as efficiently as they could have been).