What is the exact definition of the strategy design pattern?

Question

I had a geek fight with someone over what the strategy pattern really is and I need a expert to settle the matter.

We both agree that the strategy pattern allows for the guts of a class (e.g., the behavior) to be swapped out at runtime while maintaining the same interface. However, her contention is that "For [the algorithms] to be a strategy, You would have to get the same results". My contention is that swapping an "algorithm" or logic of a class could mean that the results of the overridden operation are different, but that it still meets the purpose, intent (and classification) of the strategy pattern.

Her code example with comments:

---

By your definition, any subclasses of a class would be a strategy. They have the same method definitions (signatures), and are therefore interchangeable.

Interface Strategy
{
    DoArithmatic(int[] a)
}

Class A : Strategy
public int DoArithmatic(int[]a)
{
     int temp = 0;
     for(int i =0; i< a.length; i++)
          temp += a[i]
}

Class B : Strategy
public int DoArithmaticB(int[]a)
{
     int temp = 0;
     for(int i =a.length -1; i>-1; i--)
          temp += a[i]
}

Class C : Strategy
public int DoArithmatic(int[]a)
{
     int temp = 0;
     for(int i =0; i< a.length; i++)
          temp -= a;
}

int[] a = { 1,2,3 }
ClassA.DoArithmatic(a) = 6
ClassB.DoArithmatic(a) = 6
ClassC.DoArithmatic(a) = -6//This one is not interchangeable

The first two are strategies. Because for any input they will give you the EXACT same answer. the last one is not. Just because it gives you an int does not make it a strategy. They have to "DO" the same thing.

You can't use a "higher" abstraction term just to make them a strategy.

These all do "MATH" but they are not all doing the "same" thing in a different way. That is the essence of a strategy.

So, who's right?

Answer 1

You sir are correct and your coworker needs to read the GoF.

"The strategy pattern lets the algorithms vary independently from clients that use them."

See:

http://www.dofactory.com/Patterns/PatternStrategy.aspx