Advantages (+)

• Avoids compile-time implementation dependencies.
  • Clients refer to an interface (Strategy) and are independent of an implementation.
• Provides a flexible alternative to subclassing.
  • Subclassing provides a way to change the algorithm of a class (at compile-time).
    When a subclass is instantiated, its algorithm is fixed and can't be changed for the life-time of the object.
  • Strategy provides a way to change the algorithm of an object (at run-time) by delegating to different Strategy objects.
• Avoids conditional statements for switching between algorithms.
  • Conditional statements that switch between different algorithms are replaced by delegating to different Strategy objects.
  • "Code containing many conditional state­ments often indicates the need to apply the Strategy pattern." [GoF, p318]

Disadvantages (–)

• Can make the common Strategy interface complex.
  • The Strategy interface may get complex because it must pass in the needed data for all supported algorithms (whether they are simple or complex; see Implementation).
• Requires that clients understand how strategies differ.
  • "The pattern has a potential drawback in that a client must understand how Strategies differ before it can select the appropriate one. Clients might be exposed to implementation issues." [GoF, p318]
• Introduces an additional level of indirection.
  • Strategy achieves flexibility by introducing an additional level of indirection (clients delegate an algorithm to a separate Strategy object), which makes clients dependent on a Strategy object.
  • This "can complicate a design and/or cost you some performance. A design pattern should only be applied when the flexibility it affords is actually needed." [GoF, p31]