Strategy pattern

In computer programming, the strategy pattern (also known as the policy pattern) is a behavioral software design pattern that enables selecting an algorithm at runtime. Instead of implementing a single algorithm directly, code receives run-time instructions as to which in a family of algorithms to use.

Strategy lets the algorithm vary independently from clients that use it. Strategy is one of the patterns included in the influential book '' Design Patterns'' by Gamma et al. that popularized the concept of using design patterns to describe how to design flexible and reusable object-oriented software. Deferring the decision about which algorithm to use until runtime allows the calling code to be more flexible and reusable.

For instance, a class that performs validation on incoming data may use the strategy pattern to select a validation algorithm depending on the type of data, the source of the data, user choice, or other discriminating factors. These factors are not known until run-time and may require radically different validation to be performed. The validation algorithms (strategies), encapsulated separately from the validating object, may be used by other validating objects in different areas of the system (or even different systems) without code duplication.

Typically, the strategy pattern stores a reference to some code in a data structure and retrieves it. This can be achieved by mechanisms such as the native function pointer, the first-class function, classes or class instances in object-oriented programming languages, or accessing the language implementation's internal storage of code via reflection.

Structure

UML class and sequence diagram

In the above UML class diagram, the Context class doesn't implement an algorithm directly.
Instead, Context refers to the Strategy interface for performing an algorithm (strategy.algorithm()), which makes Context independent of how an algorithm is implemented.
The Strategy1 and Strategy2 classes implement the Strategy interface, that is, implement (encapsulate) an algorithm.


The UML sequence diagram
shows the run-time interactions: The Context object delegates an algorithm to different Strategy objects. First, Context calls algorithm() on a Strategy1 object,
which performs the algorithm and returns the result to Context.
Thereafter, Context changes its strategy and calls algorithm() on a Strategy2 object,
which performs the algorithm and returns the result to Context.

Class diagram

Example

C#

The following example is in C#.

public class StrategyPatternWiki

// CustomerBill as class name since it narrowly pertains to a customer's bill
class CustomerBill

interface IBillingStrategy

// Normal billing strategy (unchanged price)
class NormalStrategy : IBillingStrategy

// Strategy for Happy hour (50% discount)
class HappyHourStrategy : IBillingStrategy

Python

The following example is equivalent to the C# example above, but in Python.

import abc
from typing import List

class Bill:
def __init__(self, billing_strategy: "BillingStrategy"):
self.drinks: list loat= []
self._billing_strategy = billing_strategy

@property
def billing_strategy(self) -> "BillingStrategy":
return self._billing_strategy

@billing_strategy.setter
def billing_strategy(self, billing_strategy: "BillingStrategy") -> None:
self._billing_strategy = billing_strategy

def add(self, price: float, quantity: int) -> None:
self.drinks.append(self.billing_strategy.get_act_price(price * quantity))

def __str__(self) -> str:
return str(f"£")

class BillingStrategy(abc.ABC):
@abc.abstractmethod
def get_act_price(self, raw_price: float) -> float:
raise NotImplementedError

class NormalStrategy(BillingStrategy):
def get_act_price(self, raw_price: float) -> float:
return raw_price

class HappyHourStrategy(BillingStrategy):
def get_act_price(self, raw_price: float) -> float:
return raw_price * 0.5

def main() -> None:
normal_strategy = NormalStrategy()
happy_hour_strategy = HappyHourStrategy()

customer_1 = Bill(normal_strategy)
customer_2 = Bill(normal_strategy)

# Normal billing
customer_1.add(2.50, 3)
customer_1.add(2.50, 2)

# Start happy hour
customer_1.billing_strategy = happy_hour_strategy
customer_2.billing_strategy = happy_hour_strategy
customer_1.add(3.40, 6)
customer_2.add(3.10, 2)

# End happy hour
customer_1.billing_strategy = normal_strategy
customer_2.billing_strategy = normal_strategy
customer_1.add(3.10, 6)
customer_2.add(3.10, 2)

# Print the bills;
print(customer_1)
print(customer_2)

if __name__

"__main__":
main()

Java

The following example is in Java.

import java.util.ArrayList;
import java.util.List;
import java.util.function.IntUnaryOperator;

enum BillingStrategy

class CustomerBill

public class StrategyPattern

Strategy and open/closed principle

According to the strategy pattern, the behaviors of a class should not be inherited. Instead, they should be encapsulated using interfaces. This is compatible with the open/closed principle (OCP), which proposes that classes should be open for extension but closed for modification.

As an example, consider a car class. Two possible functionalities for car are ''brake'' and ''accelerate''. Since accelerate and brake behaviors change frequently between models, a common approach is to implement these behaviors in subclasses. This approach has significant drawbacks: accelerate and brake behaviors must be declared in each new Car model. The work of managing these behaviors increases greatly as the number of models increases, and requires code to be duplicated across models. Additionally, it is not easy to determine the exact nature of the behavior for each model without investigating the code in each.

The strategy pattern uses composition instead of inheritance. In the strategy pattern, behaviors are defined as separate interfaces and specific classes that implement these interfaces. This allows better decoupling between the behavior and the class that uses the behavior. The behavior can be changed without breaking the classes that use it, and the classes can switch between behaviors by changing the specific implementation used without requiring any significant code changes. Behaviors can also be changed at run-time as well as at design-time. For instance, a car object's brake behavior can be changed from BrakeWithABS() to Brake() by changing the brakeBehavior member to:

brakeBehavior = new Brake();

/* Encapsulated family of Algorithms
* Interface and its implementations
*/
public interface IBrakeBehavior

public class BrakeWithABS implements IBrakeBehavior

public class Brake implements IBrakeBehavior

/* Client that can use the algorithms above interchangeably */
public abstract class Car

/* Client 1 uses one algorithm (Brake) in the constructor */
public class Sedan extends Car

/* Client 2 uses another algorithm (BrakeWithABS) in the constructor */
public class SUV extends Car

/* Using the Car example */
public class CarExample

See also

* Dependency injection
* Higher-order function
* List of object-oriented programming terms
* Mixin
* Policy-based design
* Type class
* Entity–component–system
* Composition over inheritance

References

External links

Strategy Pattern in UML

*

Strategy Pattern for C article


* Implementation of the Strategy pattern in JavaScript

{{Design Patterns Patterns

Software design patterns
Articles with example C Sharp code
Articles with example Java code