Design Patterns - Strategy Pattern
17 Jul 2018
Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from the clients that use it.
| Intent | Enables you to use different business rules or algorithms depending on the context in which they occur |
| Problem | The selection of an algorithm that needs to be applied depends on the client making the request or the data being acted on. |
| Solution | Separate the selection of algorithm from the implementation of the algorithm. Allows for the selection to be made based upon context. |
| Participants and Collaborators | Strategy declares an interface common to all supported algorithms, and specifies how the different algorithms are used. Concrete Strategy implements these different algorithms using Strategy interface. Context uses a specific Concrete Strategy with a reference of the type Strategy |
| Consequences | The Strategy pattern defines a family of algorithms. Switch or Conditional can be eliminated |
Example
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| package com.art.designpatterns.strategy; | |
| import com.google.common.collect.Lists; | |
| import java.util.List; | |
| public class StrategyDemo { | |
| public static void main(String[] args) { | |
| PromotionStrategy halfOffPromo = new HalfOffPromotionStrategy(); | |
| PromotionStrategy clearancePromo = new ClearancePromotionStrategy(); | |
| Customer customer1 = new Customer(halfOffPromo); | |
| customer1.addItem(100.0); | |
| customer1.addItem(50.0); | |
| customer1.addItem(20.0); | |
| System.out.println("Customer 1"); | |
| System.out.println("Total: " + customer1.getTotalBeforePromo()); | |
| System.out.println("Total (After Promo): " + customer1.getTotalAfterPromo()); | |
| Customer customer2 = new Customer(clearancePromo); | |
| customer2.addItem(100.0); | |
| customer2.addItem(50.0); | |
| customer2.addItem(20.0); | |
| System.out.println("Customer 2"); | |
| System.out.println("Total: " + customer2.getTotalBeforePromo()); | |
| System.out.println("Total (After Promo):" + customer2.getTotalAfterPromo()); | |
| } | |
| } | |
| class Customer { | |
| private List<Double> items; | |
| private PromotionStrategy promotionStrategy; | |
| public Customer(PromotionStrategy promotionStrategy) { | |
| this.promotionStrategy = promotionStrategy; | |
| this.items = Lists.newArrayList(); | |
| } | |
| public void addItem(Double item){ | |
| items.add(item); | |
| } | |
| public double getTotalBeforePromo(){ | |
| double total = items.stream().mapToDouble(i -> i).sum(); | |
| return total; | |
| } | |
| public double getTotalAfterPromo(){ | |
| double total = getTotalBeforePromo(); | |
| double promotion = promotionStrategy.getPromotion(total); | |
| return total - promotion; | |
| } | |
| } | |
| interface PromotionStrategy{ | |
| double getPromotion(Double totalAmount); | |
| } | |
| class HalfOffPromotionStrategy implements PromotionStrategy { | |
| @Override | |
| public double getPromotion(Double totalAmount) { | |
| return totalAmount * 0.5; | |
| } | |
| } | |
| class ClearancePromotionStrategy implements PromotionStrategy{ | |
| @Override | |
| public double getPromotion(Double totalAmount) { | |
| return totalAmount * 0.75; | |
| } | |
| } |