How can you use the strategy design pattern to define a family of algorithms?

1 What is the strategy design pattern?

The strategy design pattern is a behavioral design pattern that defines a set of algorithms that can be used interchangeably to perform a specific task. The algorithms are encapsulated in separate classes that implement a common interface, and the interface defines the contract for the algorithm. The context class is responsible for selecting and executing the appropriate strategy based on some criteria, and it can vary the strategy at runtime without changing its own structure. The strategy design pattern follows the open-closed principle, which states that classes should be open for extension but closed for modification.

2 Why use the strategy design pattern?

The strategy design pattern has several benefits for software engineering. First, it promotes loose coupling between the context and the strategies, as they only depend on the interface and not on the concrete implementation. Second, it enhances modularity and reusability, as the strategies can be defined and tested independently and used in different contexts. Third, it improves readability and maintainability, as the code becomes more clear and consistent, and new strategies can be added or modified easily without affecting the existing ones. Fourth, it supports dynamic behavior, as the context can switch between strategies at runtime based on the situation.

3 How to implement the strategy design pattern?

To implement the strategy design pattern, you need to define a strategy interface that declares the method or methods that the algorithms will use to perform the task. Then, create concrete strategy classes that inherit from the strategy interface and provide different implementations of the algorithm. After that, define a context class that holds a reference to a strategy object and provides a method to set or change the strategy at runtime. Lastly, use the context class to invoke the strategy method and execute the algorithm.

4 An example of the strategy design pattern

Let's see an example of how to use the strategy design pattern to sort a collection of objects. Suppose you have a class called Product that represents a product with attributes such as name, price, and rating. You want to sort a list of products by different criteria, such as by name, by price, or by rating. You can use the strategy design pattern to define a family of sorting algorithms that can be applied to the list of products.

First, you need to define a strategy interface that declares a method to sort a list of products. For example:

public interface SortStrategy {
  void sort(List<Product> products);
}        

Next, you need to implement concrete strategy classes that inherit from the strategy interface and provide different sorting algorithms. For example:

public class SortByName implements SortStrategy {
  @Override
  public void sort(List<Product> products) {
    // Sort products by name using some algorithm
  }
}
public class SortByPrice implements SortStrategy {
  @Override
  public void sort(List<Product> products) {
    // Sort products by price using some algorithm
  }
}
public class SortByRating implements SortStrategy {
  @Override
  public void sort(List<Product> products) {
    // Sort products by rating using some algorithm
  }
}        

Then, you need to define a context class that holds a reference to a strategy object and provides a method to set or change the strategy at runtime. For example:

public class ProductSorter {
  private SortStrategy strategy;
  public void setStrategy(SortStrategy strategy) {
    this.strategy = strategy;
  }
  public void sort(List<Product> products) {
    strategy.sort(products);
  }
}        

Finally, you can use the context class to invoke the strategy method and execute the sorting algorithm. For example:

public class Main {
  public static void main(String[] args) {
    List<Product> products = ...; // Create a list of products
    ProductSorter sorter = new ProductSorter(); // Create a context object
    sorter.setStrategy(new SortByName()); // Set the strategy to sort by name
    sorter.sort(products); // Sort the products by name
    sorter.setStrategy(new SortByPrice()); // Set the strategy to sort by price
    sorter.sort(products); // Sort the products by price
    sorter.setStrategy(new SortByRating()); // Set the strategy to sort by rating
    sorter.sort(products); // Sort the products by rating
  }
}        

5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?