Strategy Design Pattern

Last updated on Feb 4, 2023

The purpose of this blog is to provide an in-depth explanation of the Strategy Design Pattern and how it can be used to improve the flexibility and maintainability of software. The blog covers the key concepts of the pattern, including the strategy interface, concrete strategy classes, and the context class. It also includes code examples and a UML diagram to help illustrate the pattern in action.

I. Introduction

Definition of Strategy Design Pattern

The Strategy Design Pattern is a behavioural design pattern that enables an algorithm's behaviour to be selected at runtime. It provides a flexible and reusable solution for defining a set of algorithms and choosing the appropriate one to use based on the situation. The pattern involves encapsulating each algorithm in a separate class and allowing them to be interchangeable. This allows for easy modification and maintenance of the algorithms, as well as the ability to add new algorithms without affecting existing code. The Strategy Design Pattern promotes the separation of concerns and promotes the Single Responsibility Principle by decoupling the algorithms from the class that uses them.

Importance of Strategy Design Pattern in software development

The Strategy Design Pattern is a fundamental concept in software development that helps to manage complex algorithms and behaviour in a flexible and maintainable manner. It has become a crucial tool for software developers to manage and organise code in an efficient and effective way. In this article, we will discuss the importance of the Strategy Design Pattern in software development and how it can help to solve common software development challenges.

Separation of Concerns One of the main advantages of the Strategy Design Pattern is that it promotes the separation of concerns. This means that the implementation of different algorithms is separated from the context in which they are used. This allows for a clean separation of responsibilities and reduces the coupling between the algorithms and the rest of the code. This, in turn, makes it easier to maintain and change the code, as well as to add new algorithms without affecting existing code.
Flexibility and Re-usability The Strategy Design Pattern also enables greater flexibility and re-usability of code. By encapsulating each algorithm in a separate class, it becomes easier to modify and reuse the algorithms in different contexts. This can help to reduce the amount of redundant code and increase the efficiency of software development.
Algorithm Interchangeability One of the key benefits of the Strategy Design Pattern is the ability to interchange algorithms at runtime. This means that the algorithm used by a particular context can be selected based on the situation. This provides great flexibility and can help to solve complex problems in a more elegant and efficient manner.
Improved Code Readability and Maintainability By using the Strategy Design Pattern, code becomes more readable and maintainable. The encapsulation of algorithms in separate classes makes it easier to understand the code, and the clean separation of concerns makes it easier to maintain and update the code as needed.
Easier Testing and Debugging Finally, the Strategy Design Pattern can also make testing and debugging easier. By encapsulating algorithms in separate classes, it becomes easier to test and debug each algorithm individually. This can help to reduce the time and effort required to test and debug the code, and can lead to higher quality software.

In conclusion, the Strategy Design Pattern is a crucial concept in software development that provides a flexible and reusable solution for managing complex algorithms and behavior. It promotes the separation of concerns, enables greater flexibility and reusability of code, and improves the readability and maintainability of code. By using the Strategy Design Pattern, software developers can solve complex problems in a more elegant and efficient manner, and can produce high-quality software that is easier to maintain and update.

II. Understanding the Strategy Design Pattern

Concept of algorithm interchangeability

Algorithm interchangeability is a key concept in the Strategy Design Pattern. It refers to the ability to switch between different algorithms at runtime, depending on the situation. This provides great flexibility and can help to solve complex problems in a more elegant and efficient manner.

The main idea behind algorithm interchangeability is to encapsulate each algorithm in a separate class, known as a strategy. The strategy classes are then interchangeable, meaning that the context in which they are used can choose the appropriate strategy to use based on the situation. This allows for easy modification and maintenance of the algorithms, as well as the ability to add new algorithms without affecting existing code.

For example, consider a scenario in which a software application needs to sort a list of numbers. A naive approach would be to write the sorting algorithm directly into the code. However, this would limit the flexibility of the code, as it would not be easy to modify or add new sorting algorithms. With algorithm interchangeability, the sorting algorithm is encapsulated in a separate class, and the context in which it is used can choose the appropriate sorting algorithm based on the situation.

In addition to providing greater flexibility, algorithm interchangeability also promotes the separation of concerns and the Single Responsibility Principle. By encapsulating each algorithm in a separate class, the implementation of the algorithms is separated from the context in which they are used. This reduces the coupling between the algorithms and the rest of the code, making it easier to maintain and change the code.

In conclusion, algorithm interchangeability is a key concept in the Strategy Design Pattern that provides great flexibility and helps to solve complex problems in a more elegant and efficient manner.

How Strategy Design Pattern works

The Strategy Design Pattern is a behavioural design pattern that provides a flexible and reusable solution for defining a set of algorithms and choosing the appropriate one to use based on the situation. It works by encapsulating each algorithm in a separate class, known as a strategy, and allowing them to be interchangeable. This provides greater flexibility and enables the algorithms to be modified and reused in different contexts.

The key components of the Strategy Design Pattern are the context class, the strategy interface, and the strategy classes. The context class represents the class that uses the algorithms, while the strategy interface defines the common interface for all strategies. The strategy classes implement the strategy interface and define the specific algorithm for each strategy.

The context class uses the strategy interface to define a reference to the strategy object. This reference can be set to any of the strategy classes at runtime, depending on the situation. When the context class needs to use an algorithm, it simply delegates the work to the strategy object through the strategy interface. This allows the context class to use any of the strategies interchangeably, without having to know the specifics of each algorithm.

For example, consider a scenario in which a software application needs to compress a file. The context class could define a reference to a strategy object that implements the file compression algorithm. The context class could then use the strategy object to compress the file, without having to know the specifics of the compression algorithm. This allows the context class to use different compression algorithms interchangeably, without having to modify the code.

UML diagram of the pattern

                 +--------------+
                 |  Context     |
                 +--------------+
                 | -strategy    |
                 +--------------+
                 | +setStrategy(Strategy strategy)|
                 | +execute()                   |
                 +--------------+
                                 |
           +----------------------------+
           |                            |
           v                            |
+--------------+                        |
|  Strategy    |                        |
+--------------+                        |
|              |                        |
+--------------+                        |
| +execute()   |                        |
+--------------+                        |
                                 |
            +---------------------+
            |                     |
            v                     |
+--------------+    +--------------+
| ConcreteStrategyA |  ConcreteStrategyB |
+--------------+    +--------------+
|                                        |
+--------------+                        |
| +execute()                             |
+--------------+                        |

In this UML diagram, Context is the class that uses the algorithms and Strategy is the interface that defines the common interface for all strategies. ConcreteStrategyA and ConcreteStrategyB are the classes that implement the Strategy interface and define the specific algorithms for each strategy. The Context class has a reference to the Strategy object and uses it to delegate the work to the appropriate strategy.

Note that this UML diagram is a simple representation of the Strategy Design Pattern and may vary depending on the specific implementation.

III. Implementing the Strategy Design Pattern in Java

Creating an interface for different strategies

Creating an interface for different strategies is a crucial step in implementing the Strategy Design Pattern. The interface serves as a blueprint for the individual strategies and defines the methods that each strategy must implement.

Here's an example of creating an interface for file compression strategies in Java:

public interface CompressionStrategy {
    void compressFile(String fileName);
}

In this example, the CompressionStrategy interface defines the compressFile method, which takes a fileName as a parameter. Any class that implements the CompressionStrategy interface must provide an implementation of the compressFile method.

This interface can then be used to create individual classes for different file compression strategies. For example, you could create classes for ZipCompression, RarCompression, and SevenZipCompression that implement the CompressionStrategy interface and define their own implementation of the compressFile method.

By creating an interface for the different strategies, you can ensure that each strategy provides a consistent interface and can be used interchangeably with other strategies. This makes it easy to switch between strategies at runtime, depending on the specific requirements of the application.

Implementing concrete strategies

Implementing concrete strategies is the next step in implementing the Strategy Design Pattern. Concrete strategies are classes that implement the strategy interface and define the specific algorithms for each strategy.

Here's an example of implementing two concrete file compression strategies in Java:

public class ZipCompression implements CompressionStrategy {
    public void compressFile(String fileName) {
        // Code to compress the file using the ZIP algorithm
    }
}

public class RarCompression implements CompressionStrategy {
    public void compressFile(String fileName) {
        // Code to compress the file using the RAR algorithm
    }
}

In this example, the ZipCompression and RarCompression classes implement the CompressionStrategy interface and provide their own implementation of the compressFile method.

By implementing concrete strategies, you can encapsulate each algorithm in a separate class, making it easy to modify and reuse the algorithms in different contexts. This also makes it easier to maintain and update the code, as changes to one algorithm will not affect the others.

It's important to note that the number of concrete strategies and the specific algorithms they implement will vary depending on the requirements of the application. You can add or remove strategies as needed, making the Strategy Design Pattern highly flexible and adaptable to changing requirements.

Using the Context class to execute a strategy

The Context class is used to execute a strategy in the Strategy Design Pattern. The Context class holds a reference to the strategy object and delegates the work to the appropriate strategy.

Here's an example of using the Context class to execute a file compression strategy in Java:

public class FileCompressionContext {
    private CompressionStrategy compressionStrategy;

    public FileCompressionContext(CompressionStrategy compressionStrategy) {
        this.compressionStrategy = compressionStrategy;
    }

    public void setCompressionStrategy(CompressionStrategy compressionStrategy) {
        this.compressionStrategy = compressionStrategy;
    }

    public void compressFile(String fileName) {
        compressionStrategy.compressFile(fileName);
    }
}

In this example, the FileCompressionContext class holds a reference to a CompressionStrategy object and provides a compressFile method that delegates the work to the appropriate strategy. The setCompressionStrategy method can be used to switch between strategies at runtime, depending on the specific requirements of the application.

To use the FileCompressionContext, you would create a FileCompressionContext object and pass in the desired strategy. For example:

FileCompressionContext context = new FileCompressionContext(new ZipCompression());
context.compressFile("file.txt");

In this example, the FileCompressionContext is created with a ZipCompression strategy. The compressFile method is then called, which delegates the work to the ZipCompression strategy.

By using the Context class to execute a strategy, you can separate the algorithm from the specific context in which it is used. This makes it easy to switch between strategies at runtime, depending on the requirements of the application, and makes the code easier to maintain and update.

+---------------------+        +------------------+
| CompressionStrategy |<-------| FileCompressionContext |
+---------------------+        +------------------+
| compressFile(fileName: String)                   |
+---------------------+        | +compressionStrategy: CompressionStrategy  
                              | +compressFile(fileName: String)
                              | +setCompressionStrategy(compressionStrategy: CompressionStrategy)
                              +------------------+

IV. Real-life examples of Strategy Design Pattern

Example 1: Sorting algorithms

Here's an example of using the Strategy Design Pattern to implement different sorting algorithms in Java:

public interface SortStrategy {
    void sort(int[] numbers);
}

public class BubbleSort implements SortStrategy {
    public void sort(int[] numbers) {
        // Code to perform bubble sort
    }
}

public class QuickSort implements SortStrategy {
    public void sort(int[] numbers) {
        // Code to perform quick sort
    }
}

public class Sorter {
    private SortStrategy sortStrategy;

    public Sorter(SortStrategy sortStrategy) {
        this.sortStrategy = sortStrategy;
    }

    public void setSortStrategy(SortStrategy sortStrategy) {
        this.sortStrategy = sortStrategy;
    }

    public void sort(int[] numbers) {
        sortStrategy.sort(numbers);
    }
}

In this example, the SortStrategy interface defines the method for sorting an array of numbers. The BubbleSort and QuickSort classes are concrete implementations of the SortStrategy interface and provide their own implementations of the sort method. The Sorter class holds a reference to a SortStrategy object and provides a sort method that delegates the work to the appropriate strategy. The setSortStrategy method can be used to switch between strategies at runtime, depending on the specific requirements of the application.

Here's a UML diagram for this example:

+--------------+        +----------+
| SortStrategy |<-------| Sorter    |
+--------------+        +----------+
| sort(numbers: int[])  | +sortStrategy: SortStrategy
                        | +sort(numbers: int[])
                        | +setSortStrategy(sortStrategy: SortStrategy)
                        +----------+

Example 2: Payment methods in an e-commerce website

Here's an example of using the Strategy Design Pattern to implement different payment methods in an e-commerce website:

public interface PaymentMethod {
    void pay(double amount);
}

public class CreditCard implements PaymentMethod {
    public void pay(double amount) {
        // Code to perform payment using credit card
    }
}

public class PayPal implements PaymentMethod {
    public void pay(double amount) {
        // Code to perform payment using PayPal
    }
}

public class ShoppingCart {
    private PaymentMethod paymentMethod;

    public ShoppingCart(PaymentMethod paymentMethod) {
        this.paymentMethod = paymentMethod;
    }

    public void setPaymentMethod(PaymentMethod paymentMethod) {
        this.paymentMethod = paymentMethod;
    }

    public void checkout(double amount) {
        paymentMethod.pay(amount);
    }
}

In this example, the PaymentMethod interface defines the method for paying an amount. The CreditCard and PayPal classes are concrete implementations of the PaymentMethod interface and provide their own implementations of the pay method. The ShoppingCart class holds a reference to a PaymentMethod object and provides a checkout method that delegates the work to the appropriate payment method. The setPaymentMethod method can be used to switch between payment methods at runtime, depending on the specific requirements of the customer.

Here's a UML diagram for this example:

+-----------------+        +---------------+
| PaymentMethod   |<-------| ShoppingCart   |
+-----------------+        +---------------+
| pay(amount: double)   | +paymentMethod: PaymentMethod
                          | +checkout(amount: double)
                          | +setPaymentMethod(paymentMethod: PaymentMethod)
                          +---------------+

V. Best practices for using the Strategy Design Pattern

Choosing the right strategy

Choosing the right strategy is a critical step in implementing the Strategy Design Pattern. The right strategy depends on the specific requirements of the software being developed, and different strategies may be appropriate for different parts of the system. The choice of strategy should be based on the following considerations:

Problem Domain: The problem domain should be understood well enough to determine the appropriate strategy. For example, in an e-commerce website, different payment methods may be appropriate for different countries.
Performance: The performance of different strategies should be evaluated to determine which one is the best for the specific requirements. For example, a sorting algorithm that is efficient for small datasets may not be the best choice for large datasets.
Reusability: The strategy should be easily reusable in other parts of the system, making it easy to maintain and update.
Flexibility: The strategy should be flexible enough to accommodate future changes in the system.

Once the appropriate strategy has been chosen, it should be implemented and tested thoroughly to ensure that it works correctly in all scenarios. The choice of strategy should also be reviewed regularly to ensure that it remains the best choice for the specific requirements of the software being developed.

Avoiding unnecessary complexity

The Strategy Design Pattern can be a powerful tool for organizing code and promoting flexibility, but it can also add unnecessary complexity to a system if not used properly. Here are some tips for avoiding unnecessary complexity:

Keep it simple: The pattern should only be used when it provides a clear benefit in terms of flexibility or maintainability. If a simple switch statement or if-else construct can achieve the same results, it may be more appropriate.
Minimize the number of strategies: The number of strategies should be kept to a minimum to reduce complexity and increase maintainability. Consider combining strategies where possible to reduce the number of classes and interfaces required.
Clearly define responsibilities: Each strategy should have a clear and well-defined responsibility. A strategy should only be used if it is the right tool for the job, and not just because it is available.
Use encapsulation: The strategy objects should be encapsulated and hidden from the rest of the system to minimize the impact of changes to the strategy. The context should only know about the interface provided by the strategy, and not the specific implementation details.

By following these guidelines, you can help ensure that the Strategy Design Pattern is used effectively and avoids unnecessary complexity in your software.

Using the pattern in a flexible and reusable way

Using the Strategy Design Pattern in a flexible and reusable way requires careful consideration and planning. Here are some tips for using the pattern effectively:

Use interfaces: The strategy interface should be well-defined and flexible enough to accommodate future changes. The interface should clearly define the responsibilities of the strategy and provide a stable and reusable foundation for implementation.
Encapsulate implementation details: The implementation details of the strategy should be encapsulated and hidden from the rest of the system. This helps to minimize the impact of changes to the strategy and promote flexibility.
Consider using the Factory Method pattern: The Factory Method pattern can be used in conjunction with the Strategy Design Pattern to provide a flexible and reusable way of instantiating the correct strategy at runtime.
Use composition over inheritance: The Strategy Design Pattern is based on composition, which allows for greater flexibility and reuse compared to inheritance. Avoid using inheritance to implement strategies, as it can lead to tight coupling and inflexibility.
Provide a clear API: The API provided by the context and strategy should be clear and easy to use, promoting reuse and making it easier for other developers to understand and use the code.

By following these guidelines, you can help ensure that the Strategy Design Pattern is used in a flexible and reusable way, making it easier to maintain and update your software over time.

VI. Conclusion

Summary of the blog

The Strategy Design Pattern is a design pattern used in software development to promote flexibility and maintainability. The pattern is based on the idea of encapsulating algorithms as objects, allowing them to be swapped in and out of a program as needed. This helps to promote code reuse, reduce coupling, and make it easier to change the behavior of a system.

The pattern consists of three main components: the strategy interface, concrete strategy classes, and the context class. The strategy interface defines the responsibilities of the strategy, the concrete strategy classes implement the interface, and the context class uses the interface to execute a strategy.

The choice of strategy is a critical step in implementing the pattern. The right strategy should be chosen based on the specific requirements of the software being developed, including performance, re usability, and flexibility. The pattern can add unnecessary complexity to a system if not used properly, so it's important to follow guidelines for avoiding complexity and using the pattern in a flexible and reusable way.

Final thoughts on the use of the Strategy Design Pattern in software development

The Strategy Design Pattern is a powerful tool in software development that can help to promote flexibility and maintainability. When used correctly, it can make it easier to change the behavior of a system, reduce coupling, and promote code reuse.

However, it's important to use the pattern judiciously and avoid over-complicating the system. The choice of strategy should be based on the specific requirements of the software being developed, and the pattern should only be used if it provides a clear benefit in terms of flexibility or maintainability.

In conclusion, the Strategy Design Pattern is a useful tool for organizing code and solving specific design problems in software development. When used effectively, it can help to promote flexibility, reduce coupling, and make it easier to maintain and update software over time.

Suggestions for further reading

If you're interested in learning more about the Strategy Design Pattern and related topics, here are some suggestions for further reading:

Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides: This classic book provides an in-depth introduction to design patterns and includes an explanation of the Strategy Design Pattern.
Head First Design Patterns by Eric Freeman, Bert Bates, Kathy Sierra, and Elisabeth Robson: This book provides a practical and engaging introduction to design patterns and includes a comprehensive explanation of the Strategy Design Pattern.
Refactoring to Patterns by Joshua Kerievsky: This book provides practical guidance on how to use patterns to improve the quality of existing code and includes a discussion of the Strategy Design Pattern.
Design Patterns Explained: A New Perspective on Object-Oriented Design by Alan Shalloway and James Trott: This book provides a clear and concise introduction to design patterns and includes an explanation of the Strategy Design Pattern.
"Design Patterns" course on Pluralsight: This online course provides a comprehensive introduction to design patterns and includes a section on the Strategy Design Pattern.

These resources will provide you with a deeper understanding of the Strategy Design Pattern and help you to implement it effectively in your software development projects.