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Strategy Pattern

Introduction#

The Strategy Design Pattern is a popular design pattern used in object-oriented programming. It is a behavioral pattern that allows the behavior of an object to be selected at runtime. The Strategy pattern is based on the idea of encapsulating a family of algorithms into separate classes that implement a common interface. This makes it easy to switch between different algorithms at runtime, without having to change the code that uses them.

Strategy pattern

Java is a popular programming language that is widely used in the development of enterprise applications. The Strategy Design Pattern is a key pattern in Java that is used to implement complex algorithms in a clean and efficient way. Java 8 introduced lambdas, which made it even easier to implement the Strategy pattern in Java. By using lambdas, developers can reduce the verbosity of their code, making it more readable and maintainable. In this article, we will explore the Strategy Design Pattern in Java, and how it can be used to implement complex algorithms in a clean and efficient way. We will also look at how the pattern has evolved over time, and how it can be used with Java 8 lambdas.

Understanding Strategy Design Pattern#

The Strategy Design Pattern is a behavioral design pattern that allows an algorithm's behavior to be selected at runtime. This pattern is part of the Design Patterns developed by the Gang of Four, which are a set of solutions to common software design problems.

The Strategy Design Pattern is used when there are multiple algorithms that can be used to solve a problem, and the choice of algorithm depends on the context. By encapsulating each algorithm in a separate class, the Strategy Design Pattern allows the algorithms to be interchangeable at runtime.

The Strategy Design Pattern consists of three primary components:

  • Context: The object that will delegate its behavior to one of the contained strategies. The context maintains a reference to a strategy object and interacts with it through a common interface.

  • Strategy Interface: The interface that defines the behavior for all strategies. Each strategy implements this interface, providing a different implementation of the algorithm.

  • Concrete Strategies: The classes that implement the Strategy Interface. Each concrete strategy encapsulates a different algorithm that can be used to solve the problem.

The Strategy Design Pattern is particularly useful when there are multiple variations of an algorithm, and the choice of algorithm depends on the context. By encapsulating each variation in a separate class, the Strategy Design Pattern allows the variations to be interchangeable at runtime.

In Java, the Strategy Design Pattern can be implemented using interfaces and abstract classes. The Strategy Interface is defined as an interface, and each concrete strategy is implemented as a separate class that implements this interface. The Context class maintains a reference to the Strategy Interface, and can be passed different concrete strategies at runtime.

Overall, the Strategy Design Pattern is a powerful tool for implementing flexible and extensible algorithms in Java. By encapsulating each algorithm in a separate class, the Strategy Design Pattern allows the algorithms to be easily interchangeable at runtime, making it a valuable tool for solving complex software design problems.

Core Components of Strategy Design Pattern#

Strategy Design Pattern is a behavioral design pattern that enables an object to alter its behavior dynamically by choosing from a family of algorithms at runtime. This pattern defines a set of encapsulated algorithms that can be interchanged to carry out a specific task. The pattern consists of four core components: Strategy Interface, Concrete Strategies, Context Object, and Client Code.

Strategy Interface#

The Strategy Interface is an abstract interface that defines the set of operations that the Concrete Strategies must implement. It declares a set of methods that the Concrete Strategies will implement. The interface allows the Context Object to interact with all Concrete Strategies in a uniform way, without knowing their specific implementation details.

Concrete Strategies#

Concrete Strategies are the concrete implementations of the Strategy Interface. Each Concrete Strategy implements the operations defined in the Strategy Interface. The Concrete Strategies encapsulate the algorithm that the Context Object uses to perform a specific task. The Concrete Strategies are interchangeable and can be switched at runtime.

Context Object#

The Context Object is the object that uses the Concrete Strategies to perform a specific task. The Context Object maintains a reference to a Concrete Strategy object and interacts with it through the Strategy Interface. The Context Object delegates the task to the Concrete Strategy object and does not know the specific implementation details of the Concrete Strategy.

Client Code#

The Client Code is the code that creates the Context Object and sets the Concrete Strategy object that the Context Object will use. The Client Code creates the Concrete Strategies and sets them into the Context Object. The Client Code interacts with the Context Object to perform the task.

In summary, the Strategy Design Pattern is a useful pattern for situations where an object's behavior needs to be altered dynamically at runtime. The pattern consists of four core components: Strategy Interface, Concrete Strategies, Context Object, and Client Code. The Strategy Interface is an abstract interface that defines the set of operations that the Concrete Strategies must implement. Concrete Strategies are the concrete implementations of the Strategy Interface. The Context Object is the object that uses the Concrete Strategies to perform a specific task. The Client Code creates the Concrete Strategies and sets them into the Context Object.

Working of Strategy Design Pattern#

The Strategy Design Pattern is a behavioral design pattern that allows the behavior of an object to be selected at runtime. It is based on the idea of encapsulating a family of algorithms into separate classes that implement a common interface. The pattern is used when there are multiple algorithms for a specific task, and the client decides the actual implementation to be used at runtime.

The pattern consists of three main entities: the Context, the Strategy, and the ConcreteStrategy. The Context is the object that utilizes the Strategy interface to execute a particular algorithm. The Strategy interface defines the common interface for all the ConcreteStrategy classes. The ConcreteStrategy classes implement the Strategy interface and provide the actual implementation of the algorithm.

The Context object is responsible for selecting the appropriate ConcreteStrategy object at runtime. The Strategy object is passed to the Context object, which then executes the algorithm using the execute() method of the Strategy object. The execute() method of the ConcreteStrategy object is then called, which provides the actual implementation of the algorithm.

The ConcreteStrategy objects are interchangeable at runtime, allowing the Context object to switch between different algorithms as required. This makes the pattern very flexible, as it allows new algorithms to be added without modifying the Context object.

In summary, the Strategy Design Pattern allows for the separation of algorithm implementation from the client code. By encapsulating the algorithms in separate classes, the pattern allows for interchangeable algorithms to be used at runtime. This makes the pattern very flexible and easy to extend.

Implementing Strategy Design Pattern in Java#

Strategy Design Pattern is a behavioral design pattern used to define a family of algorithms, encapsulate each one of them, and make them interchangeable. In Java, the Strategy Design Pattern is implemented using interfaces and abstract classes.

To implement the Strategy Design Pattern in Java, the first step is to define an interface that represents the strategy. This interface defines the method(s) that the client will use to interact with the strategy. The next step is to define concrete classes that implement the interface. Each concrete class represents a different strategy that the client can choose from.

In Java 8, the implementation of the Strategy Design Pattern has become even easier with the introduction of lambdas. Lambdas are anonymous functions that can be used in place of a single-method interface. This means that instead of creating a separate class for each strategy, a lambda can be used to define the strategy inline.

To use lambdas in the implementation of the Strategy Design Pattern, the interface representing the strategy must be a functional interface. A functional interface is an interface that has only one abstract method. The @FunctionalInterface annotation can be used to indicate that an interface is a functional interface.

In addition to defining the strategy and its implementation, the client must also have a way to set the current strategy. This can be done using a setter method. The setter method takes an instance of the strategy interface and sets it as the current strategy.

Overall, the implementation of the Strategy Design Pattern in Java is a straightforward process that involves defining an interface for the strategy, implementing concrete classes for each strategy, and using a setter method to set the current strategy. With the introduction of lambdas in Java 8, the verbosity of the code has been reduced, making the implementation even easier.

Advantages of Using Strategy Design Pattern#

The Strategy Design Pattern provides several advantages over other design patterns. Here are some of the key benefits of using the Strategy Design Pattern:

Flexibility#

One of the primary benefits of using the Strategy Design Pattern is that it provides flexibility to the code. By encapsulating the behavior of an object into different strategies, the code becomes more flexible and easier to modify. It allows developers to change the behavior of an algorithm at runtime without changing the implementation of the algorithm. This makes the code more adaptable to changes in requirements.

Encapsulated#

The Strategy Design Pattern is based on the principle of encapsulation. Each algorithm is encapsulated into a separate class that implements a common interface. This makes it easier to manage and maintain the code. It also ensures that each algorithm is independent and can be modified without affecting the other algorithms.

Clean and Maintainable#

The Strategy Design Pattern promotes clean and maintainable code. It separates the algorithm implementation from the client code, making it easier to understand and modify. It also reduces the amount of conditional statements in the code, making it more readable and less error-prone.

Reusability#

The Strategy Design Pattern promotes code reusability. Each algorithm is encapsulated into a separate class, making it easier to reuse the code in other parts of the application. This reduces the amount of duplicate code in the application, making it easier to maintain and modify.

Open-Closed Principle#

The Strategy Design Pattern follows the Open-Closed Principle, which states that a class should be open for extension but closed for modification. The Strategy Design Pattern achieves this by encapsulating the behavior of an object into separate classes. This makes it easier to add new algorithms without modifying the existing code.

In summary, the Strategy Design Pattern provides several advantages over other design patterns. It promotes flexibility, encapsulation, clean and maintainable code, reusability, and follows the Open-Closed Principle. These benefits make it a popular choice for developers working on complex projects.

Practical Example: Payment Methods#

The Strategy Design Pattern can be applied to various scenarios, including payment methods in a shopping cart. This section will provide a practical example of how to implement payment strategies using the Strategy Design Pattern in Java.

Setting Up Payment Strategies#

To implement payment strategies, a PaymentStrategy interface is created to define the behavior of all payment methods. This interface has a single method, pay(), that takes in parameters such as price, name, email, and any other necessary information for the payment method.

Next, concrete classes are created for each payment method, such as Paypal and CreditCard, that implement the PaymentStrategy interface and define how the payment is processed for each method. For example, the CreditCard class may require parameters such as the credit card number, CVV, and password.

Shopping Cart Implementation#

In the shopping cart implementation, the PaymentStrategy interface is used to allow the customer to choose their preferred payment method. The ShoppingCart class has a pay(PaymentStrategy paymentStrategy) method that takes in a PaymentStrategy parameter and delegates the payment behavior to the selected payment method.

Sorting the Shopping Cart#

To sort the items in the shopping cart, the Collections.sort() method can be used along with a Comparator interface. The ShoppingCart class can implement the Comparator interface and define the sorting algorithm based on the desired criteria, such as price or name.

Overall, implementing payment methods using the Strategy Design Pattern allows for flexibility in adding and modifying payment methods without having to modify existing code. Additionally, sorting the shopping cart using the Collections.sort() method and a Comparator interface allows for easy customization of the sorting algorithm.

Comparing Strategy Design Pattern with Other Patterns#

Strategy vs State Pattern#

The State pattern and the Strategy pattern are both behavioral design patterns that allow objects to be flexible and interchangeable. However, they differ in their purpose and implementation.

The State pattern is used when an object needs to change its behavior based on its internal state. This pattern encapsulates the state-specific behavior into separate classes, which can be changed at runtime. In contrast, the Strategy pattern is used when an object needs to change its behavior based on external conditions. This pattern encapsulates the behavior into separate classes, which can be selected at runtime.

In the State pattern, the object's behavior changes automatically when its state changes. In the Strategy pattern, the object's behavior changes only when a new strategy is selected.

Strategy vs Policy Pattern#

The Policy pattern and the Strategy pattern are both used to encapsulate algorithms and make them interchangeable. However, they differ in their focus and implementation.

The Policy pattern is used to encapsulate a set of rules or policies that govern an object's behavior. These policies are usually defined in a separate class and can be changed at runtime. In contrast, the Strategy pattern is used to encapsulate a specific algorithm or strategy that an object can use.

In the Policy pattern, the object's behavior is determined by a set of policies that are enforced at runtime. In the Strategy pattern, the object's behavior is determined by a selected strategy.

Overall, the State pattern and the Policy pattern are more focused on encapsulating rules and policies, while the Strategy pattern is more focused on encapsulating algorithms and strategies. However, all three patterns provide flexibility and interchangeability to objects, making them valuable tools in software design.

Conclusion#

In conclusion, the Strategy Design Pattern is a powerful tool for object-oriented programming that allows developers to encapsulate a task in an abstract class and make it interchangeable with other components at runtime. This pattern promotes flexibility and ease of maintenance, making it a popular choice for software design patterns.

By using delegates and lambda expressions, developers can create a flexible class hierarchy that can be easily extended and modified to suit changing requirements. This approach is particularly useful for data structures, such as databases, where the number of possible relationships and interactions can be difficult to count.

One example of the Strategy Design Pattern in action is the PalPay payment method, which allows users to choose from a range of payment options at checkout. By encapsulating each payment method in a separate class, developers can easily add or remove options without affecting the rest of the code.

Another example is in game programming, where the Strategy Design Pattern can be used to create different AI behaviors for NPCs. By encapsulating each behavior in a separate class, developers can easily switch between different strategies depending on the player's actions.

Overall, the Strategy Design Pattern is a valuable tool for any developer looking to create flexible, maintainable code. By using abstract classes, delegates, and lambda expressions, developers can create powerful, adaptable systems that can evolve with changing requirements.