Strategy Pattern

The Strategy Pattern is a fundamental design pattern that promotes flexibility and maintainability in software design by defining a family of algorithms, encapsulating each one, and making them interchangeable. This pattern is particularly valuable when dealing with varying algorithms that need to be selected dynamically at runtime.

Core Components

1. Context

   • Maintains a reference to the current strategy object
   • Provides an interface for clients to interact with the strategy
   • May pass necessary data to the strategy during execution

2. Strategy Interface

   • Declares a common interface for all supported algorithms
   • Typically defines a single method that encapsulates the algorithm

3. Concrete Strategies

   • Implement specific algorithms while adhering to the strategy interface
   • Can be swapped dynamically without affecting the context or client code

Key Benefits

• Promotes encapsulation of algorithms
• Enables loose coupling between algorithm implementation and code that uses the algorithm
• Facilitates unit testing by allowing mock implementations
• Supports the Open-Closed Principle by allowing new strategies without modifying existing code

Common Use Cases

The Strategy Pattern is frequently employed in scenarios such as:

• Payment processing systems (different payment methods)
• Compression algorithms (various compression techniques)
• Sorting implementations (different sorting algorithms)
• Authentication mechanisms (multiple auth strategies)

Implementation Example

// Strategy interface
interface SortStrategy {
    void sort(int[] array);
}

// Concrete strategies
class QuickSort implements SortStrategy {
    public void sort(int[] array) {
        // QuickSort implementation
    }
}

class MergeSort implements SortStrategy {
    public void sort(int[] array) {
        // MergeSort implementation
    }
}

// Context
class Sorter {
    private SortStrategy strategy;
    
    public void setStrategy(SortStrategy strategy) {
        this.strategy = strategy;
    }
    
    public void performSort(int[] array) {
        strategy.sort(array);
    }
}

Related Patterns

The Strategy Pattern often works in conjunction with:

• Factory Pattern - for creating strategy objects
• Template Method Pattern - offering an alternative approach to algorithm variation
• Command Pattern - sharing similar encapsulation principles

Considerations

When implementing the Strategy Pattern, consider:

1. Performance Impact

   • Strategy objects may introduce additional memory overhead
   • Dynamic strategy switching might affect runtime performance

2. Complexity Trade-offs

   • Increases number of classes in the system
   • May be overkill for simple algorithmic variations

3. Data Sharing

   • Consider how data will be passed between context and strategy
   • Evaluate whether strategies need access to context state

Best Practices

• Keep strategy interfaces focused and cohesive
• Document the purpose and constraints of each strategy
• Consider using lambda expressions for simple strategies in supported languages
• Implement sensible default strategies where appropriate

The Strategy Pattern embodies the principle of composition over inheritance and promotes flexible, maintainable code design by separating the "what" from the "how" in algorithm implementation.