Design Patterns | Strategy

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1. References
2. Example
   • The Subject
   • The Strategy
   • The Concrete Strategy
   • The Strategy Context
   • A simple class to test the Strategy design pattern

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1. References
   • Definition: (source: http://en.wikipedia.org/wiki/Strategy_pattern)
     
     The strategy pattern (also known as the policy pattern) enables an algorithm's behavior to be selected at runtime.
     
     The strategy pattern:
     ► defines a family of algorithms,
     ► encapsulates each algorithm, and
     ► makes the algorithms interchangeable within that family.
     
     Strategy lets the algorithm vary independently from clients that use it.
     
     
   • Class diagram: (source: http://en.wikipedia.org/wiki/Strategy_pattern)
     
     
2. Example
   • The Subject:
     
     public class Server { private final String id; public Server(final String id) { this.id = id; } public void doRequest() { System.out.println("Server: " + id); } }
     
   • The Strategy:
     
     public interface ProviderStrategy { Server getInstance(final List<Server> servers); }
     
   • The Concrete Strategy:
     
     
     • Implementation 1:
       
       public class RandomStrategy implements ProviderStrategy { private final Random random = new Random(); public Server getInstance(final List<Server> servers) { final int index = random.nextInt(servers.size()); return servers.get(index); } }
       
     • Implementation 2:
       
       public class RoundRobinStrategy implements ProviderStrategy { private final AtomicInteger index = new AtomicInteger(0); public Server getInstance(final List<Server> servers) { final int index = Math.abs(this.index.getAndIncrement()); return servers.get(index % servers.size()); } }
       
   • The Strategy Context:
     
     public class ProviderStrategyContext { private final ProviderStrategy providerStrategy; public ProviderStrategyContext(ProviderStrategy providerStrategy) { this.providerStrategy = providerStrategy; } public void doRequest(final List<Server> servers) { providerStrategy.getInstance(servers).doRequest(); } }
     
   • A simple class to test the Strategy design pattern:
     
     public class StrategyPatternTest { public static void main(String[] args) { // list of available servers final List<Server> servers = new ArrayList<>(); servers.add(new Server("server1")); servers.add(new Server("server2")); servers.add(new Server("server3")); // Strategies: RoundRobin, Random final ProviderStrategyContext providerStrategyContext1 = new ProviderStrategyContext(new RoundRobinStrategy()); final ProviderStrategyContext providerStrategyContext2 = new ProviderStrategyContext(new RandomStrategy()); System.out.println("-- RoundRobin Strategy:"); IntStream.iterate(0, i -> i + 2).limit(6).forEach(i -> providerStrategyContext1.doRequest(servers)); System.out.println("-- Random Strategy:"); IntStream.iterate(0, i -> i + 2).limit(6).forEach(i -> providerStrategyContext2.doRequest(servers)); } }