Strategy design pattern | Nagacharan Teja Tangirala

Strategy pattern helps selecting from multiple implementations based on a certain policy. A simple example is the contextual based search selected that selects from the multiple available search algorithms. If the data is sorted, then binary search can be used. Otherwise, a linear search is unavoidable.

Consider a simple tool that needs the search feature, which is represented by an interface:

interface SearchStrategy {
    int searchKey([T] data, T key);
}

There can be multiple search strategies:

class BinarySearch implements SearchStrategy {

    ...
    @Override
    int searchKey([T] data, T key) {
        // Implement binary search to find the position of the key.
        return position;
    }
    ...
}

class LinearSearch implements SearchStrategy {

    ...
    @Override
    int searchKey([T] data, T key) {
        // Implement linear search to find the position of the key.
        return position;
    }
    ...
}

Now consider a simple Service that needs this search feature. Service does not bother about which search strategy is being used, it just calls the search function.

class Service {

    ...
    private SearchStrategy searchStrategy;
    private boolean isDataSorted;
    ...
    void searchForKey([T] data, T key) {
        this.searchStrategy.searchKey(data, key);
    }
}

Handling the switch between different search strategies is dependent on Context. The Context is not explicitly handled in the Service. Instead, Policy is used to monitor the Context and accordingly initialize the SearchStrategy.

class ServicePolicy {

    ...
    private Service service;
    ...
    ServicePolicy(Service service) {
        this.service = service;
    }

    void configure() {
        if (service.isDataSorted()) {
            service.setStrategy(new BinarySearch());
        } else {
            service.setStrategy(new LinearSearch());
        }
    ...
}

Each time a new Service is created, a corresponding ServicePolicy can be created. Depending on the state of the Service, an appropriate SearchStrategy instance is set to the Service. The Service can then just call the searchKey() method and it will call the appropriate concrete implementation.

main() {

    ...
    Service service = new Service();
    ServicePolicy policy = new ServicePolicy(service);
    policy.configure();
    ...
}

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