Difference between Strategy pattern and Command pa

问题:

What is the difference between the Strategy pattern and the Command pattern? I am also looking for some examples in Java.

回答1:

Typically the Command pattern is used to make an object out of what needs to be done -- to take an operation and its arguments and wrap them up in an object to be logged, held for undo, sent to a remote site, etc. There will tend to be a large number of distinct Command objects that pass through a given point in a system over time, and the Command objects will hold varying parameters describing the operation requested.

The Strategy pattern, on the other hand, is used to specify how something should be done, and plugs into a larger object or method to provide a specific algorithm. A Strategy for sorting might be a merge sort, might be an insertion sort, or perhaps something more complex like only using merge sort if the list is larger than some minimum size. Strategy objects are rarely subjected to the sort of mass shuffling about that Command objects are, instead often being used for configuration or tuning purposes.

Both patterns involve factoring the code and possibly parameters for individual operations out of the original class that contained them into another object to provide for independent variability. The differences are in the use cases encountered in practice and the intent behind each pattern.

回答2:

Words are already given. Here is the difference in concrete code.

public class ConcreteStrategy implements BaseStrategy {

    @Override
    public void execute(Object argument) {
        // Work with passed-in argument.
    }

}

---

public class ConcreteCommand implements BaseCommand {

    private Object argument;

    public ConcreteCommand(Object argument) {
        this.argument = argument;
    }

    @Override
    public void execute() {
        // Work with own state.
    }

}

回答3:

Strategy - Quicksort or Mergesort [algo change]

Command - Open or Close [action change]

回答4:

The main difference is , the command does some action over the object. It may change the state of an object.

While Strategy decides how to process the object. It encapsulates some business logic.

回答5:

Strategy pattern is useful when you have multiple implementations (algorithms) for a given feature and you want to change the algorithm at runtime depending on parameter type.

One good example from HttpServlet code:

service() method will direct user's request to doGet() or doPost() or some other method depending on method type.

protected void service(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
    {
    String method = req.getMethod();

    if (method.equals(METHOD_GET)) {
        long lastModified = getLastModified(req);
        if (lastModified == -1) {
        // servlet doesn't support if-modified-since, no reason
        // to go through further expensive logic
        doGet(req, resp);
        } else {
        long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
        if (ifModifiedSince < (lastModified / 1000 * 1000)) {
            // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
            maybeSetLastModified(resp, lastModified);
            doGet(req, resp);
        } else {
            resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
        }
        }

    } else if (method.equals(METHOD_HEAD)) {
        long lastModified = getLastModified(req);
        maybeSetLastModified(resp, lastModified);
        doHead(req, resp);

    } else if (method.equals(METHOD_POST)) {
        doPost(req, resp);

    } else if (method.equals(METHOD_PUT)) {
        doPut(req, resp);   

    } else if (method.equals(METHOD_DELETE)) {
        doDelete(req, resp);

    } else if (method.equals(METHOD_OPTIONS)) {
        doOptions(req,resp);

    } else if (method.equals(METHOD_TRACE)) {
        doTrace(req,resp);

    } else {
        //
        // Note that this means NO servlet supports whatever
        // method was requested, anywhere on this server.
        //

        String errMsg = lStrings.getString("http.method_not_implemented");
        Object[] errArgs = new Object[1];
        errArgs[0] = method;
        errMsg = MessageFormat.format(errMsg, errArgs);

        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
    }
    }

Salient features of Strategy pattern

1. It's a behavioural pattern
2. It's based on delegation
3. It changes guts of the object by modifying method behaviour
4. It's used to switch between family of algorithms
5. It changes the behaviour of the object at run time

Command pattern is used to enable loose coupling between Invoker and Receiver. Command, ConcreteCommand, Receiver, Invoker and Client are major components of this pattern.

Different Receivers will execute same Command through Invoker & Concrete Command but the implementation of Command will vary in each Receiver.

e.g. You have to implement "On" and "Off" functionality for TV & DVDPlayer. But TV and DVDPlayer will have different implementation for these commands.

Have a look at below posts with code examples :

Real World Example of the Strategy Pattern

Using Command Design pattern