command.md

The Command Pattern

The pattern belongs to the behavioral category of the design patterns.

Idea

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Explanation

Wikipedia says:

In object-oriented programming, the command pattern is a behavioral design pattern in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time. This information includes the method name, the object that owns the method and values for the method parameters.

In plain words:

Using command objects makes it easier to construct general components that need to delegate, sequence or execute method calls at a time of their choosing without the need to know the class of the method or the method parameters.

Class Diagram

The class diagram will be:

Example

The task:

Let's consider to create an action command that will encapsulate a storage and provide an action to perform.

Let's create a storage object:

final class Storage {
    private int value = 0;

    int push() {
        value = 1000;
        return 1;
    }

    int pop() {
        return value;
    }
}

And then let's create a specification of an action:

public interface Action {

    int execute();
}

We need an executor to register and perform actions:

final class ActionExecutor {
    private final Map<String, Action> actions = new HashMap<>();

    public void register(final String actionName, final Action action) {
        actions.put(actionName, action);
    }

    public int execute(final String actionName) {
        final var action = actions.get(actionName);
        if (action == null) {
            throw new IllegalStateException("No action registered: " + actionName);
        }

        return action.execute();
    }
}

Implementations of actions will look like:

final class PopAction implements Action {
    private final Storage storage;

    public PopAction(final Storage storage) {
        this.storage = storage;
    }

    @Override
    public int execute() {
        return storage.pop();
    }
}

And:

final class PushAction implements Action {
    private final Storage storage;

    public PushAction(final Storage storage) {
        this.storage = storage;
    }

    @Override
    public int execute() {
        return storage.push();
    }
}

And then it can be used as:

final var storage = new Storage();

final var executor = new ActionExecutor();
executor.register("push", storage::push);
executor.register("pop", storage::pop);

assertEquals(0, executor.execute("pop"));
assertEquals(1, executor.execute("push"));
assertEquals(1000, executor.execute("pop"));

More Examples

• java.lang.Runnable
• Netflix Hystrix
• javax.swing.Action

Links

• Command Pattern