Why does the client have no reference to the invoker in the Command pattern?

Question

Why does the client have no reference to the invoker when it has references to receivers and concretecommands?

public static void main(String[] args) 
{
    StockTrade stock = new StockTrade();
    BuyStockOrder bsc = new BuyStockOrder (stock);
    SellStockOrder ssc = new SellStockOrder (stock);
    Agent agent = new Agent(); //<-- The invoker is right there yet it is not
                               // in the diagram?

    agent.placeOrder(bsc);
    agent.placeOrder(ssc);
}

score 2 · Accepted Answer · answered Jan 18 '10 at 19:34

2

The wiki article explains this as:

The client instantiates the command object and provides the information required to call the method at a later time. The invoker decides when the method should be called. The receiver is an instance of the class that contains the method's code.

answered Jan 18 '10 at 19:34

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Dances around the root of the question, the point is that Client and Invoker can be decoupled, the "main" just happens create and call them both. – Ted Johnson Jan 19 '10 at 01:48

score 1 · Answer 2 · answered Jan 18 '10 at 19:44

If I understand you question correctly, the answer is this.

There is a reference in that diagram that can be traced from the Client to Invoker. If you look you can see there is a dotted line from Client to ConcreteCommand called "instantiate" and then a line from ConcreteCommand to Command and then a line from Command to Invoker.

As I understand it -- this dotted line represents creating the objects for later use (as you can see from the code -- they are created in main.)

Why does the client have no reference to the invoker in the Command pattern?

2 Answers2