Question

Link to command pattern

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);
}

Answer 1

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.