Behavioral Design Patterns: Mediator

Previously, we had a look at the iterator pattern. The mediator pattern is very different in what it tries to achieve. It is one of the behavioral patterns and its purpose is to alter the way objects communicate with each other. Instead of the objects communicating with each other directly, the mediator will handle the objects' interactions.

For example, imagine the scenario of a financial exchange. You do want to trade and buy but you don't buy directly from the one that makes the offer. Instead, the exchange is in the middle, in order for you to make the transaction.

People would like to sell and buy. This shall be facilitated by the exchange. First, you have the Order object.

package com.gkatzioura.design.behavioural.mediator;

public class Order {

    private String stock;
    private Integer quantity;
    private Double price;

    public String getStock() {
        return stock;
    }

    public void setStock(String stock) {
        this.stock = stock;
    }

    public Integer getQuantity() {
        return quantity;
    }

    public void setQuantity(Integer quantity) {
        this.quantity = quantity;
    }

    public Double getPrice() {
        return price;
    }

    public void setPrice(Double price) {
        this.price = price;
    }

}

The next object will be the FinancialEntity that sells the stocks.

package com.gkatzioura.design.behavioural.mediator;

public class FinancialEntity {

    public boolean sell(Order order) {

        /**
         * Supposing the sale was successful return true
         */
        return true;
    }

}

Then, we create the Exchange object. We won't get further into commissions but imagine that things can be way more complex. The Exchange is actually our mediator.

package com.gkatzioura.design.behavioural.mediator;

public class Exchange {

    private FinancialEntity financialEntity;

    public Exchange(FinancialEntity financialEntity) {
        this.financialEntity = financialEntity;
    }

    public void serve(Order order) {

        /**
         * Choose the financial entity suitable for the order
         */
        financialEntity.sell(order);
    }

}

And the last step is creating the Trader object.

package com.gkatzioura.design.behavioural.mediator;

public class Trader {

    private Exchange exchange;

    public Trader(Exchange exchange) {
        this.exchange = exchange;
    }

    public void buy(String stock,Integer quantity,Double price) {
        Order order = new Order();
        order.setStock(stock);
        order.setQuantity(quantity);
        order.setPrice(price);
        exchange.serve(order);
    }

}

As you can see, the Trader object is not interacting directly with the financial entity that provides the stocks.

Let's put them all together in the main class.

package com.gkatzioura.design.behavioural.mediator;

public class Mediator {

    public static void main(String[] args) {

        final FinancialEntity financialEntity = new FinancialEntity();
        final Exchange exchange = new Exchange(financialEntity);
        Trader trader = new Trader(exchange);
        trader.buy("stock_a",2,32.2d);
    }
}

That's it, you just used the mediator pattern for an exchange application! You can also find the source code on GitHub.