中介者模式：只有一个仲裁者

中介者模式是一种行为型设计模式，它的主要目的是减少多个对象之间的直接通信，并通过一个中介者对象来协调它们之间的交互。这样可以降低系统的复杂性，增加可维护性和灵活性。

示例代码

// 中介者接口
interface Mediator {
    void sendMessage(String message, Colleague colleague);
}

// 抽象同事类
abstract class Colleague {
    protected Mediator mediator;

    public Colleague(Mediator mediator) {
        this.mediator = mediator;
    }

    public abstract void sendMessage(String message);

    public abstract void receiveMessage(String message);
}

// 具体同事类A
class ConcreteColleagueA extends Colleague {
    public ConcreteColleagueA(Mediator mediator) {
        super(mediator);
    }

    @Override
    public void sendMessage(String message) {
        mediator.sendMessage(message, this);
    }

    @Override
    public void receiveMessage(String message) {
        System.out.println("ConcreteColleagueA received message: " + message);
    }
}

// 具体同事类B
class ConcreteColleagueB extends Colleague {
    public ConcreteColleagueB(Mediator mediator) {
        super(mediator);
    }

    @Override
    public void sendMessage(String message) {
        mediator.sendMessage(message, this);
    }

    @Override
    public void receiveMessage(String message) {
        System.out.println("ConcreteColleagueB received message: " + message);
    }
}

// 具体中介者类
class ConcreteMediator implements Mediator {
    private ConcreteColleagueA colleagueA;
    private ConcreteColleagueB colleagueB;

    public void setColleagueA(ConcreteColleagueA colleagueA) {
        this.colleagueA = colleagueA;
    }

    public void setColleagueB(ConcreteColleagueB colleagueB) {
        this.colleagueB = colleagueB;
    }

    @Override
    public void sendMessage(String message, Colleague colleague) {
        if (colleague == colleagueA) {
            colleagueB.receiveMessage(message);
        } else if (colleague == colleagueB) {
            colleagueA.receiveMessage(message);
        }
    }
}

// 测试代码
public class MediatorPatternDemo {
    public static void main(String[] args) {
        ConcreteMediator mediator = new ConcreteMediator();

        ConcreteColleagueA colleagueA = new ConcreteColleagueA(mediator);
        ConcreteColleagueB colleagueB = new ConcreteColleagueB(mediator);

        mediator.setColleagueA(colleagueA);
        mediator.setColleagueB(colleagueB);

        colleagueA.sendMessage("Hello, I am A.");
        colleagueB.sendMessage("Hi, I am B.");
    }
}

Mediator接口定义了一个方法sendMessage，用于同事之间的通信。Colleague抽象类表示同事类，其中sendMessage方法用于向中介者发送消息，receiveMessage方法用于接收来自中介者的消息。ConcreteColleagueA和ConcreteColleagueB是具体的同事类，它们实现了Colleague类，并重写了其抽象方法。ConcreteMediator是具体的中介者类，它实现了Mediator接口，并持有对所有同事对象的引用，以便在收到消息时将其转发给正确的同事对象。

简单来说，就是Colleague需要持有Mediator对象，并直接与Mediator对象通信，Mediator对象通过通信内容进行相关处理。其中Mediator对象也需要持有ConcreteColleague[X]对象;