解决的问题

所有做过前端的人都应该使用过该模式。你要开发一个界面，界面由选项列表（OptionList），文本框（TextBox）和按钮（Button）组成。在选项列表完成选择后，文本框就会显示选择的内容，按钮会变成可点击的。

代码

EventHandler:

package com.cong.designpattern.mediator;

public interface EventHandler {
    public void handle();
}

UIControl:

package com.cong.designpattern.mediator;

import java.util.ArrayList;
import java.util.List;

public class UIControl {
    private List<EventHandler> eventHandlers = new ArrayList<>();
    public void addEventHandler(EventHandler eventHandler) {
        this.eventHandlers.add(eventHandler);
    }
    public void removeEventHandler(EventHandler eventHandler) {
        this.eventHandlers.remove(eventHandler);
    }
    protected void notifyEventHandlers() {
        for (EventHandler eventHandler : this.eventHandlers) eventHandler.handle();
    }
}

OptionList:

package com.cong.designpattern.mediator;

public class OptionList extends UIControl {
    private String selection;
    public String getSelection() {
        return selection;
    }
    public void setSelection(String selection) {
        this.selection = selection;
        this.notifyEventHandlers();
    }
}

TextBox:

package com.cong.designpattern.mediator;

public class TextBox extends UIControl {
    private String content;
    public String getContent() {
        return content;
    }
    public void setContent(String content) {
        this.content = content;
        this.notifyEventHandlers();
    }
}

Button:

package com.cong.designpattern.mediator;

public class Button extends UIControl{
    private boolean isEnabled = false;
    public boolean isEnabled() {
        return isEnabled;
    }
    public void setEnabled(boolean enabled) {
        isEnabled = enabled;
        this.notifyEventHandlers();
    }
}

DialogBox:

package com.cong.designpattern.mediator;

public class DialogBox {
    private Button button = new Button();
    private TextBox textBox = new TextBox();
    private OptionList optionList = new OptionList();
    public DialogBox() {
        this.optionList.addEventHandler(() -> {
            String selection = this.optionList.getSelection();
            this.textBox.setContent(selection);
            this.button.setEnabled(selection != null && !selection.isEmpty());
        });
    }
    public void simulateUserInteraction() {
        this.optionList.setSelection("Hello Mediator Pattern!");
        System.out.println("Text box content: " + this.textBox.getContent());
        System.out.println("Button is enable: " + this.button.isEnabled());
    }
}

Test code:

DialogBox dialogBox = new DialogBox();
dialogBox.simulateUserInteraction();

UML

Mediator Pattern UML