Message
public class Message {
	private String from;
	private String to;
	private String content;
	public String getFrom() {
		return from;
	}
	public void setFrom(String from) {
		this.from = from;
	}
	public String getTo() {
		return to;
	}
	public void setTo(String to) {
		this.to = to;
	}
	public String getContent() {
		return content;
	}
	public void setContent(String content) {
		this.content = content;
	}
	
}

QQInterface
public interface QQInterface {
	void sendMsg(Message msg);
	Message receiveMsg(String name);

}

QQServer
import java.util.HashMap;
import java.util.Map;

public class QQServer {
	static Map<String,Message> serverMap = new HashMap<String,Message>();
	public static void serverReceiveMsg(Message msg){
		serverMap.put(msg.getTo(),msg);
		System.out.println(msg.getFrom()+" "+" "+msg.getContent());
	}
	public static Message asked(String name){
		Message msg = serverMap.get(name);
		return msg;
	}
}

QQ1
public class QQ1 implements QQInterface{
	public static void main(String[] args){
		Message msg = new Message();
		msg.setFrom("QQ1");
		msg.setTo("QQ2");
		msg.setContent("I am QQ1 。");
		QQ1 q = new QQ1();
		q.sendMsg(msg);
	}
	public void sendMsg(Message msg){
		QQServer.serverReceiveMsg(msg);
	}
	@Override
	public Message receiveMsg(String name) {
		Message msg = QQServer.asked(name);
		System.out.println("******"+msg.getContent());
		return msg;
	}

}

QQ2
public class QQ2 implements QQInterface{
	public static void main(String[] args){
		Message msg = new Message();
		msg.setFrom("QQ2");
		msg.setTo("QQ1");
		msg.setContent("I am QQ2 。");
		QQ2 q = new QQ2();
		q.sendMsg(msg);
	}
	public void sendMsg(Message msg){
		QQServer.serverReceiveMsg(msg);
	}
	@Override
	public Message receiveMsg(String name) {
		Message msg = QQServer.asked(name);
		System.out.println("******"+msg.getContent());
		return msg;
	}
}

Test
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Test {
	public static void main(String[] args){
		Map<String,List<Message>> QQMap = new HashMap<String,List<Message>>();
		List<Message> msgList = new ArrayList<Message>();
		
//		第一次發送消息
		Message msg1 = new Message();
		msg1.setFrom("QQ1");
		msg1.setTo("QQ2");
		msg1.setContent("HI!");
		msgList.add(msg1);
		QQMap.put(msg1.getTo(), msgList);
//		第二次發送消息
		Message msg2 = new Message();
		msg2.setFrom("QQ1");
		msg2.setTo("QQ2");
		msg2.setContent("Hello World !");
		msgList.add(msg2);
		QQMap.put(msg2.getTo(), msgList);
		
        List<Message> qq2MsgList = QQMap.get("QQ2");
		for(Message m:qq2MsgList){//打印QQ1發給QQ2消息
			System.out.println(m.getContent());
		}
	}
}