Java arraylist 元素更新，Jade答案

【问题标题】：Java arraylist elements update, JadeJava arraylist 元素更新，Jade
【发布时间】：2021-05-09 21:08:35
【问题描述】：

现在我想构建一个 ArrayList 消息队列，它会自动填充来自 MQTT 代理的消息。例如，如果从 MQTT 发送了 3 条消息，那么在 MessageQueue 中，应该自动有 3 条消息。我当前的代码是

import java.util.ArrayList;
import TestMQTT.SubscribeSample;
import jade.core.Agent;
import jade.core.behaviours.TickerBehaviour;
import jade.lang.acl.ACLMessage;

public class ServerAgent extends Agent {

private static final long serialVersionUID = 1L;
protected ArrayList<ACLMessage> MessageQueue = null;
// protected ArrayList<String> myProcess = new ArrayList<>();

public void setup() {
    
    SubscribeSample subscribeSample = new SubscribeSample();
    subscribeSample.subscribe("Json");

    //repeat the following actions every 5 seconds
    this.addBehaviour(new TickerBehaviour(this, 5000) {

        @Override
        protected void onTick() {
            
            if (subscribeSample.getMsg() != null && MessageQueue != null) {
                MessageQueue.add(subscribeSample.getMsg());
                System.out.println("Current MessageQueue is：" + MessageQueue);


            } else {
                System.out.println("No message yet");
            }
            
            System.out.println("The whole MessageQueue are：" + MessageQueue);

        }

subscribesample类定义如下：

public class SubscribeSample {

public static String arrivedMessage;
public static ArrayList<Object> ReceivedRequests = new ArrayList<>();
public static ACLMessage msg = null;


public static void subscribe(String TOPIC) {
    
    String broker = "tcp://192.168.137.100:1883";
    
    int qos = 1;
    String clientid = "mqtt-explorer-3260c410";
    MaintestOutput m = new MaintestOutput();
    
    try {
        
        MqttClient client = new MqttClient(broker, clientid, new MemoryPersistence());
        MqttConnectOptions options = new MqttConnectOptions();
        options.setCleanSession(true);
        options.setConnectionTimeout(10);
        options.setKeepAliveInterval(20);

        client.setCallback(new MqttCallback() {

            public void connectionLost(Throwable cause) {
                System.out.println("connectionLost");
            }

            public void messageArrived(String TOPIC, MqttMessage message)
                    throws ClassNotFoundException, IOException {
                System.out.println("======get message from [" + TOPIC + "]======");
                System.out.println("message content:" + new String(message.getPayload()));

                String Json = new String(message.getPayload());
                Gson gson = new Gson();
                msg = gson.fromJson(Json, ACLMessage.class);

            public void deliveryComplete(IMqttDeliveryToken token) {
                System.out.println("deliveryComplete---------" + token.isComplete());
            }

        
        }

        );

        // estblished connection
        System.out.println("conneted to the broker: " + broker);
        client.connect(options);

        System.out.println("connected successfully");
        client.subscribe(TOPIC, qos);
        System.out.println("start listening" + TOPIC);
    } catch (

    Exception e) {
        e.printStackTrace();
    }
}

在我的ServerAgent中，System.out.println("Current MessageQueue is：" + MessageQueue)只会显示当前收到的消息，不会显示之前的消息，而System.out.println("整个MessageQueue的输出是：" + MessageQueue) 永远为零。

我想实现ServerAgent收到的每一条消息，都会自动发送到MessageQueue，System.out.println("The whole MessageQueue are：" + MessageQueue)的输出就是所有的消息

【问题讨论】：

标签： java arrays arraylist pug mqtt

【解决方案1】：

列表MessageQueue 始终为null；您没有为其分配新列表。

将声明更改为：

protected ArrayList<ACLMessage> MessageQueue = new ArrayList<>();

另外，最好将其命名为 messageQueue 与 Java Naming Conventions 一致，后者建议字段/变量/参数使用驼峰命名法。

另外，总是最好使用抽象类型，所以List<ACLMessage> 而不是ArrayList<ACLMessage>。

推荐的形式是：

protected List<ACLMessage> messageQueue = new ArrayList<>();

【讨论】：