我想从类 1 访问类 2 的对象，**对象**来自另一个类，比如说类 3

Question

下面是声明为"client"的对象，我导入的另一个类称为mqttAndroidClient

public class connection{
public void onCtreate(){
String clientId = MqttClient.generateClientId();
MqttAndroidClient client =
new MqttAndroidClient(getApplicationContext(), "tcp://" + TEXT + ":" + TEXT2,
clientId);
}
} 

the object "client" of connection class, i wanna access it from a main class 

public class main extends AppComptActivity{
//
//Attributes
//
public void onCreate(){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//onclick of button
button1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String topic = TOPIC;
String payload = PAYLOAD;
byte[] encodedPayload = new byte[0];
try {
encodedPayload = payload.getBytes("UTF-8");
MqttMessage message = new MqttMessage(encodedPayload);
message.setRetained(true);
client.publish(topic, message);
} catch (UnsupportedEncodingException | MqttException e) {
e.printStackTrace();
}
}
});
}

在上面的代码中，它无法将客户端标识为连接类中的对象

client.publish(topic, message);

i have tried to call it in main using

connection myobject = new connection(); myobject.onCreate();

but its throwing a declaration error i am very new to java and oops and 
if the info is not efficient i will post the full code

Answer 1

首先，将 Connection 类中的客户端对象作为全局变量，并调用初始化对象的构造函数。

public class Connection {
MqttAndroidClient client;
Context context;
public Connection(Context context) {

this.context = context;
initClient(context);
}
public void initClient(Context context){
client =
new MqttAndroidClient(getApplicationContext(), "tcp://" + TEXT1 + ":" + TEXT2,
clientId);
}
public MqttAndroidClient getClient(){
return client;
}
}

现在MainClass首先获取Connection class的对象，然后通过调用返回客户端对象的方法从连接类中获取客户端对象。

主类内部

Connection connection= new Connection(this);
MqttAndroidClient client= connection.getClient();

Answer 2

让我猜猜问题 - 您想从onClick方法中访问client，以便可以调用client.publish(topic, message);，但现在您遇到编译错误。 我说的对吗？

原因 ：MqttAndroidClient clientonCtreate方法中的connection是一个局部变量，在调用onCtreate之后创建和销毁。 因此它是不可访问的。

解决方案：使MqttAndroidClient client成为connection类的属性。

更新连接类，如下所示：

public class connection{
private MqttAndroidClient client;
public MqttAndroidClient getClient(){
return this.client;
}
public void onCreate(){
String clientId = MqttClient.generateClientId();
this.client =
new MqttAndroidClient(getApplicationContext(), "tcp://" + TEXT + ":" + TEXT2,
clientId);
}

现在在下面的方法中，您可以添加

connection myobject = new connection(); 
myobject.onCreate();
myobject.getClient().publish(topic, message);