How to wait inside a method, till other method is completed

Question

Let's say we have this code:

int number;

public void onMessageReceived(int num) {
    number = num;
}

public int getNumber() {
    sendMessage("number")
    return number;
}

you get the Message only after lets say 1 second.

How do you "wait" until you get the "number" without freezing the Main-Thread?

public int getNumber() {
    sendMessage("number")
    //WAIT TILL I GET THE "NUMBER"
    return number;
}

Answer 1

This kind of asynchronous calls need some callback mechanisms.

You may need to use an observer design pattern that subscribe into an event call.

See this link

Or try to use CallbackTask like in this question:

java-executors-how-to-be-notified-without-blocking-when-a-task-completes

Answer 2

You probably want to use something like Java Future.

Have a look at the docs or read up this simple example.

Answer 3

May be recursive methodlogy

public int getNumber() {

 while(number!=0)
 getNumber(); //Assuming number will not be null till you get positive integer greater than a zero same method will get recalled.
 sendMessage("number")

return number;
}

When onMessageReceived(int num) called then you can find a positive integer in number so that it will countinue the execution

Answer 4

You should use join method...

When a thread calls join() on another thread, the currently running thread will wait until the thread it joins with has completed.