Returning Values from Thread Operations

In multithreaded programming, the interaction between threads often requires the exchange of data. One common scenario is attempting to retrieve the result of an operation performed within a separate thread.

Consider the example code below:

public void test() {
    Thread uiThread = new HandlerThread("UIHandler") {
        public synchronized void run() {
            int value = 2; // To be returned to test()
        }
    };
    uiThread.start();
}

In this instance, a value is modified within a separate thread (in this case, the "UIHandler"). The challenge lies in returning this value to the caller method, which needs to retrieve the modified data.

Leveraging an Object's State

One approach to this problem is to use an object's state to store and retrieve the required data. For instance, you can create a custom class that implements the Runnable interface, allowing it to be executed as a thread. Within this class, you can have a field to store the value calculated by the thread:

public class Foo implements Runnable {
    private volatile int value;

    @Override
    public void run() {
        value = 2;
    }

    public int getValue() {
        return value;
    }
}

With this implementation, you can separate the thread creation and the retrieval of the calculated value. Here's an example:

Foo foo = new Foo();
Thread thread = new Thread(foo);
thread.start();
thread.join();
int value = foo.getValue(); // Retrieve the modified value

Key Considerations

It's important to note that threads do not natively return values. By referencing a thread like an ordinary class and requesting its value using methods like getValue(), you can bridge this gap. Additionally, you should ensure synchronization mechanisms to prevent data race conditions and maintain thread safety.