How Do You Retrieve Values Modified Within a Thread in Java?

Accessing Return Values from Threads

When utilizing multithreading in programming, the concept of returning values from threads is often encountered. This article will delve into an instance where a value is modified within a thread, and we aim to retrieve it within the caller method.

Consider a scenario where a HandlerThread is employed within a test() method to execute a task. A variable named value is modified within the thread, and we seek to send its updated value back to the test() method.

To achieve this goal, the thread cannot directly return a value. Instead, we can employ an alternative approach. We create a class that implements the Runnable interface, as demonstrated in the code snippet below:

public class Foo implements Runnable {

    private volatile int value;

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

    public int getValue() {
        return value;
    }
}

Within this class, we declare a volatile integer variable value and override the run() method to update its value. Additionally, we define the getValue() method to retrieve the updated value.

To utilize this approach, we create an instance of the Foo class and instantiate a Thread object using this instance as the target Runnable:

Foo foo = new Foo();
Thread thread = new Thread(foo);

We initiate the thread and join it with the main thread:

thread.start();
thread.join();

Finally, we can retrieve the updated value by accessing the getValue() method of the Foo instance:

int value = foo.getValue();

Through this approach, we effectively return a value from the thread by creating a Runnable class that encapsulates the desired value and exposes it via a getter method. This allows us to access the updated value within the caller method.

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