How to Retrieve the Output of a Thread in Java?

How can I capture the output or return value from a thread in Java?

public class MyThread extends Thread {
    private String result;
    
    public void run() {
        result = "Hello from thread";
    }
    
    public String getResult() {
        return result;
    }
}

In Java, threads run independently, and retrieving a result directly from a thread is not straightforward because they execute asynchronously. However, you can obtain output from a thread in several ways: using `Callable` and `Future`, storing output in shared variables, or using thread-safe collections. This article explores these methods in detail.

import java.util.concurrent.Callable;
import java.util.concurrent.Future;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Main {
    public static void main(String[] args) throws Exception {
        ExecutorService executor = Executors.newSingleThreadExecutor();

        Callable<String> task = () -> {
            // Simulate computation
            Thread.sleep(1000);
            return "Output from Callable thread";
        };

        Future<String> future = executor.submit(task);
        String result = future.get(); // Blocking call until the result is available
        System.out.println(result);
        executor.shutdown();
    }
}

Causes

• Threads are designed for parallel execution and don't return values directly.
• Using Runnable allows only void returns, while Callable can return values.

Solutions

• Use the `Callable` interface along with `Future` to get results from threads.
• Implementing custom methods in a thread to store results for later retrieval.
• Utilizing thread-safe data structures like `ConcurrentHashMap` or `BlockingQueue` to share data between threads.

Mistake: Directly modifying shared variables within threads without proper synchronization.

Solution: Use synchronized blocks or Atomic variables to ensure thread safety.

Mistake: Not handling InterruptedException when calling get() on a Future object.

Solution: Always use try-catch blocks to gracefully handle exceptions.