Java Lambda Expressions Not Capturing Outer Variable in Multi-threaded Environment
After trying multiple solutions online, I still can't figure this out. This might be a silly question, but I'm working with a peculiar scenario where a lambda expression in my Java application is not properly capturing an outer variable when called from multiple threads. I have a simple scenario where I'm trying to increment a counter using a lambda within a Runnable. The code looks something like this: ```java import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class CounterExample { private int counter = 0; public void incrementCounter() { ExecutorService executor = Executors.newFixedThreadPool(5); for (int i = 0; i < 10; i++) { executor.submit(() -> { int temp = counter; temp++; counter = temp; }); } executor.shutdown(); } public int getCounter() { return counter; } public static void main(String[] args) { CounterExample example = new CounterExample(); example.incrementCounter(); // Wait for tasks to finish try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Final Counter: " + example.getCounter()); } } ``` I expected the final output of the counter to be 10, but instead, I am getting values like 6 or 7. I suspect that the operations on `counter` are not atomic and possibly leading to race conditions. I've tried using `AtomicInteger` to ensure thread safety, but I'm not sure if I implemented it correctly. Here’s how I modified my code: ```java import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.atomic.AtomicInteger; public class CounterExample { private AtomicInteger counter = new AtomicInteger(0); public void incrementCounter() { ExecutorService executor = Executors.newFixedThreadPool(5); for (int i = 0; i < 10; i++) { executor.submit(() -> { counter.incrementAndGet(); }); } executor.shutdown(); } public int getCounter() { return counter.get(); } public static void main(String[] args) { CounterExample example = new CounterExample(); example.incrementCounter(); // Wait for tasks to finish try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Final Counter: " + example.getCounter()); } } ``` Even with `AtomicInteger`, I still occasionally see the counter not reaching 10. Could someone please guide to understand if I'm missing something with my execution context or if there's a better approach to this question? My Java version is 11. Thank you! My development environment is Linux. Any advice would be much appreciated.