I’m trying to test and occur multi-thread concurrency problem.

What i expected is that result should be less than 0, because it is not synchronized.
But the test below fails.

import static org.assertj.core.api.Assertions.assertThat;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import org.junit.jupiter.api.Test;

class OptionTest {

    static class Option {

        private int quantity;

        public Option(int quantity) {
            this.quantity = quantity;
        }

        public int getQuantity() {
            return quantity;
        }

        public boolean subtract(int quantity){
            if(quantity >= this.quantity){
                return false;
            }

            // System.out.println("quantity = " + quantity);
            this.quantity -= quantity;
            return true;
        }
    }

    @Test
    void asyncSubtract() throws InterruptedException {
        int numberOfThreads = 100; // Increased number of threads
        ExecutorService executorService = Executors.newFixedThreadPool(numberOfThreads);
        List<Callable<Boolean>> tasks = new ArrayList<>();

        Option option = new Option(201);
        for (int i = 0; i < numberOfThreads; i++) {
            tasks.add(() -> option.subtract(50));
        }

        executorService.invokeAll(tasks);
        executorService.shutdown();

        assertThat(option.getQuantity()).isLessThan(1);
    }
}

When after add print on quantity in the substract method, it passed.

import static org.assertj.core.api.Assertions.assertThat;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import org.junit.jupiter.api.Test;

class OptionTest {

    static class Option {

        private int quantity;

        public Option(int quantity) {
            this.quantity = quantity;
        }

        public int getQuantity() {
            return quantity;
        }

        public boolean subtract(int quantity){
            if(quantity >= this.quantity){
                return false;
            }

            System.out.println("quantity = " + quantity);
            this.quantity -= quantity;
            return true;
        }
    }

    @Test
    void asyncSubtract() throws InterruptedException {
        int numberOfThreads = 100; // Increased number of threads
        ExecutorService executorService = Executors.newFixedThreadPool(numberOfThreads);
        List<Callable<Boolean>> tasks = new ArrayList<>();

        Option option = new Option(201);
        for (int i = 0; i < numberOfThreads; i++) {
            tasks.add(() -> option.subtract(50));
        }

        executorService.invokeAll(tasks);
        executorService.shutdown();

        assertThat(option.getQuantity()).isLessThan(1);
    }
}

Can anyone explain about this code execution result? Is there any JVM magic?