test: add unit tests for multithreading scenarios

src/test/java/banking/TestC.java

@@ -1,4 +1,98 @@
 package banking;
 
+import dev.banking.model.BankAccount;
+import org.junit.jupiter.api.Test;
+
+import java.time.Duration;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+
+import static org.junit.jupiter.api.Assertions.*;
+
 public class TestC {
-}
+
+    @Test
+    void transferConservation() throws Exception {
+
+        BankAccount a =
+                new BankAccount(1, 100_000);
+
+        BankAccount b =
+                new BankAccount(2, 100_000);
+
+        ExecutorService executor =
+                Executors.newFixedThreadPool(50);
+
+        CountDownLatch latch =
+                new CountDownLatch(100);
+
+        for (int i = 0; i < 50; i++) {
+
+            executor.submit(() -> {
+
+                for (int j = 0; j < 1000; j++) {
+                    a.transfer(b, 1);
+                }
+
+                latch.countDown();
+            });
+
+            executor.submit(() -> {
+
+                for (int j = 0; j < 1000; j++) {
+                    b.transfer(a, 1);
+                }
+
+                latch.countDown();
+            });
+        }
+
+        latch.await();
+
+        executor.shutdown();
+
+        long total =
+                a.getBalance() + b.getBalance();
+
+        assertEquals(200_000, total);
+    }
+
+    @Test
+    void deadlockFreeTransfers() {
+
+        assertTimeoutPreemptively(
+                Duration.ofSeconds(5),
+                () -> {
+
+                    BankAccount a =
+                            new BankAccount(1, 1_000_000);
+
+                    BankAccount b =
+                            new BankAccount(2, 1_000_000);
+
+                    ExecutorService executor =
+                            Executors.newFixedThreadPool(50);
+
+                    for (int i = 0; i < 50000; i++) {
+
+                        executor.submit(() ->
+                                a.transfer(b, 1));
+
+                        executor.submit(() ->
+                                b.transfer(a, 1));
+                    }
+
+                    executor.shutdown();
+
+                    assertTrue(
+                            executor.awaitTermination(
+                                    5,
+                                    TimeUnit.SECONDS
+                            )
+                    );
+                }
+        );
+    }
+}