test: add unit tests for multithreading scenarios

.idea/.gitignore

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+# Default ignored files
+/shelf/
+/workspace.xml
+# Editor-based HTTP Client requests
+/httpRequests/
+# Datasource local storage ignored files
+/dataSources/
+/dataSources.local.xml

.idea/compiler.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="CompilerConfiguration">
+    <annotationProcessing>
+      <profile name="Maven default annotation processors profile" enabled="true">
+        <sourceOutputDir name="target/generated-sources/annotations" />
+        <sourceTestOutputDir name="target/generated-test-sources/test-annotations" />
+        <outputRelativeToContentRoot value="true" />
+        <module name="HW-09-Advanced-Multithreading" />
+      </profile>
+    </annotationProcessing>
+  </component>
+</project>

.idea/encodings.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="Encoding">
+    <file url="file://$PROJECT_DIR$/src/main/java" charset="UTF-8" />
+    <file url="file://$PROJECT_DIR$/src/main/resources" charset="UTF-8" />
+  </component>
+</project>

.idea/jarRepositories.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="RemoteRepositoriesConfiguration">
+    <remote-repository>
+      <option name="id" value="central" />
+      <option name="name" value="Central Repository" />
+      <option name="url" value="https://repo.maven.apache.org/maven2" />
+    </remote-repository>
+    <remote-repository>
+      <option name="id" value="central" />
+      <option name="name" value="Maven Central repository" />
+      <option name="url" value="https://repo1.maven.org/maven2" />
+    </remote-repository>
+    <remote-repository>
+      <option name="id" value="jboss.community" />
+      <option name="name" value="JBoss Community repository" />
+      <option name="url" value="https://repository.jboss.org/nexus/content/repositories/public/" />
+    </remote-repository>
+  </component>
+</project>

.idea/misc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ExternalStorageConfigurationManager" enabled="true" />
+  <component name="MavenProjectsManager">
+    <option name="originalFiles">
+      <list>
+        <option value="$PROJECT_DIR$/pom.xml" />
+      </list>
+    </option>
+  </component>
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_X" default="true" project-jdk-name="homebrew-24" project-jdk-type="JavaSDK">
+    <output url="file://$PROJECT_DIR$/out" />
+  </component>
+</project>

.idea/vcs.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="" vcs="Git" />
+  </component>
+</project>

pom.xml

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         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     </properties>
 
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <version>5.12.2</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>3.5.3</version>
+            </plugin>
+        </plugins>
+    </build>
+
 </project>

src/test/java/banking/TestA.java

@@ -1,4 +1,49 @@
 package banking;
 
+import dev.banking.model.BankAccount;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
 public class TestA {
+
+    @Test
+    void concurrentDeposits() throws Exception {
+
+        BankAccount account = new BankAccount(1, 0);
+
+        int threadCount = 100;
+        int depositsPerThread = 1000;
+
+        ExecutorService executor =
+                Executors.newFixedThreadPool(threadCount);
+
+        CountDownLatch latch =
+                new CountDownLatch(threadCount);
+
+        for (int i = 0; i < threadCount; i++) {
+
+            executor.submit(() -> {
+
+                for (int j = 0; j < depositsPerThread; j++) {
+                    account.deposit(1);
+                }
+
+                latch.countDown();
+            });
+        }
+
+        latch.await();
+
+        executor.shutdown();
+
+        assertEquals(
+                threadCount * depositsPerThread,
+                account.getBalance()
+        );
+    }
 }

src/test/java/banking/TestB.java

@@ -1,4 +1,56 @@
 package banking;
 
+import dev.banking.model.BankAccount;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
 public class TestB {
+
+    @Test
+    void concurrentDepositWithdraw() throws Exception {
+
+        BankAccount account =
+                new BankAccount(1, 1_000_000);
+
+        ExecutorService executor =
+                Executors.newFixedThreadPool(100);
+
+        CountDownLatch latch =
+                new CountDownLatch(100);
+
+        for (int i = 0; i < 50; i++) {
+
+            executor.submit(() -> {
+
+                for (int j = 0; j < 10000; j++) {
+                    account.deposit(1);
+                }
+
+                latch.countDown();
+            });
+
+            executor.submit(() -> {
+
+                for (int j = 0; j < 10000; j++) {
+                    account.withdraw(1);
+                }
+
+                latch.countDown();
+            });
+        }
+
+        latch.await();
+
+        executor.shutdown();
+
+        assertEquals(
+                1_000_000,
+                account.getBalance()
+        );
+    }
 }

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
+                            )
+                    );
+                }
+        );
+    }
 }