Add unit tests for EventHandler functionality and implement UserEvent…

src/test/java/com/neroyun/mediator/EventHandlerTest.java

@@ -0,0 +1,158 @@
+package com.neroyun.mediator;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.Executors;
+import java.util.stream.Stream;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Unit tests for Event Handler functionality.
+ * Tests event handling, event publishing, and multiple handlers for a single event.
+ */
+public class EventHandlerTest {
+
+    private Mediator mediator;
+    private UserEventCounterHandler userEventCounter;
+
+    @BeforeEach
+    void setUp() {
+        // Clear any previous state
+        UserStore.getInstance().clear();
+
+        // Create handler instances
+        userEventCounter = new UserEventCounterHandler();
+
+        // Setup mediator with handlers
+        mediator = new PipelinedMediator()
+                .use(() -> Stream.of(
+                        new UserCreatedEventHandler(),
+                        userEventCounter
+                ))
+                .use(() -> Executors.newFixedThreadPool(2));
+    }
+
+    @Test
+    void testUserCreatedEventHandler() {
+        // Arrange
+        UserCreatedEvent event = new UserCreatedEvent(123L, "Jane Doe");
+        UserCreatedEventHandler handler = new UserCreatedEventHandler();
+
+        // Act
+        Void result = handler.handle(event);
+
+        // Assert
+        assertNull(result, "Event handler should return null (Void)");
+    }
+
+    @Test
+    void testEventHandlerReturnsVoid() {
+        // Arrange
+        UserCreatedEvent event = new UserCreatedEvent(456L, "John Smith");
+        UserCreatedEventHandler handler = new UserCreatedEventHandler();
+
+        // Act
+        Void result = handler.handle(event);
+
+        // Assert
+        assertNull(result, "Event handler should return null (Void)");
+    }
+
+    @Test
+    void testEventHandlerMatches() {
+        // Arrange
+        UserCreatedEvent event = new UserCreatedEvent(1L, "Test User");
+        UserCreatedEventHandler handler = new UserCreatedEventHandler();
+
+        // Act
+        boolean matches = handler.matches(event);
+
+        // Assert
+        assertTrue(matches, "Handler should match its own event type");
+    }
+
+    @Test
+    void testEventPublishDoesNotThrow() {
+        // Arrange
+        UserCreatedEvent event = new UserCreatedEvent(999L, "No Exception User");
+
+        // Act & Assert
+        assertDoesNotThrow(() -> mediator.publish(event), "Publishing an event should not throw an exception");
+    }
+
+    @Test
+    void testMultipleHandlersForSameEvent() {
+        // Arrange
+        UserCreatedEvent event = new UserCreatedEvent(100L, "Multiple Handlers User");
+
+        // Act
+        mediator.publish(event);
+
+        // Give handlers time to process (since they run asynchronously)
+        try {
+            Thread.sleep(100);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+
+        // Assert
+        assertTrue(userEventCounter.getCount() > 0, "Counter handler should have been invoked");
+    }
+
+    @Test
+    void testMultipleEventsHandledInOrder() {
+        // Arrange
+        UserCreatedEvent event1 = new UserCreatedEvent(1L, "First User");
+        UserCreatedEvent event2 = new UserCreatedEvent(2L, "Second User");
+        UserCreatedEvent event3 = new UserCreatedEvent(3L, "Third User");
+
+        // Act
+        mediator.publish(event1);
+        mediator.publish(event2);
+        mediator.publish(event3);
+
+        // Give handlers time to process
+        try {
+            Thread.sleep(200);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+
+        // Assert
+        assertEquals(3, userEventCounter.getCount(), "Handler should have handled 3 events");
+    }
+
+    @Test
+    void testEventHandlerWithNullParameters() {
+        // Arrange & Act & Assert
+        assertThrows(IllegalArgumentException.class,
+            () -> mediator.publish(null),
+            "Publishing null event should throw IllegalArgumentException");
+    }
+
+    @Test
+    void testUserCreatedEventImmutability() {
+        // Arrange
+        Long id = 42L;
+        String name = "Test User";
+
+        // Act
+        UserCreatedEvent event = new UserCreatedEvent(id, name);
+
+        // Assert
+        assertEquals(id, event.id(), "Event should store the correct id");
+        assertEquals(name, event.name(), "Event should store the correct name");
+    }
+
+    @Test
+    void testEventImplementsEventInterface() {
+        // Arrange
+        UserCreatedEvent event = new UserCreatedEvent(1L, "Test");
+
+        // Assert
+        assertInstanceOf(Event.class, event, "UserCreatedEvent should implement Event interface");
+    }
+}
+

src/test/java/com/neroyun/mediator/PipelinedMediatorTest.java

@@ -17,11 +17,23 @@ public PipelinedMediatorTest() {
     }
 
     @Test
-    public void testMediator() {
+    void testMediator() {
         mediator.send(new UserCreateCommand("John Doe", "johndoe@sample.com"));
 
         var users = UserStore.getInstance().getUsers();
         assert users.size() == 1;
         assert users.get(0).name().equals("John Doe");
     }
+
+    @Test
+    void testEventPublish() {
+        // Arrange
+        UserCreatedEvent event = new UserCreatedEvent(1234L, "Event Test User");
+
+        // Act & Assert - should not throw exception
+        mediator.publish(event);
+
+        // Event publishing is asynchronous, so we just verify it doesn't throw
+        assert true;
+    }
 }

src/test/java/com/neroyun/mediator/UserEventCounterHandler.java

@@ -0,0 +1,28 @@
+package com.neroyun.mediator;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+/**
+ * A test event handler that counts how many UserCreatedEvent instances it has handled.
+ * Uses AtomicInteger for thread-safe counting.
+ */
+public class UserEventCounterHandler implements Handler<UserCreatedEvent, Void> {
+    private final AtomicInteger counter = new AtomicInteger(0);
+
+    @Override
+    public Void handle(UserCreatedEvent message) {
+        counter.incrementAndGet();
+        System.out.printf("UserEventCounterHandler: Counted event for user %s (Total: %d)\n",
+                         message.name(), counter.get());
+        return null;
+    }
+
+    public int getCount() {
+        return counter.get();
+    }
+
+    public void reset() {
+        counter.set(0);
+    }
+}
+