Add tests showing commands vs events

CleanCode/src/main/java/pl/mperor/lab/java/clean/code/ddd/Command.java

@@ -0,0 +1,12 @@
+package pl.mperor.lab.java.clean.code.ddd;
+
+interface Command {
+
+    interface CommandHandler<C extends Command> {
+        Status handle(C command);
+    }
+
+    enum Status {
+        DONE
+    }
+}

CleanCode/src/main/java/pl/mperor/lab/java/clean/code/ddd/Event.java

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+package pl.mperor.lab.java.clean.code.ddd;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.function.Consumer;
+
+interface Event {
+
+    record Publisher(Collection<EventConsumer> consumers) {
+
+        public Publisher {
+            consumers = List.copyOf(consumers);
+        }
+
+        Publisher onPublish(EventConsumer consumer) {
+            var newConsumers = new ArrayList<>(consumers);
+            newConsumers.add(consumer);
+            return new Publisher(newConsumers);
+        }
+
+        void publish() {
+            var event = new Event() {};
+            consumers.forEach(consumer -> consumer.accept(event));
+        }
+
+        interface EventConsumer extends Consumer<Event> {
+        }
+    }
+}

CleanCode/src/test/java/pl/mperor/lab/java/clean/code/ddd/CommandVsEventTest.java

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+package pl.mperor.lab.java.clean.code.ddd;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.Collections;
+import java.util.UUID;
+
+/**
+ * ⚡ Command vs Event 🎯
+ *
+ * <p><b>Command:</b> An instruction to perform an action, usually leading to a state change.
+ * - Imperative 🏗️
+ * - Synchronous or Asynchronous ⏳
+ * - Expect a specific effect 🔄
+ *
+ * <p><b>Event:</b> A fact that something has happened in the past.
+ * - Declarative 📢
+ * - Asynchronous-friendly ⏩
+ * - Captures history 🕰️
+ */
+class CommandVsEventTest {
+
+    private final Command.CommandHandler handler = command -> {
+        System.out.printf("🫵 Command: %s%n", command);
+        return Command.Status.DONE;
+    };
+
+    private final Event.Publisher publisher = new Event.Publisher(Collections.emptyList());
+
+    @Test
+    void commandShouldExecuteImmediately() {
+        Assertions.assertEquals(Command.Status.DONE, handler.handle(new Command() {}));
+    }
+
+    @Test
+    void commandsCanBeHandledByTheSameHandler() {
+        record CreateOrderCommand(UUID uuid) implements Command {}
+        record CreatePaymentCommand(UUID uuid) implements Command {}
+        Assertions.assertEquals(Command.Status.DONE, handler.handle(new CreateOrderCommand(UUID.randomUUID())));
+        Assertions.assertEquals(Command.Status.DONE, handler.handle(new CreatePaymentCommand(UUID.randomUUID())));
+    }
+
+    @Test
+    void eventShouldNotBeConsumeWithoutPublishing() {
+        publisher.onPublish(_ -> Assertions.fail());
+    }
+
+    @Test
+    void eventShouldBeConsumeByManyConsumers() {
+        publisher.onPublish(event -> System.out.printf("❗Event: %s%n", event))
+                .onPublish(Assertions::assertNotNull)
+                .publish();
+    }
+}