feat: add publish-subscribe design pattern (#2898) (#3223)

* feat: add publisher-subscriber design pattern (#2898)

* fix: check styles on publisher-subscriber design pattern (#2898)

* fix: check styles on publisher-subscriber design pattern (#2898)

* fix: check styles on publisher-subscriber design pattern (#2898)

* fix: improve coverage on publisher-subscriber design pattern (#2898)

* fix: fix dependancy on publisher-subscriber design pattern (#2898)

* fix: fix documentation on publisher-subscriber design pattern (#2898)

* fix: apply spotless on publisher-subscriber design pattern (#2898)

* fix: changes after review on publisher-subscriber design pattern (#2898)

* fix: changes after review on publisher-subscriber design pattern (#2898)

* fix: fix sonar on publisher-subscriber design pattern (#2898)

* fix: fix sonar on publisher-subscriber design pattern (#2898)

---------

Co-authored-by: Sanura Hettiarachchi <sanura.hettiarachchi@tipico.com>

publish-subscribe/src/test/java/com/iluwatar/publish/subscribe/LoggerExtension.java

@@ -0,0 +1,40 @@
+package com.iluwatar.publish.subscribe;
+
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.read.ListAppender;
+import java.util.List;
+import java.util.stream.Collectors;
+import org.junit.jupiter.api.extension.AfterEachCallback;
+import org.junit.jupiter.api.extension.BeforeEachCallback;
+import org.junit.jupiter.api.extension.ExtensionContext;
+import org.slf4j.LoggerFactory;
+
+public class LoggerExtension implements BeforeEachCallback, AfterEachCallback {
+
+  private final ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
+  private final Logger logger = (Logger) LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME);
+
+  @Override
+  public void afterEach(ExtensionContext extensionContext) throws Exception {
+    listAppender.stop();
+    listAppender.list.clear();
+    logger.detachAppender(listAppender);
+  }
+
+  @Override
+  public void beforeEach(ExtensionContext extensionContext) throws Exception {
+    logger.addAppender(listAppender);
+    listAppender.start();
+  }
+
+  public List<String> getMessages() {
+    return listAppender.list.stream().map(e -> e.getMessage()).collect(Collectors.toList());
+  }
+
+  public List<String> getFormattedMessages() {
+    return listAppender.list.stream()
+        .map(e -> e.getFormattedMessage())
+        .collect(Collectors.toList());
+  }
+}