design patterns : observer

design-pattern/gof/src/observer/after/ChatServer.java

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+package observer.after;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public class ChatServer {
+
+    private Map<String, List<Subscriber>> subscribers = new HashMap<>();
+
+    public void register(String subject, Subscriber subscriber) {
+        if (this.subscribers.containsKey(subject)) {
+            this.subscribers.get(subject).add(subscriber);
+        } else {
+            List<Subscriber> list = new ArrayList<>();
+            list.add(subscriber);
+            this.subscribers.put(subject, list);
+        }
+    }
+
+    public void unregister(String subject, Subscriber subscriber) {
+        if (this.subscribers.containsKey(subject)) {
+            this.subscribers.get(subject).remove(subscriber);
+        }
+    }
+
+    public void sendMessage(User user, String subject, String message) {
+        if (this.subscribers.containsKey(subject)) {
+            String userMessage = user.getName() + ": " + message;
+            this.subscribers.get(subject).forEach(s -> s.handleMessage(userMessage));
+        }
+
+    }
+}

design-pattern/gof/src/observer/after/Client.java

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+package observer.after;
+
+public class Client {
+
+    public static void main(String[] args) {
+        ChatServer chatServer = new ChatServer();
+        User user1 = new User("kim");
+        User user2 = new User("lee");
+
+        chatServer.register("오징어게임", user1);
+        chatServer.register("오징어게임", user2);
+
+        chatServer.register("디자인패턴", user1);
+
+        chatServer.sendMessage(user1, "오징어게임", "아.. 이름이 기억났어.. 일남이야.. 오일남");
+        chatServer.sendMessage(user2, "디자인패턴", "옵저버 패턴으로 만든 채팅");
+    }
+}

design-pattern/gof/src/observer/after/Subscriber.java

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+package observer.after;
+
+public interface Subscriber {
+
+    void handleMessage(String message);
+}

design-pattern/gof/src/observer/after/User.java

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+package observer.after;
+
+public class User implements Subscriber {
+
+    private String name;
+
+    public User(String name) {
+        this.name = name;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    @Override
+    public void handleMessage(String message) {
+        System.out.println(message);
+    }
+}

design-pattern/gof/src/observer/before/ChatServer.java

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+package observer.before;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public class ChatServer {
+
+    private Map<String, List<String>> messages;
+
+    public ChatServer() {
+        this.messages = new HashMap<>();
+    }
+
+
+    public void add(String subject, String message) {
+        if (messages.containsKey(subject)) {
+            messages.get(subject).add(message);
+        } else {
+            List<String> messageList = new ArrayList<>();
+            messageList.add(message);
+            messages.put(subject, messageList);
+        }
+    }
+
+    public List<String> getMessage(String subject) {
+        return messages.get(subject);
+    }
+}

design-pattern/gof/src/observer/before/Client.java

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+package observer.before;
+
+public class Client {
+
+    public static void main(String[] args) {
+        ChatServer chatServer = new ChatServer();
+
+        User user1 = new User(chatServer);
+        user1.sendMessage("디자인패턴", "이번엔 옵저버 패턴입니다.");
+        user1.sendMessage("롤드컵2021", "LCK 화이팅!");
+
+        User user2 = new User(chatServer);
+        System.out.println(user2.getMessage("디자인패턴"));
+
+        user1.sendMessage("디자인패턴", "예제 코드 보는 중..");
+        System.out.println(user2.getMessage("디자인패턴"));
+    }
+}

design-pattern/gof/src/observer/before/User.java

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+package observer.before;
+
+import java.util.List;
+
+public class User {
+
+    private ChatServer chatServer;
+
+    public User(ChatServer chatServer) {
+        this.chatServer = chatServer;
+    }
+
+
+    public void sendMessage(String subject, String message) {
+        chatServer.add(subject, message);
+    }
+
+    public List<String> getMessage(String subject) {
+        return chatServer.getMessage(subject);
+    }
+}