Observer design pattern implementation

Observer/Observer.java

@@ -0,0 +1,5 @@
+package Observer;
+
+public interface Observer {
+    void update(String weather);
+}

Observer/PhoneDisplay.java

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+package Observer;
+
+public class PhoneDisplay implements Observer {
+    private String weather;
+
+    @Override
+    public void update(String weather) {
+        this.weather = weather;
+        display();
+    }
+
+    private void display() {
+        System.out.println("Phone Display: Weather updated - " + weather);
+    }
+}

Observer/Subject.java

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+package Observer;
+
+public interface Subject {
+    void addObserver(Observer observer);
+    void removeObserver(Observer observer);
+    void notifyObservers();
+}

Observer/TVDisplay.java

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+package Observer;
+
+class TVDisplay implements Observer {
+    private String weather;
+
+    @Override
+    public void update(String weather) {
+        this.weather = weather;
+        display();
+    }
+
+    private void display() {
+        System.out.println("TV Display: Weather updated - " + weather);
+    }
+}
+

Observer/WeatherApp.java

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+package Observer;
+
+public class WeatherApp {
+    public static void main(String[] args) {
+        WeatherStation weatherStation = new WeatherStation();
+
+        Observer phoneDisplay = new PhoneDisplay();
+        Observer tvDisplay = new TVDisplay();
+
+        weatherStation.addObserver(phoneDisplay);
+        weatherStation.addObserver(tvDisplay);
+
+
+        weatherStation.setWeather("Sunny");
+
+
+    }
+}

Observer/WeatherStation.java

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+package Observer;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class WeatherStation implements Subject {
+    private List<Observer> observers = new ArrayList<>();
+    private String weather;
+
+    @Override
+    public void addObserver(Observer observer) {
+        observers.add(observer);
+    }
+
+    @Override
+    public void removeObserver(Observer observer) {
+        observers.remove(observer);
+    }
+
+    @Override
+    public void notifyObservers() {
+        for (Observer observer : observers) {
+            observer.update(weather);
+        }
+    }
+
+    public void setWeather(String newWeather) {
+        this.weather = newWeather;
+        notifyObservers();
+    }
+}

Observer/WeatherStationTest.java

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+package Observer;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class WeatherStationTest {
+    private WeatherStation weatherStation;
+    private TestObserver phoneDisplay;
+    private TestObserver tvDisplay;
+
+    @BeforeEach
+    void setUp() {
+        weatherStation = new WeatherStation();
+        phoneDisplay = new TestObserver();
+        tvDisplay = new TestObserver();
+    }
+
+    @Test
+    void testAddObserverAndNotifyObservers() {
+        // Add observers
+        weatherStation.addObserver(phoneDisplay);
+        weatherStation.addObserver(tvDisplay);
+
+        // Simulate weather change
+        weatherStation.setWeather("Rainy");
+
+        // Verify that all observers received the update
+        assertEquals("Rainy", phoneDisplay.getWeather());
+        assertEquals("Rainy", tvDisplay.getWeather());
+    }
+
+    @Test
+    void testRemoveObserver() {
+        // Add observers
+        weatherStation.addObserver(phoneDisplay);
+        weatherStation.addObserver(tvDisplay);
+
+        // Remove one observer
+        weatherStation.removeObserver(phoneDisplay);
+
+        // Simulate weather change
+        weatherStation.setWeather("Cloudy");
+
+        // Verify only the remaining observer received the update
+        assertEquals(null, phoneDisplay.getWeather());
+        assertEquals("Cloudy", tvDisplay.getWeather());
+    }
+
+    @Test
+    void testNoObservers() {
+        // Simulate weather change without any observers
+        weatherStation.setWeather("Windy");
+
+        // No exceptions should occur, nothing to verify
+    }
+
+    // Custom TestObserver class
+    private static class TestObserver implements Observer {
+        private String weather;
+
+        @Override
+        public void update(String weather) {
+            this.weather = weather;
+        }
+
+        public String getWeather() {
+            return weather;
+        }
+    }
+}
+

README.md

@@ -126,6 +126,8 @@ It is very easy to contribute, you may follow these steps -
 99.[RotateLinkedList](https://github.com/PrajaktaSathe/Java/blob/main/Programs/RotateLinkedList.java)-Program to demo rotating a linked list
 100. [ReverseString](https://github.com/PrajaktaSathe/Java/blob/main/ReverseString.java) -Program to reverse a String using the java method substring.
 101.[Overriding](https://github.com/PrajaktaSathe/Java/blob/main/Programs/Overriding.java)-Program to demo overriding in java
+102. [Observer-pattern](https://github.com/PrajaktaSathe/Java/blob/main/Programs/Observer/WeatherApp.java)- Observer design pattern in java
+
 
 # Contributors - 
 ## A big thank you to all our contributors!!!