Observer

1// Observer interface
2interface Observer {
3    void update(String stockSymbol, double price);
4}
5
6// Subject interface
7interface Subject {
8    void registerObserver(Observer observer);
9    void removeObserver(Observer observer);
10    void notifyObservers();
11}
12
13// Concrete Subject
14class StockMarket implements Subject {
15    private Map<String, Double> stocks = new HashMap<>();
16    private List<Observer> observers = new ArrayList<>();
17    private String lastUpdatedStock;
18    
19    @Override
20    public void registerObserver(Observer observer) {
21        observers.add(observer);
22    }
23    
24    @Override
25    public void removeObserver(Observer observer) {
26        observers.remove(observer);
27    }
28    
29    @Override
30    public void notifyObservers() {
31        double price = stocks.get(lastUpdatedStock);
32        for (Observer observer : observers) {
33            observer.update(lastUpdatedStock, price);
34        }
35    }
36    
37    public void setStockPrice(String symbol, double price) {
38        stocks.put(symbol, price);
39        lastUpdatedStock = symbol;
40        notifyObservers();
41    }
42}
43
44// Concrete Observers
45class StockAlert implements Observer {
46    private String name;
47    private Map<String, Double> watchList = new HashMap<>();
48    
49    public StockAlert(String name) {
50        this.name = name;
51    }
52    
53    public void addToWatchList(String symbol, double targetPrice) {
54        watchList.put(symbol, targetPrice);
55    }
56    
57    @Override
58    public void update(String symbol, double price) {
59        if (watchList.containsKey(symbol)) {
60            double target = watchList.get(symbol);
61            if (price <= target) {
62                System.out.println(name + " ALERT: " + symbol + 
63                    " is now $" + price + " (target: $" + target + ")");
64            }
65        }
66    }
67}
68
69class StockDisplay implements Observer {
70    private String displayName;
71    
72    public StockDisplay(String name) {
73        this.displayName = name;
74    }
75    
76    @Override
77    public void update(String symbol, double price) {
78        System.out.println(displayName + " Display: " + symbol + " = $" + price);
79    }
80}
81
82// Usage
83public class Main {
84    public static void main(String[] args) {
85        StockMarket market = new StockMarket();
86        
87        // Create observers
88        StockAlert investor1 = new StockAlert("John");
89        investor1.addToWatchList("AAPL", 150.00);
90        
91        StockDisplay display = new StockDisplay("Main");
92        
93        // Register observers
94        market.registerObserver(investor1);
95        market.registerObserver(display);
96        
97        // Update stock prices - observers notified automatically
98        market.setStockPrice("AAPL", 175.00);  // Above target
99        market.setStockPrice("AAPL", 145.00);  // Below target - alert!
100        market.setStockPrice("GOOG", 2800.00);
101    }
102}