ObserverPattern

import java.util.ArrayList;
import java.util.List;

// Observer interface
interface Observer {
    void update(String weather);
}

// Subject interface
interface Subject {
    void addObserver(Observer observer);
    void removeObserver(Observer observer);
    void notifyObservers();
}

// Concrete Subject
class WeatherStation implements Subject {
    private List<Observer> observers = new ArrayList<>();
    private String currentWeather;

    public void setWeather(String weather) {
        this.currentWeather = weather;
        notifyObservers();
    }

    @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(currentWeather);
        }
    }
}

// Concrete Observer
class DisplayDevice implements Observer {
    private String deviceName;

    public DisplayDevice(String deviceName) {
        this.deviceName = deviceName;
    }

    @Override
    public void update(String weather) {
        System.out.println("Display device " + deviceName + " updated with weather: " + weather);
    }
}

public class WeatherStationExample {
    public static void main(String[] args) {
        // Creating a weather station
        WeatherStation weatherStation = new WeatherStation();

        // Creating display devices (observers)
        DisplayDevice phoneDisplay = new DisplayDevice("Phone");
        DisplayDevice tvDisplay = new DisplayDevice("TV");

        // Registering observers with the subject (weather station)
        weatherStation.addObserver(phoneDisplay);
        weatherStation.addObserver(tvDisplay);

        // Simulating a change in weather
        weatherStation.setWeather("Sunny");

        // Unregistering an observer
        weatherStation.removeObserver(tvDisplay);

        // Simulating another change in weather
        weatherStation.setWeather("Rainy");
    }
}

StockMarket

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

// Observer interface
interface Observer {
    void update();
}

// Subject interface
interface Subject {
    void addObserver(Observer observer);
    void removeObserver(Observer observer);
    void notifyObservers();
}

// Concrete Subject
class StockMarket implements Subject {
    private List<Observer> observers = new ArrayList<>();
    private String stockSymbol;
    private double stockPrice;

    public StockMarket(String stockSymbol, double initialPrice) {
        this.stockSymbol = stockSymbol;
        this.stockPrice = initialPrice;
    }

    public void simulateMarketChanges() {
        // Simulate changes in stock prices
        Random random = new Random();
        double percentageChange = (random.nextDouble() - 0.5) * 5; // +/- 2.5%
        stockPrice += (stockPrice * percentageChange) / 100;
        notifyObservers();
    }

    @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();
        }
    }

    public String getStockSymbol() {
        return stockSymbol;
    }

    public double getStockPrice() {
        return stockPrice;
    }
}

// Concrete Observer: Financial Institution
class FinancialInstitution implements Observer {
    private String institutionName;
    private StockMarket stockMarket;

    public FinancialInstitution(String institutionName, StockMarket stockMarket) {
        this.institutionName = institutionName;
        this.stockMarket = stockMarket;
    }

    @Override
    public void update() {
        System.out.println("Financial Institution " + institutionName +
                " received update for stock " + stockMarket.getStockSymbol() + ": $" + stockMarket.getStockPrice());
        // Implement custom strategy for responding to stock changes (e.g., buy/sell decisions)
    }
}

// Concrete Observer: Individual Investor
class IndividualInvestor implements Observer {
    private String investorName;
    private StockMarket stockMarket;

    public IndividualInvestor(String investorName, StockMarket stockMarket) {
        this.investorName = investorName;
        this.stockMarket = stockMarket;
    }

    @Override
    public void update() {
        System.out.println("Individual Investor " + investorName +
                " received update for stock " + stockMarket.getStockSymbol() + ": $" + stockMarket.getStockPrice());
        // Implement custom strategy for responding to stock changes (e.g., portfolio adjustments)
    }
}

public class StockMarketExample {
    public static void main(String[] args) {
        // Creating a stock market with an initial stock
        StockMarket stockMarket = new StockMarket("XYZ", 100.0);

        // Creating observers (financial institutions and individual investors)
        FinancialInstitution institution1 = new FinancialInstitution("Bank A", stockMarket);
        FinancialInstitution institution2 = new FinancialInstitution("Investment Firm B", stockMarket);
        IndividualInvestor investor1 = new IndividualInvestor("John Doe", stockMarket);
        IndividualInvestor investor2 = new IndividualInvestor("Jane Smith", stockMarket);

        // Registering observers with the subject (stock market)
        stockMarket.addObserver(institution1);
        stockMarket.addObserver(institution2);
        stockMarket.addObserver(investor1);
        stockMarket.addObserver(investor2);

        // Simulating changes in the stock market
        for (int i = 0; i < 5; i++) {
            stockMarket.simulateMarketChanges();
            System.out.println("-----------------------------");
        }
    }
}