Observer Pattern: Stock Price Alerts

Problem

Design a StockMarket system where multiple observers (mobile app, email alert, dashboard widget) are notified whenever a stock price changes. Observers can subscribe and unsubscribe at runtime without the StockMarket knowing what they are.

Requirements

StockObserver interface with: onPriceChange(symbol, oldPrice, newPrice)
StockMarket (subject) with: subscribe(observer), unsubscribe(observer), updatePrice(symbol, price)
MobileAlertObserver, EmailAlertObserver, DashboardObserver each implement StockObserver
When price changes, all subscribed observers are notified automatically
Show: add observer, trigger price update, remove observer, trigger again — removed observer should NOT fire

Constraints

–StockMarket must not import or reference any concrete Observer class
–Notifications happen synchronously in order of subscription

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