You need to implement a simple caching system that stores and retrieves string values based on string keys. The cache should support basic operations like storing values, retrieving values, and checking if a key exists.
Examples
// Store values
cache.put('user_123', 'John Doe');
cache.put('session_456', 'active');
// Retrieve values
String userName = cache.get('user_123'); // Returns 'John Doe'
String session = cache.get('session_456'); // Returns 'active'
// Check existence
Boolean hasUser = cache.containsKey('user_123'); // Returns true
Boolean hasOrder = cache.containsKey('order_789'); // Returns false
// Retrieve non-existent key
String missing = cache.get('nonexistent'); // Returns null
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