Reuses expensive-to-create objects instead of creating and destroying them repeatedly.

Common for database connections, thread pools, etc.

class Connection {
  void query(String sql) => print('Executing: $sql');
}

class ConnectionPool {
  final List<Connection> _available = [];
  final int _maxSize;

  ConnectionPool(this._maxSize);

  Connection acquire() {
    if (_available.isNotEmpty) return _available.removeLast();
    return Connection(); // create new if pool isn't full
  }

  void release(Connection conn) {
    if (_available.length < _maxSize) _available.add(conn);
  }
}

// Usage
final pool = ConnectionPool(5);
final conn = pool.acquire();
conn.query('SELECT * FROM users');
pool.release(conn);