Java: Volatile variable

In a book i'm reading, it shows below code and states: if 'data' isn't defined as a volatile var, other classes might access it before it's completely initialized.  Can someone explain this statement to me. There's nothing else written about it.
class StockData {}
class StockExchange {
      static volatile StockData data = null;
      static void loadStockData () {
            //load and initialize reference variable data. the variable data is written to only once
      }
}
class BuyStocks {
      public void buy () {
            if (StockExchange.data != null) {
                     //analyze data
                    //buy stock
            }
      }
}