java - 过滤器上的 map 给出编译问题

Question

这里可能出现什么问题？
我在编译时收到以下错误:

error: cannot find symbol
                                .collect(toList())
                                         ^
  symbol:   method toList()
  location: class stocks
 error: invalid method reference
                                .map(Stock::getSymbol)
                                     ^
  cannot find symbol
    symbol:   method getSymbol()
    location: class Stock 

Stock 类有一个公共(public)方法 getSymbol

   static class StockUtil{

            public static Predicate<Stock> isLessThanLimit(Double limit){
                return s -> s.tick < limit;
            }

            public static String toUpperCase(String stock){
                return stock.toUpperCase();
            }
        }

        public static void main(String[] args) {
            // TODO Auto-generated method stub
            List<Stock> stocks = new ArrayList<>(Arrays.asList(new Stock("AMZ",312.0),
                new Stock("Yahoo",212.0),
                new Stock("ORCL",512.0),
                new Stock("SAP",112.0),
                new Stock("Google",412.0),
                new Stock("FB",12.0),
                new Stock("TWT",315.0)));


            //print list of stock names below 400 in uppercase
            System.out.println(stocks.stream().filter(StockUtil.isLessThanLimit(400.0))
                    .map(Stock::getSymbol)
                    .map(StockUtil::toUpperCase)
                    .collect(toList())
                    );

Answer 1

请确保使用对该方法的完整引用:

Collectors.toList()

或静态导入

import static java.util.stream.Collectors.toList;

以及一个Stock类，有点像:

class Stock {
    String symbol;
    ...
    public String getSymbol() {...} //getter for symbol
    ...
}

Ideone it!