Question

这可能是什么问题？
我在编译时遇到以下错误：

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

Class Stock有一个公共方法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

要么确保使用方法的完整参考

Collectors.toList()

或静态导入

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

以及Stock类有点像：

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

Ideone it!

Answer 2

您尚未定义方法toList()，或者您忘记执行静态导入，请尝试：

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

您的代码中不需要进行任何其他更改。

如果您不想进行静态导入：

System.out.println(stocks.stream().filter(StockUtil.isLessThanLimit(400.0))
                    .map(Stock::getSymbol)
                    .map(StockUtil::toUpperCase)
                    .collect(Collectors.toList())
                    );

有关详细信息，请参阅收集器的java documentation。

映射过滤器给出编译问题

2 个答案: