如何在Java 8流API中实现不变性

Question

Consider the following code. I read it is important to achieve immutability in the code while dealing with Stream API. How can we get a list of all the item in lower case with immutability?

    List<String> stockList = Arrays.asList("GOOG", "AAPL", "MSFT", "INTC");
    List<String> stockList2 = new ArrayList<>();
    stockList.parallelStream()
            .filter(e -> !e.contains("M"))
            .map(String::toLowerCase)
            .map(e -> stockList2.add(e))
            .collect(toList());
   stockList2.forEach(System.out::println);

Answer 1

You shouldn't be using map() to add elements to a List . map() is supposed to transform a Stream of elements of one type to a Stream of elements of another type.

Use the List returned by collect(Collectors.toList()) :

List<String> stockList2 = 
    stockList.stream()
             .filter(e -> !e.contains("M"))
             .map(String::toLowerCase)
             .collect(Collectors.toList());
stockList2.forEach(System.out::println);

You are not mutating any objects. The String s are already immutable ( toLowerCase produces a new String if necessary) and the original List ( stockList ) is not mutated by the creation of the output List ( stockList2 ).