java - 如何将过滤后的流数组输出收集为列表

Question

我是 Java8 语法的新手，我们如何在过滤后得到列表的输出。在我的例子中，过滤器返回一个数组。

已添加到评论中，有没有更好的方法可以得到它

Config config = new Config("ooo", "wc", "code");
    Config config1 = new Config("ppp", "wc", "code");
    Config config2 = new Config("ooo", "wc", "code");

    Config[] configs = {config, config1, config2};

    Config config4 = new Config("ooo", "REG", "code");
    Config config5 = new Config("ppp", "REG", "code");
    Config config6 = new Config("ooo", "REG", "code");

    Config[] _configs = {config4, config5, config6};

    PromoCode promoCode = new PromoCode(121, "VOUCHER", "121", configs);
    PromoCode promoCode1 = new PromoCode(122, "VOUCHER", "122", null);
    PromoCode promoCode2 = new PromoCode(123, "LINK", "123", configs);
    PromoCode promoCode3 = new PromoCode(124, "VOUCHER", "124", null);
    PromoCode promoCode4 = new PromoCode(125, "LINK", "125", _configs);
    PromoCode promoCode5 = new PromoCode(126, "LINK", "126", _configs);

    List<String> resultantValues = new ArrayList<String>();
    PromoCode[] promoCodes = {promoCode, promoCode1, promoCode2, promoCode3, promoCode4, promoCode5};
    Stream<PromoCode> stream = Stream.of(promoCodes);
    stream.parallel()
        .filter(x -> x.getCode().equalsIgnoreCase("VOUCHER"))
        .collect(Collectors.toList())
        .parallelStream()
        .forEach(x-> {
            Stream.of(x.getConfigs())
                .filter(t -> t.getOccasion().equals("wc"))
            //after filter, how can we get the output
             // List of list of strings format
                .forEach(o -> {
                    resultantValues.add(o.getProduct()+"_"+o.getProduct());
                });
        });

    System.out.println(resultantValues);

Answer 1

检索 List<List<T>>可以按如下方式完成:

 Stream.of(promoCodes)
       .parallel() // is this really needed?
       .filter(x -> x.getCode().equalsIgnoreCase("VOUCHER"))          
       .map(x-> 
            Stream.of(x.getConfigs())
                .filter(t -> t.getOccasion().equals("wc"))
                .map(o ->  o.getProduct()+"_"+o.getProduct())
                .collect(Collectors.toList())
       )
       .collect(Collectors.toList());

或者如果你想要 List<T>格式化然后使用 flatMap :

 Stream.of(promoCodes)
       .parallel() // is this really needed?
       .filter(x -> x.getCode().equalsIgnoreCase("VOUCHER"))          
       .flatMap(x-> 
            Stream.of(x.getConfigs())
                  .filter(t -> t.getOccasion().equals("wc"))
                  .map(o ->  o.getProduct()+"_"+o.getProduct())
        )
        .collect(Collectors.toList());

或者正如@Holger 提到的那样，对于第二种方法，您可以避免嵌套在 flatMap 中。与:

 Stream.of(promoCodes)
       .parallel() // is this really needed?
       .filter(x -> x.getCode().equalsIgnoreCase("VOUCHER"))
       .flatMap(x -> Arrays.stream(x.getConfigs()))
       .map(x -> x.getProduct() + "_" + x.getProduct())
       .collect(Collectors.toList());

这绝对更具可读性:

---

请注意，我还删除了一些不必要的方法调用，例如对列表的中间收集 .collect(Collectors.toList()) , .parallelStream()等