Java 8 Streams 从 List<String[]> 中过滤出空 String[]？

Question

假设我们有 2 个测试 String[]，类似于：

String[] test = {"","","","",""};
String[] test2 = {"Test","Name", "5.00", "NY", "Single"};

然后我们将它们附加到这样的列表中：

List<String[]> testList = new ArrayList<>();
testList.add(test);
testList.add(test2);

我试图做的目标是使用 Java 8 流在每个 String[] 的索引 2 中找到支付的美元总和。但我似乎无法过滤掉每个索引中包含空值的 String[]。这是我的尝试：

public static void main(String[] args) {
        String[] test = {"","","","",""};
        String[] test2 = {"Test","Name", "5.00", "NY", "Single"};
        List<String[]> testList = new ArrayList<>();
        testList.add(test);
        testList.add(test2);
        System.out.println(testList);
        testList.stream().map(Arrays::asList).filter(i -> !i.isEmpty())
                                             .mapToDouble(columnsPerRow -> 
                                                Double.parseDouble(columnsPerRow.get(2)))
                                             .sum();
        System.out.println(testList);
    }

我尝试过滤它是使用 filter(i -> !i.isEmpty())

它抛出的错误仍然是：

Exception in thread "main" java.lang.NumberFormatException: empty String
    at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1842)
    at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
    at java.lang.Double.parseDouble(Double.java:538)
    at Main.lambda$main$0(Main.java:23)
    at java.util.stream.ReferencePipeline$6$1.accept(ReferencePipeline.java:244)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.DoublePipeline.collect(DoublePipeline.java:500)
    at java.util.stream.DoublePipeline.sum(DoublePipeline.java:411)
    at Main.main(Main.java:23)

Answer 1

在i -> !i.isEmpty()中，i是ArrayList，它包含("","","","","")，它不为空，你可以检查索引2处的值，你可以这样做

double r = testList.stream().map(Arrays::asList)
                   .filter(i -> !i.isEmpty())              // ensure list not empty
                   .map(i -> i.get(2))                     // keep only 3rd element
                   .filter(i -> !i.isEmpty())              // ensure string isn't empty
                   .mapToDouble(Double::parseDouble)       // map to double
                   .sum();

第一个.filter(i -> !i.isEmpty())可以替换为.filter(i -> i.size() >= 3)

Answer 2

您过滤了错误的实体：

public static void main(String[] args)
{
    String[] test = {"","","","",""};
    String[] test2 = {"Test","Name", "5.00", "NY", "Single"};
    String[] test3 = {"Test","Name", "13.00", "NY", "Single"};
    List<String[]> testList = new ArrayList<>();
    testList.add(test);
    testList.add(test2);
    testList.add(test3);
    double sum = testList.stream()
                    .map(Arrays::asList)
                    .filter(i -> i.size() > 2 && !i.get(2).isEmpty())
                    .mapToDouble(columnsPerRow -> Double.parseDouble(columnsPerRow.get(2)))
                    .sum();
    System.out.println(sum);
}

按预期生成18.0。

过滤阶段会移除少于 2 个元素或第 2 个元素为空的数组。

Answer 3

相信你的话，过滤掉每个索引中包含空值的 String[]。

    String[] test = {"","","","",""};
    String[] test2 = {"Test","Name", "5.00", "NY", "Single"};

    double total = Stream.of(test, test2)
            .filter(arr -> !Arrays.stream(arr).allMatch(String::isEmpty))
            .mapToDouble(arr -> Double.parseDouble(arr[2]))
            .sum();
    
    System.out.println(total);

输出是：

5.0

我的代码只过滤出 all 值为空字符串的字符串数组。因此，如果美元值为 "" 并且数组包含其他非空字符串，它将出现异常。没有美元值的数组需要只保存空字符串，这就是您应该想要的。为了验证。您可能想要捕获NumerFormatException 并报告验证错误。或者在进行计算之前验证流。