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java - Java 8 Streams 从列表中过滤出空字符串 []?

问题描述

假设我们有 2 个测试字符串 []，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)

标签： javajava-8streamjava-stream

解决方案

在i -> !i.isEmpty(),i是ArrayList, 并且它包含("","","","","")si 它不是空的, 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)

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