将两个列表与一个列表进行比较 JAVA 流不起作用

Question

我有一个任务：

A sequence of positive integers numbers and a sequence of strings stringList are given. Get a new sequence of strings according to the following rule: for each value n from sequence numbers, select a string from sequence stringList that starts with a digit and has length n.

If there are several required strings in the stringList sequence, return the first; if there are none,then return the string "Not found"

例如：

input: {1, 3, 4}, {"1aa", "aaa", "1", "a"}
output: {"1", "1aa", "Not Found"}

我的输出：

[java.util.stream.ReferencePipeline@7f690630, 
java.util.stream.ReferencePipeline@edf4efb, 
java.util.stream.ReferencePipeline@2f7a2457]

我的代码：

(List<Integer> numbers, List<String> stringList) {
return numbers.stream()
        .map(Object::toString)
        .map(value -> (stringList.stream().filter(e -> (Character.isDigit(e.charAt(0)))).map(s -> {
            if (((Object) (s.length())).toString().equals(value)) return s;
            return "Not Found";
}))).map(Object::toString).collect(Collectors.toList());

请帮忙。

Answer 1

以下代码使用正则表达式 "\d.*" 来匹配以数字开头的字符串应该有效。

此外，无需将整数值转换为字符串，然后再将此字符串转换回 int 以匹配长度。

public static List<String> task(List<Integer> nums, List<String> strs) {
    return nums.stream()
        .map(i -> strs.stream()
                      //.filter(s -> s.length() == i && i > 0 && Character.isDigit(s.charAt(0)))
                      .filter(s -> s.length() == i && s.matches("\d.*"))
                      .findFirst() // Optional<String>
                      .orElse("Not Found")
        )
        .collect(Collectors.toList());
}

测试

System.out.println(task(Arrays.asList(1, 3, 4), Arrays.asList("1aa", "aaa", "1", "a")));

输出：

[1, 1aa, Not Found]

更新

要摆脱嵌套流，应将字符串列表转换为过滤映射，其中长度是键，第一个匹配字符串是值，然后在流式传输整数列表时使用此映射：

public static List<String> taskFast(List<Integer> nums, List<String> strs) {
    Map<Integer, String> strByLength = strs
        .stream()
        .filter(s -> s.length() > 0 && Character.isDigit(s.charAt(0)))
        .collect(Collectors.toMap(
            String::length,
            s -> s,
            (s1, s2) -> s1 // keep the first match for the same length
        ));

    return nums.stream()
        .map(i -> strByLength.getOrDefault(i, "Not Found"))
        .collect(Collectors.toList());
}

将两个列表与一个列表进行比较 JAVA 流不起作用

Compare two lists with one JAVA Stream do not work

java

lambda

list

java-stream