Java：高效提取字符串中指定长度单词的方法

import java.util.Arrays;
import java.util.Objects; // 用于Objects.requireNonNullElseGet，处理null或空字符串

public class WordExtractor {

    /**
     * 从给定字符串中提取所有指定长度的单词。
     *
     * @param sentence   输入的句子字符串。
     * @param wordLength 目标单词的长度。
     * @return 包含所有符合长度要求的单词的字符串数组。
     *         如果输入句子为空或null，则返回空数组。
     */
    public String[] findWordsByLength(String sentence, int wordLength) {
        // 1. 处理null或空字符串输入，避免NullPointerException
        // Objects.requireNonNullElseGet(sentence, () -> "") ensures sentence is not null
        // .trim() removes leading/trailing spaces
        // .split("\s+") splits by one or more whitespace characters
        String[] words = Objects.requireNonNullElseGet(sentence, () -> "")
                                .trim()
                                .split("\s+");

        // 2. 将单词数组转换为流，并进行过滤和收集
        return Arrays.stream(words)
                     .filter(word -> !word.isEmpty() && word.length() == wordLength) // 过滤空字符串和符合长度的单词
                     .toArray(String[]::new); // 将结果收集到新的字符串数组中
    }

    public static void main(String[] args) {
        WordExtractor extractor = new WordExtractor();

        // 示例1
        String s1 = "Monday is a new day";
        int n1 = 3; // 3字母单词
        String[] result1 = extractor.findWordsByLength(s1, n1);
        System.out.println("Input: "" + s1 + "", Length: " + n1 + " -> Result: " + Arrays.toString(result1)); // 预期: {"new", "day"}

        // 示例2
        String s2 = "Monday is a new day";
        int n2 = 2; // 2字母单词
        String[] result2 = extractor.findWordsByLength(s2, n2);
        System.out.println("Input: "" + s2 + "", Length: " + n2 + " -> Result: " + Arrays.toString(result2)); // 预期: {"is"}

        // 示例3：包含多个空格
        String s3 = "  hello   world  java  ";
        int n3 = 5;
        String[] result3 = extractor.findWordsByLength(s3, n3);
        System.out.println("Input: "" + s3 + "", Length: " + n3 + " -> Result: " + Arrays.toString(result3)); // 预期: {"hello", "world"}

        // 示例4：空字符串或null输入
        String s4 = "";
        int n4 = 3;
        String[] result4 = extractor.findWordsByLength(s4, n4);
        System.out.println("Input: "" + s4 + "", Length: " + n4 + " -> Result: " + Arrays.toString(result4)); // 预期: {}

        String s5 = null;
        int n5 = 3;
        String[] result5 = extractor.findWordsByLength(s5, n5);
        System.out.println("Input: "" + s5 + "", Length: " + n5 + " -> Result: " + Arrays.toString(result5)); // 预期: {}
    }
}