1､ 使用IntStream实现的几种方法

intStream文档：https://docs.oracle.com/javase/8/docs/api/java/util/stream/IntStream.html

1) IntStream.range

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class Main {
  public static void main(String[] args) {
    System.out.println(getIndexList("Hello world!",'l'));
    System.exit(0); //success
  }
  public static List<Integer> getIndexList(String s, char c) {
    return IntStream.range(0, s.length())
                    .filter(index -> s.charAt(index) == c)
                    .boxed()
                    .collect(Collectors.toList());
 }
}

2) IntStream.iterate

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class Main {
  public static void main(String[] args) {
    System.out.println(getIndexList("Hello world!",'l'));
    System.exit(0); //success
  }
 public static List<Integer> getIndexList(String s, char c) {
    return IntStream.iterate(s.indexOf(c), i -> s.indexOf(c, i + 1))
            .takeWhile(i -> i > -1)
            .boxed()
            .collect(Collectors.toList());
 }
}

或

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class Main {
  public static void main(String[] args) {
    System.out.println(getIndexList("Hello world!",'l'));
    System.exit(0); //success
  }
 private static List<Integer> getIndexList(String word, char c) {
  return IntStream
          .iterate(word.indexOf(c), index -> index >= 0, index -> word.indexOf(c, index + 1))
          .boxed()
          .collect(Collectors.toList());
 }
}

2､使用Stream.of实现

import java.util.Scanner;
import java.util.regex.MatchResult;
import java.util.stream.Stream;

public class Main {
  public static void main(String[] args) {
        Stream.of("Hello world!")
                .map(Scanner::new) // 将字符串映射为 Scanner 对象
                .flatMap(s -> s.findAll("l")) // 查找所有匹配字符 "l" 的位置
                .map(MatchResult::start) // 获取匹配结果的开始位置
                .forEach(System.out::println); // 打印每个位置索引
    System.exit(0); //success
  }
}