import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
 
public class Main {
    public static void main(String[] args) {
        final List<Character> list = Arrays.asList('A', 'B', 'A', 'C', 'E', 'A');
        final Map<Character, List<Integer>> indexMap = IntStream.range(0, list.size()).boxed()
                .collect(Collectors.groupingBy(list::get));
        System.out.println(indexMap);
    }
}

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