ЯсноПонятно24

import java.util.List;

public class App {

    public static int findIndexOfNearest(List<Integer> list, int target) {
        return list.stream()
                .min((a, b) -> Integer.compare(Math.abs(a - target), Math.abs(b - target)))
                .map(list::indexOf)
                .orElse(-1);
    }

    public static void main(String[] args) {
        List<Integer> list = List.of(1, 3, 5, 7, 9);
        int target = 6;
        
        int index = findIndexOfNearest(list, target);
        System.out.println("Index of the nearest number to " + target + " is: " + index);
    }
}