如何在TreeMap中获取索引键

import java.util.TreeMap;

public class TreeMapExample {
    public static void main(String[] args) {
        // 创建一个TreeMap
        TreeMap<Integer, String> treeMap = new TreeMap<>();

        // 添加键值对到TreeMap
        treeMap.put(1, "Apple");
        treeMap.put(3, "Banana");
        treeMap.put(5, "Orange");
        treeMap.put(7, "Grape");
        treeMap.put(9, "Mango");

        // 获取索引键
        int indexKey = 4;
        Integer key = treeMap.ceilingKey(indexKey);

        if (key != null) {
            // 打印结果
            System.out.println("索引键 " + indexKey + " 的下一个键是 " + key);
            System.out.println("对应的值是 " + treeMap.get(key));
        } else {
            System.out.println("索引键 " + indexKey + " 不存在");
        }
    }
}

索引键 4 的下一个键是 5
对应的值是 Orange