你可以使用 Hashmap 以及评论中描述的一些简单操作：

import java.util.HashMap;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        System.out.println(isSubset("light", "lig"));
        System.out.println(isSubset("light", "gil"));
        System.out.println(isSubset("light", "liig"));
    }
    private static boolean isSubset(String str, String sub) {
        // change strings to lower case
        str = str.toLowerCase();
        sub = sub.toLowerCase();
        // initialze hashmap to store character frequencies in substring
        Map<Character, Integer> map = new HashMap<>();
        // iterate over the substring and store number of occurrences per character
        for(char c : sub.toCharArray()) {
            Integer count = map.get(c);
            map.put(c, count!=null ? count+1 : 1);
        }
        // iterate over the original string and decrement the count of each character present in the map
        for(char c : str.toCharArray()) {
            Integer count = map.get(c);
            if(count!=null)
                map.put(c, count-1);
        }
        // check if all characters count has reached to zero, i.e., present in the string
        for(int count : map.values())
            if(count != 0)
                return false;
        return true;
    }
}

输出：

true
true
false

改用substring（）方法