Ransom Note: HashMap

First Thoughts: should return false if the letter does not exist in the first place, or letter used exceeds letter stored. Using hashmap can shorten time of retrieval.

My Solution:

class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {
        HashMap<Character, Integer> map = new HashMap<>();
        for (int i=0; i<magazine.length(); i++) {
            char c = magazine.charAt(i);
            map.putIfAbsent(c, 0);
            map.put(c, map.get(c)+1); 
        }
        for (int j=0; j<ransomNote.length(); j++) {
            char ch = ransomNote.charAt(j);
            if (map.get(ch)==null) return false;
            map.put(ch, map.get(ch)-1);
            if (map.get(ch)<0) return false;
        }
        return true;
    }
}