Find if two given Strings are Anagrams

 
import java.util.*;
 class Anagram{
    public static void main(String args[]){
       String s1 = "abcb";
       String s2 = "bbac";
       boolean output = anagram(s1,s2);
       System.out.println(output);  
    }
    private static boolean anagram(String a, String b){
       if ((a == null || b == null)  
            || (a.length() != b.length())
            || (a.isEmpty())) return false;
       int len = a.length();
       Map<Character, Integer> fmap = new HashMap<Character, Integer>();
       for(int i=0; i<len; i++){
          char key = a.charAt(i);
          Integer value = fmap.get(key);
          if(value == null) fmap.put(key,1);
          else fmap.put(key,value+1);
       }
      //Check string s2 frequency in fmap
      for(int i=0; i<len; i++){
         char key = b.charAt(i);
         Integer value = fmap.get(key);
         if(value == null) return false;  
        else{
            if(value == 1) fmap.remove(key);
            else fmap.put(key,value-1);
        }
      }
      return fmap.size() == 0;
    }