print frequency of characters using efficient method

i/p:
 c[]={'a','a','d','a'}
o/p:
 a=3 d=1

import java.util.*;
public class NewClass {
    void frequency(HashMap<Character,Integer> a,char c[])
    {
        for(int i=0;i<c.length;i++)
        {    
            if(a.containsKey(c[i]))
            {
                a.put( c[i],a.get(c[i])+1);
            }
            else
                a.put(c[i],1);
        }
       
    }
    public static void main(String args[])
    {
        char c[]={'a','b','b','e','d'};
        HashMap<Character,Integer> a=new HashMap<Character,Integer>();
        NewClass ob=new NewClass();
        ob.frequency(a,c);
            System.out.print(a);
    }
}