Design Compressed String Iterator

/* Programmer     : Dhruv Patel
*  Problem Name   : 604. Design Compressed String Iterator 
*  Used In        : Leetcode
*  Used As        : Contest
*  Problem        :
*  Design and implement a data structure for a compressed string iterator. It should support the following operations: 
*  next and hasNext.
*  The given compressed string will be in the form of each letter followed by a positive integer representing the 
*  number of this letter existing in the original uncompressed string.
*  next() - if the original string still has uncompressed characters, return the next letter; Otherwise return a white space.
*  hasNext() - Judge whether there is any letter needs to be uncompressed.
*  Note:
*  Please remember to RESET your class variables declared in StringIterator, as static/class variables are persisted across multiple test cases. Please see here for more details
*  Thoughts     => 
*  The following code is solved with the use of two lists. One list maintains the character and another list maintain the frequency.
*  However, the hashmap seems to be a good choice for this problem. But, there are some edge cases where it fails for duplicate keys.
*  Every time next() called then we decrement the frequency by 1 and rest of operations are self-explanatory.
*/
public class StringIterator {
    public int i=0;
    public List<Character> list_;
    public List<Integer> counter;
    int frequency=0;
    public StringIterator(String compressedString) {
        list_=new ArrayList<>();
        counter=new ArrayList<>();
        int i=0;
        while(i<compressedString.length()){
            char t = compressedString.charAt(i);
            int j = i+1;
            String n = "";
            while(j<compressedString.length() && Character.isDigit(compressedString.charAt(j))){
                 n+=compressedString.charAt(j);
                 j++;
            }
            int f = Integer.parseInt(n);
            list_.add(t);
            counter.add(f);
            i=j;    
            }
    }
    
    public char next() {
        char r=' ';
        if(!hasNext()){return ' ';}
         if(counter.get(frequency)>0){
          r=list_.get(frequency);
          counter.set(frequency,counter.get(frequency)-1);
          if(counter.get(frequency)==0){
              frequency++;
          }
          return r;
          }
        
        return ' ';
    }
    public boolean hasNext() {
        return frequency<list_.size();
    }  
    
}

/**
 * Your StringIterator object will be instantiated and called as such:
 * StringIterator obj = new StringIterator(compressedString);
 * char param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */