Can't count numbers

Task: Implement countWords method in Words class.

The input parameter is a list of strings representing lines of text. Count how often the word occurs in the text. If the word "kitten" occurred in a text 23 times then its entry would be "kitten - 23\n". Return statistics as a String containing all the entries.

Omit all words which contain less than 4 letters and appear less than 10 (the words that are too small or too rare) The entries in the resulting String should be also sorted by their amount and then in alphabetical order if it is needed.

Be sure to make your code handling texts that are not in English. You may not use streams, lambdas, or method references in your code

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I am trying to do the task with this code

import java.util.*;

public class Words {

    public String countWords(List<String> lines) {
        TreeMap <String,Integer> cnt = new TreeMap<String, Integer>();
        for(int i = 0;i < lines.size();++i){
            String cur = lines.get(i);
            StringTokenizer st = new StringTokenizer(cur," ");
            ArrayList <String> a = new ArrayList <String> ();
            while(st.hasMoreTokens()){
                String s = st.nextToken();
                ArrayList <String> li = new ArrayList<>();
                li = process(s);
                for(int j = 0;j < li.size();++j){
                    a.add(li.get(j));
                }
            }
            for(int j = 0;j < a.size();++j){
                int freqs = 0;
                if(!cnt.isEmpty()){
                    if(cnt.containsKey(a.get(j))){
                        freqs = cnt.get(a.get(j));
                        cnt.remove(a.get(j));
                    }
                }
                cnt.put(a.get(j),++freqs);
            }
        }
        ArrayList <Pair> ar = new ArrayList<>();
        for(Map.Entry <String,Integer> iter : cnt.entrySet()){
            if(iter.getKey().length() < 4 || iter.getValue() < 10){
                continue;
            }
            ar.add(new Pair(iter.getKey(),iter.getValue()));
        }
        Collections.sort(ar,new Cmp());
        String ans = "";
        for(int i = 0;i < ar.size();++i){
            String t = ar.get(i).getString() + " - ";
            t += Integer.toString(ar.get(i).getFreq());
            ans += t;
            if(i < ar.size() - 1){
                ans += "\n";
            }
        }
        return ans;
    }

    ArrayList <String> process(String s){
        ArrayList <String> ret = new ArrayList<>();
        for(int i = 0;i < s.length();++i){
            if(Character.isLetter(s.charAt(i))){
                int j = i;
                String cur = "";
                while(j < s.length() && Character.isLetter(s.charAt(j))){
                    cur += s.charAt(j);
                    ++j;
                }
                if(!cur.isEmpty()){
                    cur = cur.toLowerCase();
                    ret.add(cur);
                }
                i = j;
            }
        }
        return ret;
    }

    private class Pair{
        private String s;
        private int freq;

        public Pair(String s,int freq){
            this.s = s;
            this.freq = freq;
        }

        public String getString(){
            return s;
        }

        public int getFreq(){
            return freq;
        }
    }

    class Cmp implements Comparator <Pair> {
        public int compare(Pair x,Pair y){
            int z = -Integer.compare(x.getFreq(),y.getFreq());
            if(z == 0){
                z = x.getString().compareTo(y.getString());
            }
            return z;
        }
    }
}

So the problem is I can't count numbers like 1800, what am I doing wrong?