Integer to English Word

public String numberToWords(int num) {
        String[] units = {""," Thousand"," Million"," Billion"};
        int i = 0;
        String res="";
        while(num>0) {
            int temp = num%1000;
            if(temp>0) res = convert(temp) + units[i] + (res.length()==0 ?"": " "+res);
            num /= 1000;
            i++;
        }
        return res.isEmpty()? "Zero" : res;
    }
    public String convert(int num){
        String res = "";
        String[] ten = {"Zero", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};
        String[] hundred = {"Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        String[] twenty = {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        if(num>0) {
            int temp = num/100;
            if(temp>0) {
                res += ten[temp] + " Hundred";
            }
            temp = num%100;
            if(temp>=10 && temp<20){
                if(!res.isEmpty()) res +=" ";
                res = res + twenty[temp%10];
                return res;
            }else if(temp>=20){
                temp = temp/10;
                if(!res.isEmpty()) res +=" ";
                res = res + hundred[temp-1];
            }
            temp = num%10;
            if(temp>0) {
                if(!res.isEmpty()) res +=" ";
                res = res + ten[temp];
            }
        }
        return res;
    }