Largest prime factor, Java

Largest prime factor

//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_72

import java.util.*;
import java.lang.*;
import java.math.BigInteger;
import java.util.ArrayList;

class Rextester
{  
    public static void main(String args[])
    {
        BigInteger test = new BigInteger("211"); 
        ArrayList<String> list = new ArrayList<>();
        list = calc(test);
        String res = "";
        for(int i = 0; i < list.size(); i++){
            res += list.get(i)+", ";
        }
        System.out.println(res);
    }
    
     public static Boolean isPrime(BigInteger a) {
         BigInteger b = new BigInteger("2");
         BigInteger c = new BigInteger("1");
         BigInteger d = new BigInteger("0");
         int res = 5;
        if(a.mod(b).compareTo(a) == 0) { return false; } 
        b.add(c); //3
        if(a.mod(b).compareTo(a) == 0) { return false; } 
        b.add(c);
        b.add(c); //5
        if(a.mod(b).compareTo(a) == 0) { return false; } 
        b.add(c);
        b.add(c); //7
        if(a.mod(b).compareTo(a) == 0) { return false; } 
        b.add(c);
        b.add(c);
        b.add(c);
        b.add(c); //11
        
        return true; 
    }
     public static ArrayList<String> calc(BigInteger a) {
        ArrayList<String> list = new ArrayList<>();
         list.add("(Values)");
         BigInteger count = new BigInteger("1");
         BigInteger c = new BigInteger("1");
         BigInteger d = new BigInteger("0");
         
        while (a.compareTo(count) == -1) {
            System.out.println("Made it");
            if(isPrime(count)) {
                if (a.mod(count).compareTo(d) == 0) {list.add(a.toString()); }
            }
            count.add(c);
        }
        
        return list;
    }
}


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