PE #3, Java - rextester

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//Title of this code
//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_45

import java.util.*;
import java.lang.*;

class Rextester
{  
    public static void main(String args[])
    {
        BigInteger spork = 600851475143; 
        boolean potato = isPrime(spork);
        System.out.println(potato); 
    }
    
    public static boolean isPrime(BigInteger number) { 
     String numStr = ""+number; 
     int numSum =0; 
     int sqrtNum = (int)Math.sqrt((double)(number)); 
     int bigFac = -1; 
        
        for(int i =0; i<numStr.length(); i++) { 
            numSum +=  Integer.parseInt(numStr.substring(i, i+1)); 
        }
        System.out.println("Sum: "+numSum); 
        
        if (numSum%3==0) { return false; } 
        if ((number%2==0)&&(number!=2)) { return false; } 
        if ((numStr.substring(numStr.length()-1, numStr.length()))=="5") { return false; } 
        
        for(int i=7; i<sqrtNum; i+=2) {
            if(number%i==0) {   
                System.out.println("Factor: "+i);
                return false; 
            }
        }
            
        return true; 
    }
}

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Compilation time: 0.55 sec, absolute service time: 0.71 sec