Google Billboard Puzzle

EulersNumber.java

import java.math.BigDecimal;
import java.math.MathContext;

public class EulersNumber {
  public static void main(String[] args) {
    BigDecimal e = BigDecimal.ONE;
    BigDecimal bigDecimal = BigDecimal.ONE;

    for(int i=1;i<100;i++) {
      bigDecimal = bigDecimal.multiply(new BigDecimal(i * 1.0 + ""));
      e = e.add(new BigDecimal(1.0 + "").divide(bigDecimal, new MathContext(10000)));
    }

    String strDecimalPart = (e + "").substring(2);

    for(int i=0;i<strDecimalPart.length()-10;i++) {
      long num = Long.parseLong(strDecimalPart.substring(i,i+10));
      if(isPrime(num)) {
        System.out.println("First 10 digit prime number in the decimal part of e : " + num);
        break;
      }
    }
  }

  public static boolean isPrime (long n) {
    if (n<=1) return false;
    if (n==2) return true;
    if (n%2==0) return false;
    long m = (long) Math.sqrt(n);

    for (long i=3; i<=m; i+=2)
      if (n%i==0)
        return false;

    return true;
  } 
}