Test.java

package temp;

import java.math.BigDecimal;
import java.math.BigInteger;

public class Test {
  public static void main(String[] args) {
    System.out.println(toSeventeenDigits(1000));
    System.out.println(toSeventeenDigits(100));
    System.out.println(toSeventeenDigits(10));
    System.out.println(toSeventeenDigits(1));
  }

  private static String toSeventeenDigits(long l) {
    BigDecimal d = new BigDecimal(BigInteger.valueOf(l), 17);
    d = d.divide(new BigDecimal(10).pow(17));
    return d.toPlainString();
  }
}

test.out

0.0000000000000000000000000000001
0.00000000000000000000000000000001
0.000000000000000000000000000000001
0.0000000000000000000000000000000001