Why it's possible to instantiate String and not possible for Number(Long,Double,Integer...)?

Hi why it's possible to instantiate String and not possible for Numbers .I have made an example for that

public static void main(String[] args) throws InstantiationException,
        IllegalAccessException {
    String a = "s";
    String newInstance = a.getClass().newInstance();
    System.out.println(newInstance);
    Double b = 0d;
    Double newInstance2 = b.getClass().newInstance();
    System.out.println(newInstance2);
}