How do I get a N digit random prime number?

What am doing wrong here in this piece of code ? What is a better way to generate N digit random number ?

public static boolean isPrime(long n){
    return (Math.pow(2, n-1) % n == 1) && (Math.pow(3, n-1) % n == 1);
}

public static long getPrime(int digit){
    long sample;
    long multiple = Math.round(Math.pow(10, digit));
    do{         
        sample = Math.round(Math.random()* multiple);
        //System.out.println(sample);
    }while(!isPrime(sample));
    return sample;
}