How to implement C's drand48 and srand48 in Java?

I need to implement C's drand48() and srand48() random number generator and initializer for a programming project written in Java. The program needs to mimic the generation of a pseudo random number from these functions in C given a specific seed.

According to the man pages:

All the functions work by generating a sequence of 48-bit integers, Xi, according to the linear congruential formula:

        Xn+1 = (aXn + c) mod m, where n >= 0

The parameter m = 2^48, hence 48-bit integer arithmetic is performed. Unless lcong48() is called, a and c are given by:

        a = 0x5DEECE66D
        c = 0xB

In this srand48() implementation below, I set the high order 32-bits of Xi to the argument seedval. The low order 16-bits are set to the arbitrary value 0x330E, according to the man pages. I do not understand how to apply the congruential formula to extract a random number. Any help would be greatly appreciated, thanks!

public void srand48(long seedval) {
  this.seed = seedval & 0xFFFFFFFF;
  this.seed = (this.seed << 16) | 0x330E;
}

public double drand48() {
  this.seed = (0x5DEECE66DL * this.seed + 0xBL) & ((1L << 48) - 1);
  return this.seed;
}

The numbers that come out when drand48() is called are out of the range [0.0, 1.0). Could someone also explain why 48 bits is so significant for this pseudo random number generation?