code: public float nextCentral() {
          // Average 12 uniformly-distributed random values.
          float sum = 0.0f;
          for (int j = 0; j < 12; ++j)
              sum += gen.nextFloat();
          
          // Subtract 6 to center about 0.
          return stddev*(sum - 6) + mean;
      }
docs: Compute the next random value using the Central Limit Theorem,
  which states that the averages of sets of uniformly-distributed
  random values are normally distributed.