gistfile1.java

def array = []
def tmp = ""
array = ('0'..'9') + ('a'..'z') + ('A'..'Z') + '_'

(1..16).each {
  tmp += array[Math.floor(Math.random() * array.size()) as int]
}
println tmp

gistfile2.rb

array = []
tmp = ""
array = ('0'..'9').to_a + ('a'..'z').to_a + ('A'..'Z').to_a + '_'.to_a

(1..16).each {
  tmp += array[rand(array.size())]
}
print tmp + "\n"

randomAlphabetNum16.groovy

def array = (0..9) + ('a'..'z') + ('A'..'Z') + '_'
def getRandomChar = {
  array.with {
    Collections.shuffle(it)
    head()
  }
}

println(
  (1..16).collect{ getRandomChar() }.join()
)

randomAlphabetNum16.py

import string
import random

def getRandomAlphabetNum(n):
	elements = string.digits + string.letters + '_'
	return ''.join(random.choice(elements) for i in xrange(n))

if __name__ == '__main__':
	print getRandomAlphabetNum(16)