This question isn't very clear as there is multiple equivalents to sqrt in java there is another class call strict math which has a sqrt function. See source 1 or if your looking the actual code to a sqrt function see source two which has pretty decent algorithm for this problem. Otherwise Corey's answer is correct.

Math.sqrt()

For example:

int num = 16;

System.out.println(Math.sqrt(num));

output:

4.0