sqrt
public static double sqrt(double a)

Returns the correctly rounded positive square root  of a double value. Special
cases:

· If the argument is NaN or less than zero, then the result is NaN.
· If the argument is positive infinity, then the result is positive infinity.
· If the argument is positive zero or negative zero, then the result is the same
as the argument.

Otherwise, the result is the double value closest to the true mathetmatical
square root of the argument value.

Parameters:
a - a double

public class sqrt
{
public static void main(String args[])
{
System.out.println(java.lang.Math.sqrt(4));

}
}

to get the square root  of a number,let's say 9 (w/c is 3) try this
one in your main method:

double anyNumber=9;
System.out.println("Square root: " + Math.sqrt(anyNumber));