.atan2()

The Math.atan2() method returns the counterclockwise angle, in radians, between a (x,y) point and the positive x-axis.

Math.atan2(y, x)

Note that the order of arguments is first the Y coordinate, then the X coordinate.

Use Math.atan2() to return the angle of point (0.0, 1.0) and the x-axis:

public class Main {
  public static void main(String[] args) {

    double x = 0.0;
    double y = 1.0;

    System.out.println(Math.atan2(y, x));
    // Output: 1.5707963267948966
  }
}