Here you can find the source of calculateAngle(double dx, double dy)
public static double calculateAngle(double dx, double dy)
//package com.java2s; /******************************************************************************* * Copyright C 2012, The Pistoia Alliance * //from w w w . j ava 2s. co m * Permission is hereby granted, free of charge, to any person obtaining * a copy of this software and associated documentation files (the * "Software"), to deal in the Software without restriction, including * without limitation the rights to use, copy, modify, merge, publish, * distribute, sublicense, and/or sell copies of the Software, and to * permit persons to whom the Software is furnished to do so, subject to the following conditions: * * The above copyright notice and this permission notice shall be * included in all copies or substantial portions of the Software. * * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. ******************************************************************************/ public class Main { public static double calculateAngle(double dx, double dy) { double alpha; if (Math.abs(dx) > Math.abs(dy)) { double tg = dy / dx; alpha = Math.atan(tg); if (dx < 0) { alpha += Math.PI; } } else { double ctg = dx / dy; alpha = Math.PI / 2. - Math.atan(ctg); if (dy < 0) { alpha += Math.PI; } } return alpha; } }