public class Main {

  public static void main(String[] args) {

    System.out.println(Math.atan2(0.5, 0.5));

    System.out.println(Math.atan2(-0.5, -0.5));

    System.out.println(Math.atan2(5, 5));

    System.out.println(Math.atan2(10, 20));

    System.out.println(Math.atan2(-5, -5));

    System.out.println(Math.atan2(-10, 10)); 

  }

}

​