Java Shooter Game - FieldDirector:: calculateAngleForTwoPoints(Circle base, Circle target) - http://tibor-simon.com/programming/auto-aiming-logic-for-games/

FieldDirector_calculateAngleForTwoPoints.java

public static float calculateAngleForTwoPoints(Circle base, Circle target) {

    float px = base.getCenterX();
    float py = base.getCenterY();

    float ex = target.getCenterX();
    float ey = target.getCenterY();

    float angle = 0;

    if (px < ex && py < ey) {
        angle = -(float) (Math.atan(Math.abs(px-ex) / Math.abs(py-ey))*180 / Math.PI);
    } else if (px < ex && py > ey) {
        angle = (float) (Math.atan(Math.abs(px-ex) / Math.abs(py-ey))*180 / Math.PI)-180.0f;
    } else if (px > ex && py > ey) {
        angle = -(float) (Math.atan(Math.abs(px-ex) / Math.abs(py-ey))*180 / Math.PI)-180.0f;
    } else if (px > ex && py < ey) {
        angle = (float) (Math.atan(Math.abs(px-ex) / Math.abs(py-ey))*180 / Math.PI)-360.0f;
    }
    return angle;
}