Physically more or less correct reflection of a ball on a round object

reflectExample.java

// physically more or less correct reflection of a ball on a round object (paddle)
// to simulate speed-loss at collision, multiply ballSpeed with a factor like 0.8f (line 21)

// if you wanted the paddle to bounce away, too, it would be in the reversed direction of refAngle
// but considering that both objects are moving now, you should also consider both 
// objects' mass and current speed/dir of the paddle

// check for collisions
float minDist = mBall.getWidth()/2 + mPaddle.getWidth()/2; // radius ball + paddle
float dX = mBallX - mPaddleX; // horizontal difference between ball and paddle
float dY = mBallY - mPaddleY; // vertical diff...
float dist = (float) Math.sqrt(dX*dX + dY*dY); // distance between ball and paddle

// cross product of (dX,dY)x(mballspeedX,Y) < 0: they show in different directions
// means, the ball does not go away from the paddle but to it = there may be a new collision coming
float crossProd = dX*mBallSpeedX + dY*mBallSpeedY;
if(crossProd<0 && dist <= minDist) { // collision and ball moving to paddle
    // atan2(Y,X) gives the angle of the cartesian coordinate
    double refAngle    = Math.atan2(dY, dX); // reflection angle (angle from paddle to ball)
    double comingAngle = Math.atan2(-mBallSpeedY, -mBallSpeedX); // angle ball is coming, reversed direction
    // angle ball shall go away, we take the difference between the reflectionAngle and the
    // balls direction and a add it to the reflection angle
    double goingAngle = refAngle + (refAngle-comingAngle);
    // calculate new direction
    float ballSpeed = (float) Math.sqrt(mBallSpeedX*mBallSpeedX + mBallSpeedY*mBallSpeedY);
    mBallSpeedX = (float)(ballSpeed * Math.cos(goingAngle));
    mBallSpeedY = (float)(ballSpeed * Math.sin(goingAngle));
}