位图从屏幕上掉下来

Question

我有一个位图图像，我试图保持在屏幕上它停在顶部但总是落在底部。我不知道为什么会这样做，我的书告诉我它是正确的。这是一个涉及重力和提升的游戏，但我的船可以升到屏幕的顶部，但永远不会从屏幕上掉下来。这是我的代码。

private final int GRAVITY=-12;
//stop ship from leaving the screen
private int maxY;
private int minY;
// limit the bounds of the ships speed
private  final int MIN_SPEED=1;
            private final int MAX_SPEED= 20;

private Bitmap bitmap;
private int x,y;
private int speed = 0;
// Constructor
public PlayerShip(Context context, int screenX, int screenY){
    boosting= false;
    x=50;
    y=50;
    speed=1;
    bitmap= BitmapFactory.decodeResource(
            context.getResources(), R.drawable.ship);
    maxY=bitmap.getHeight()-screenY;
    minY=00;
}
public void update (){


    //ARE WE BOOSTING
    if (boosting) {
        //speed up
        speed += 2;
            }
    else {
        //SLOW DOWN
        speed-=5;
    }
    if ( speed> MAX_SPEED){
        speed=MAX_SPEED;
    }
    // Never Stop Completely
    if (speed <MIN_SPEED){
        speed = MIN_SPEED;
    }
    //move the ship up or down
    y-=speed+ GRAVITY;
    //BUT DON'T LET SHIP STRAY OFF SCREEN
    if ( y<minY){
        y=minY;
    }
    if(y<maxY){
        y=maxY;

}x++;}

//Getters
public Bitmap getBitmap(){
    return bitmap;
}
 public int getSpeed(){
 return speed;
  }
 public int getX(){
    return x;
}
public int getY() {
return y;
}

public void setBoosting() {

     boosting = true;
}
public void stopBoosting(){
    boosting=false;
}

}