Why is getHeight() not equal to the frame height?

Question

I am making a bounc*ing cube that moves around a frame. To detect the boundary of the frame, I used getHeight() and getWidth() from the JComponent in the JFrame and calculate the possible boundary so that the cube can rebound.

It works totally fine for getWidth(). However, I cannot make it rebound on the boundary for getHeight().

When the cu*be goes to the top of the window, it keeps going for about 10 pixels before rebounding.

Here is the screenshot for this problem.

Here is my code:

public class Cube {
    final public static int DIMENSION = 100;
    private final static int D_X = 20;
    private final static int D_Y = -20;
    private final static int SPEED = 1;
    private int xVelBK;
    private int yVelBK;
    private int xVel;
    private int yVel;
    private CubePoint[] pts;

    public  Cube(int x, int y){
    xVel = SPEED;
    yVel = SPEED;

    pts = new CubePoint[8];

    pts[0] = new CubePoint(x,y);
    pts[1] = new CubePoint(x,y - DIMENSION);
    pts[2] = new CubePoint(x + DIMENSION,y - DIMENSION);
    pts[3] = new CubePoint(x + DIMENSION,y);

    pts[4] = new CubePoint(x + D_X,y + D_Y);
    pts[5] = new CubePoint(x + D_X,y + D_Y - DIMENSION);
    pts[6] = new CubePoint(x + D_X + DIMENSION,y + D_Y - DIMENSION);
    pts[7] = new CubePoint(x + D_X + DIMENSION,y + D_Y);
    }

    public void move(int componentWidth, int componentHeight){
    ......

    //bug in here
    if (pts[0].getY()  - DIMENSION - D_Y <= 0 ) {
        yVel = SPEED;
    }

    .......
    }
..........
}

Here is the component :

public class CubeComponent extends JComponent {
    ...........

    private Cube cube;
    private Timer timer;
    private CubePoint originMousePos;
    private CubePoint closest;
    private Font font;
    private int mode;


    private int delay = DEFAULT_DELAY ;

    private int mouseToClosestX;
    private int mouseToClosestY;

    class CubeMoveListener implements ActionListener{
    @Override
    public void actionPerformed(ActionEvent e){
        cube.move(getWidth(),getHeight());
        repaint();
    }

    }



    @Override
    public void paintComponent(Graphics g){
    Graphics2D g2 = (Graphics2D)g;

    cube.draw(g2);
    }

}

Answer 1

As far as I can tell:

 if (pts[0].getY()  - DIMENSION - D_Y <= 0 ) {

SHOULD be used to determine when the cube reaches the bottom of the component so you should be testing for ">"

and

if ( pts[0].getY() >= componentHeight) {

SHOULD be used for testing when the cube reaches the top. So it should be:

if ( pts[0].getY() <= 0) {

The key point is that:

1. ONE of the vertical tests needs to test for "<= 0".
2. the structure of the horizontal and vertical test should be the same, you just need to change the variable names.