How to make a random position whenever I start running the program?

问题

I have an image which moves in a random direction. My problem is everytime I start running the program, the image always appear at the upper-left corner and moves a diagonal direction and after hitting the wall, it starts to move random direction. How can I make the image appear in a random position everytime I execute or run the program? Any help would be much appreciated...

Here's the code:

public class Ball extends JPanel implements Runnable
{

private Image ball;
private Thread animator;
private int x;
private int y;
private final int DELAY = 50;
private int xVelocity = 1;
private int yVelocity = 1;
private static final int RIGHT_WALL = 400;
private static final int LEFT_WALL = 1;
private static final int DOWN_WALL = 400;
private static final int UP_WALL = 1;
private boolean showImage;

public Ball()
{
    randomPosition();
    setRandomDirection();
    setBackground(Color.BLACK);
    setDoubleBuffered(true);

    ImageIcon ii = new ImageIcon(this.getClass().getResource("ball.gif"));
    ball = ii.getImage();
    x = y = 10;

}

public void addNotify()
{
    super.addNotify();
    animator = new Thread(this);
    animator.start();
}

public void paint(Graphics g)
{
    super.paint(g);

    Graphics2D g2d = (Graphics2D) g;
    g2d.drawImage(ball, x, y, this);
    Toolkit.getDefaultToolkit().sync();
    g.dispose();
}

public void move()
{

    x += xVelocity;
    y += yVelocity;

    if (x >= RIGHT_WALL)
    {
        x = RIGHT_WALL;

        randomDirection();
    }

    if (y <= UP_WALL)
    {
        y = UP_WALL;

        randomDirection();
    }

    if (x <= LEFT_WALL)
    {
        x = LEFT_WALL;

        randomDirection();
    }
    if (y >= DOWN_WALL)
    {
        y = DOWN_WALL;

        randomDirection();
    }

}

private void randomDirection()
{
    double speed = 2.0;
    double direction = Math.random() * 2 * Math.PI;
    xVelocity = (int) (speed * Math.cos(direction));
    yVelocity = (int) (speed * Math.sin(direction));

}


private void randomPosition()
{
    x = LEFT_WALL + (int) (Math.random() * (RIGHT_WALL - LEFT_WALL));
    y = UP_WALL + (int) (Math.random() * (DOWN_WALL - UP_WALL));
}


public void run()
{
    long beforeTime, timeDiff, sleep;
    beforeTime = System.currentTimeMillis();
    while (true)
    {

        cycle();
        repaint();

        timeDiff = System.currentTimeMillis() - beforeTime;
        sleep = DELAY - timeDiff;

        if (sleep > 2)
        {
            sleep = 1;
        }
        try
        {
            Thread.sleep(sleep);
        }
        catch (InterruptedException e)
        {
            System.out.println("interrupted");
        }

        beforeTime = System.currentTimeMillis();
    }
}
}

回答1:

All you need is a setter for x and y in your moving object:

public void randomStart(){
    this.x = Math.random() * this.RIGHT_WALL;
    this.y = Math.random() * this.DOWN_WALL;
}

Then you call this from your main().

回答2:

Like in your randomDirection() method, you should initialize the x and y position of you image randomly:

private void randomPosition() {
    x = LEFT_WALL + (int) (Math.random() * (RIGHT_WALL - LEFT_WALL));
    y = UP_WALL + (int) (Math.random() * (DOWN_WALL - UP_WALL));
}

You also should call the randomDirection() method once in the constructor, so your image moves in another direction, everytime you start your program.

So the constructor should look like this:

public YourClassName() {
    randomPosition();
    randomDirection();
}

回答3:

You also have to generate initialized value randomly.

Actually you always take :

private int x, y;
private int xVelocity = 1;
private int yVelocity = 1;

so x == 0, y == 0, xVelocity == 1 and yVelocity == 1 as static initial values for your variables.

回答4:

Random r = new Random()  
public Ball()
{
    randomPosition();
    setRandomDirection();
    setBackground(Color.BLACK);
    setDoubleBuffered(true);

    ImageIcon ii = new ImageIcon(this.getClass().getResource("chicken.gif"));
    ball = ii.getImage();
    this.x = r.nextInt();
    this.y = r.nextInt();

}

来源：https://stackoverflow.com/questions/7980051/how-to-make-a-random-position-whenever-i-start-running-the-program