RandomWalk

import java.awt.*;
import java.util.*;

public class RandomWalk {

    // DrawingPanel will have dimensions DIM by DIM
    public static final int DIM = 400;

    // Start the random walk in the center of the screen
    public static final int CENTER = DIM / 2;

    // how big should the cursor appear?
    public static final int CURSOR_DIM = 10;

    // how long should the cursor saty in one place?
    public static final int SLEEP_TIME = 25; // milliseconds

    public static void main( String[] args ) {
        DrawingPanel panel = new DrawingPanel( DIM, DIM );

        Random rand = new Random();

        walkRandomly( panel, rand );
    }

    /**
     * Draw a random walk on the panel.
     * Stop as soon as you walk off the panel.
     * Each random step should go only in one
     * of these directions: left, right, up, down.
     * @param panel a DrawingPanel to draw the
     *        random walk on
     * @param rand a Java Random object to be
     *        used to generate random steps
     */
    public static void walkRandomly( DrawingPanel panel, Random rand ) {
        Graphics g = panel.getGraphics();

        // start in center of panel
        int x = CENTER;
        int y = CENTER;

        // Randomly step left, right, up, or down
        // until cursor goes off screen.
        while ( onScreen( x, y ) ) {

            // Draw the cursor in BLACK
            g.fillRect(x, y, CURSOR_DIM, CURSOR_DIM);
            // Wait a bit.
            panel.sleep( SLEEP_TIME );
            // Show a shadow version of the cursor
            g.setColor(Color.GRAY);
            g.fillRect(x, y, CURSOR_DIM, CURSOR_DIM);

            // YOUR CODE GOES BETWEEN THE STARS
            // Choose a new location for the cursor
            //*********************************

        // create a new random object
        Random move = new Random();

        // chose the next direction based on randomly
        // chosing an integer between 0 and 3
        int xmove = move.nextInt(3) - 1;
        int ymove = 0;

        if (xmove == 0) {
            ymove = (move.nextInt(2) * 2) - 1;
        }

        x += CURSOR_DIM * xmove;
        y += CURSOR_DIM * ymove;

            //*********************************
            // END OF YOUR CODE

            // draw the cursor at its new location
            g.setColor(Color.BLACK);
            g.fillRect(x, y, CURSOR_DIM, CURSOR_DIM);
        }
    }

    /**
     * determine whether (x, y) is a point on the panel.
     * @param x the x-coord of the cursor
     * @param y the y-coord of the cursor
     * @return true if (x,y) is on the screen,
     *         false if (x,y) is off the screen
     */
    public static boolean onScreen( int x, int y ) {
        // YOUR CODE GOES BETWEEN THE STARS
        // Replace "return true" with code
        // that returns true if (x,y) is still
        // on the screen and returns false if
        // (x,y) is no longer on the screen.
        //*********************************
        return ((x >= 0 && x <= DIM) && (y >= 0 && y <= DIM));
        //*********************************
        // END OF YOUR CODE
    }
}