/** * This class represents a simple picture. You can draw the picture using * the draw method. But wait, there's more: being an electronic picture, it * can be changed. You can set it to black-and-white display and back to * colors (only after it's been drawn, of course). * * This class was written as an early example for teaching Java with BlueJ. *  * @author  Michael Kolling and David J. Barnes * @version 2006.03.30 */public class Picture{    private Square wall;    private Square window;    private Triangle roof;    private Circle sun;    /**     * Constructor for objects of class Picture     */    public Picture()    {        // nothing to do... instance variables are automatically set to null    }    /**     * Draw this picture.     */    public void draw()    {        wall = new Square();        wall.moveVertical(80);        wall.changeSize(100);        wall.makeVisible();                window = new Square();        window.changeColor("black");        window.moveHorizontal(20);        window.moveVertical(100);        window.makeVisible();        roof = new Triangle();          roof.changeSize(50, 140);        roof.moveHorizontal(60);        roof.moveVertical(70);        roof.makeVisible();        sun = new Circle();        sun.changeColor("yellow");        sun.moveHorizontal(180);        sun.moveVertical(-10);        sun.changeSize(60);        sun.makeVisible();    }    /**     * Change this picture to black/white display     */    public void setBlackAndWhite()    {        if(wall != null)   // only if it's painted already...        {            wall.changeColor("black");            window.changeColor("white");            roof.changeColor("black");            sun.changeColor("black");        }    }    /**     * Change this picture to use color display     */    public void setColor()    {        if(wall != null)   // only if it's painted already...        {            wall.changeColor("red");            window.changeColor("black");            roof.changeColor("green");            sun.changeColor("yellow");        }    }}