# Java Notes

# Example - DragDemo.java

| Drag the ball around on the applet to the left. The source for this program is given below. It's written as both an applet (subclass `JApplet`) and application (it has a `main` method). The two source files are given below. The image is drawn with the Graphics method (`fillOval`), but an image could be easily displayed.

### The main/applet framework

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    /** DragDemo.java - Mouse drag example dual application/applet
        @author Fred Swartz
        @version 2004-04-15
    */
    // "appletviewer DragDemo.java" works because of the following line.
    // <applet code="DragDemo.class" height="200" width="200"></applet>
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;
    import javax.swing.event.*;
    //////////////////////////////////////////////////////////////// class DragDemo
    /** This is an application because it has a main method.
        It's also an applet because it extends JApplet.
    */
    public class DragDemo extends JApplet {
        //============================================================= method main
        public static void main(String[] args) {
            JFrame window = new JFrame();
            window.setTitle("Drag Demo");
            window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            window.setContentPane(new DragBallPanel());
            window.pack();
            window.show();
        }//end main

        //====================================================== applet constructor
        public DragDemo() {
            this.setContentPane(new DragBallPanel());
        }
    }//endclass DragDemo

### The panel that is used for the graphics

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    /** DragBallPanel.java - Panel that allows dragging a ball around.
        @author Fred Swartz
        @version 2004-04-15
    */

    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    /////////////////////////////////////////////////////////// class DragBallPanel
    /** When the mousePressed listener is called to position is tested
        to see if it's in the area of the ball.  If it is,
        (1) _canDrag is set true meaning pay attention to the MouseDragged events.
        (2) Record where in the ball (relative to the upper left coordinates)
            the mouse was clicked, because it looks best if we drag from there.
    */
    public class DragBallPanel extends JPanel implements MouseListener, MouseMotionListener {

        private static final int BALL_DIAMETER = 40; // Diameter of ball
        //--- instance variables
        /** Ball coords.  Changed by mouse listeners.  Used by paintComponent. */
        private int _ballX     = 50;   // x coord - set from drag
        private int _ballY     = 50;   // y coord - set from drag

        /** Position in ball of mouse press to make dragging look better. */
        private int _dragFromX = 0;    // pressed this far inside ball's
        private int _dragFromY = 0;    // bounding box.

        /** true means mouse was pressed in ball and still in panel.*/
        private boolean _canDrag  = false;

        //============================================================= constructor
        /** Constructor sets size, colors, and adds mouse listeners.*/
        public DragBallPanel() {
            setPreferredSize(new Dimension(300, 300));
            setBackground(Color.blue);
            setForeground(Color.yellow);
            //--- Add the mouse listeners.
            this.addMouseListener(this);
            this.addMouseMotionListener(this);
        }//endconstructor

        //=================================================== method paintComponent
        /** Ball is drawn at the last recorded mouse listener coordinates. */
        public void paintComponent(Graphics g) {
            super.paintComponent(g);   // Required for background.
            g.fillOval(_ballX, _ballY, BALL_DIAMETER, BALL_DIAMETER);
        }//end paintComponent

        //===================================================== method mousePressed
        /** Set _canDrag if the click is in the ball (or in the bounding
            box, which is lazy, but close enuf for this program).
            Remember displacement (dragFromX and Y) in the ball
            to use as relative point to display while dragging.
        */
        public void mousePressed(MouseEvent e) {
            int x = e.getX();   // Save the x coord of the click
            int y = e.getY();   // Save the y coord of the click

            if (x >= _ballX && x <= (_ballX + BALL_DIAMETER)
                    && y >= _ballY && y <= (_ballY + BALL_DIAMETER)) {
                _canDrag = true;
                _dragFromX = x - _ballX;  // how far from left
                _dragFromY = y - _ballY;  // how far from top
            } else {
                _canDrag = false;
            }
        }//end mousePressed

        //============================================================ mouseDragged
        /** Set x,y  to mouse position and repaint. */
        public void mouseDragged(MouseEvent e) {
            if (_canDrag) {   // True only if button was pressed inside ball.
                //--- Ball pos from mouse and original click displacement
                _ballX = e.getX() - _dragFromX;
                _ballY = e.getY() - _dragFromY;

                //--- Don't move the ball off the screen sides
                _ballX = Math.max(_ballX, 0);
                _ballX = Math.min(_ballX, getWidth() - BALL_DIAMETER);

                //--- Don't move the ball off top or bottom
                _ballY = Math.max(_ballY, 0);
                _ballY = Math.min(_ballY, getHeight() - BALL_DIAMETER);

                this.repaint(); // Repaint because position changed.
            }
        }//end mouseDragged

        //====================================================== method mouseExited
        /** Turn off dragging if mouse exits panel. */
        public void mouseExited(MouseEvent e) {
            _canDrag = false;
        }//end mouseExited

        //=============================================== Ignore other mouse events.
        public void mouseMoved   (MouseEvent e) {}  // ignore these events
        public void mouseEntered (MouseEvent e) {}  // ignore these events
        public void mouseClicked (MouseEvent e) {}  // ignore these events
        public void mouseReleased(MouseEvent e) {}  // ignore these events
    }//endclass DragBallPanel