growableballbouncer.java

书籍"Java_面向事件编程"的附带光盘代码

import java.awt.*;
import objectdraw.*;

// A class to draw a bunch of basketballs on the screen so
// that we can watch them bounce randomly.  Each click
// adds a ball to the display.  Leaving the canvas makes
// the balls temporarily vanish.  Moving the mouse back
// into the canvas makes the balls reappear and bounce
// for a while.
public class GrowableBallBouncer extends WindowController {
    
    // diameter of the balls
    private static final double BALLSIZE = 16;
    
    // spacing between balls
    private static final double BALLSPACING = 13;
    
    // spacing between balls and edges of canvas
    private static final double MARGIN = 20;
    
    // all the balls displayed.
    private GrowableBallCollection allTheBalls;
    
    // position for next ball added
    private double ballLeft;
    
    // common y coordinate for all balls
    double ballTop;
    
    // create the ball collection
    public void begin() {
        // make the collection size appropriate for balls that will
        // fit on the canvas
        allTheBalls =
            new GrowableBallCollection(
                                       (int) ( (canvas.getWidth() - 2*MARGIN - BALLSIZE) /
                                               (BALLSIZE + BALLSPACING)
                                               ) + 1);
        
        // x coordinate of first ball
        ballLeft = canvas.getWidth() - BALLSIZE - BALLSPACING;
        
        // center the balls vertically
        ballTop = canvas.getHeight()/2 - BALLSIZE/2;
    }
    
    // add a new ball (if it will fit) when the mouse is clicked.
    public void onMousePress( Location point ) {
        if ( ballLeft > MARGIN ) {
            allTheBalls.add( new BBall( ballLeft , ballTop, BALLSIZE, canvas));
            ballLeft = ballLeft - BALLSIZE - BALLSPACING;
        }
    }
    
    
    // hide all the balls when the mouse exits
    public void onMouseExit( Location point ) {
        if ( !allTheBalls.isEmpty() ) {
            allTheBalls.hide();
        }
    }
    
    
    // show all the balls when the mouse arrives and bounce for a while
    public void onMouseEnter( Location point ) {
        if ( !allTheBalls.isEmpty() ) {
            allTheBalls.show();
            new Bouncer( allTheBalls );
        }
    }
    
    
}