ballcollection.java

书籍"Java_面向事件编程"的附带光盘代码

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

// A class that makes and keeps track of a line of small
// basketballs that stretches from one end of the canvas
// to the other.  A method is provided to access a random
// ball from the collection.

public class BallCollection implements BallCollectionInterface {
    
    // diameter of the balls
    private static final double BALLSIZE = 20;
    
    // spacing between balls
    private static final double BALLSPACING = 12;
    
    // spacing between balls and edges of canvas
    private static final double MARGIN = 20;
    
    // the entire collection of balls.
    private BBall[] ball;
    
    // used to pick random balls from the collection
    private RandomIntGenerator picker;
    
    
    // create a new collection of basketballs that stretches across
    // the program's window
    public BallCollection ( DrawingCanvas canvas ) {
        
        // how many balls will fit
        int ballCount = (int) ( (canvas.getWidth() - 2*MARGIN ) /
                                (BALLSIZE + BALLSPACING)
                                ) ;
        
        // center the balls vertically
        double ballTop = canvas.getHeight()/2 - BALLSIZE/2;
        
        // create the array of slots for basketballs
        ball = new BBall[ballCount];
        
        double ballLeft = MARGIN;
        
        // Draw all the balls and put them in the array
        for ( int ballNumber = 0;
              ballNumber < ballCount;
              ballNumber ++){
            ball[ballNumber] = new BBall( ballLeft, ballTop, BALLSIZE, canvas);
            ballLeft = ballLeft + BALLSPACING + BALLSIZE;
        }
        
        picker = new RandomIntGenerator( 0, ballCount-1);
        
    }
    
    // Return a random ball from the collection
    public BBall pickRandomly() {
        return ball[ picker.nextValue() ];
    }
    
    // hide all the balls
    public void hide() {
        
        for ( int counter = 0; counter < ball.length; counter++) {
            ball[counter].hide();
        }
    }
    
    // show all the balls
    public void show() {
        
        for ( int counter = 0; counter < ball.length; counter++) {
            ball[counter].show();
        }
    }
}