growableballcollection.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 keeps track of a small collection of
// basketballs drawn on the canvas.

public class GrowableBallCollection implements BallCollectionInterface {
    
    // the entire collection of balls.
    private BBall[ ] balls;
    
    // how many balls at the moment
    private int ballCount = 0;
    
    // limit on number of balls
    private int maxBalls;
    
    // used to pick random balls from the collection
    private RandomIntGenerator picker;
    
    
    // create a new, initially empty collection of basketballs
    public GrowableBallCollection ( int size ) {
        
        // create the array of slots for basketballs
        balls = new BBall[size];
        maxBalls = size;
        
    }
    
    // return true if there are currently no balls
    public boolean isEmpty() {
        return ballCount == 0;
    }
    
    
    // add an additional ball to the collection (if it will fit)
    public void add( BBall newBall ) {
        if ( ballCount < maxBalls ) {
            balls[ballCount] = newBall;
            ballCount++;
            
            picker = new RandomIntGenerator( 0, ballCount-1);
            
        } else {
            System.out.println("WHOOPS!  Too many balls.");
        }
    }
    
    
    // Return a random ball from the collection
    public BBall pickRandomly() {
        if ( ballCount > 0) {
            return balls[ picker.nextValue() ];
        } else {
            return null;
        }
    }
    
    
    // hide all the balls
    public void hide() {
        for ( int counter = 0; counter < ballCount; counter++) {
            balls[counter].hide();
        }
    }
    
    // show all the balls
    public void show() {
        for ( int counter = 0; counter < ballCount; counter++) {
            balls[counter].show();
        }
    }
}