多型與動態繫結機制，在實踐中非常常見，適用於許多場景，尤其是對於設計複雜的測試用例，更能發揮強大的作用。

先實現一個demo，把思路走通再說。

/**
 * 
 */
package ball;

import java.util.InputMismatchException;
import java.util.Scanner;

/**
 * @author administrator
 *
 */
public class BallTest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		BallTest ball = new BallTest();
		ball.testMethod();
	}

	private void testMethod() {
		
		// 初始化
		BallPlay ballPlay = null;
		
		System.out.println("提示：1是乒乓球賽，2是足球賽，3是羽毛球賽，4是檯球賽！");
		
		Scanner scanner = new Scanner(System.in);
		int vChoice = 0;
		
		for (int i = 0; i < 3; i++) {
			try {
				System.out.print("現在看哪個比賽：");
				vChoice = scanner.nextInt();				// 程式執行時，才能知道具體的例項和方法呼叫，而不是在編譯時確定
				switch (vChoice) {
				case 1:
					ballPlay = new PingPangBall();
					ballPlay.playStarting("乒乓球賽");
					break;
				case 2:
					ballPlay = new FootBall();				// 子類可以自動向父類轉換
					((FootBall) ballPlay).lookGiftBox();	// 父類向子類轉換必須進行強制轉換，當然這種轉換是安全的
					ballPlay.playStarting("足球賽");
					break;
				case 3:
					ballPlay = new Badminton();
					ballPlay.playStarting("羽毛球賽");
					break;
				case 4:
					ballPlay = new BilliardBall();
					ballPlay.playStarting("檯球賽");
					break;
				default:
					throw new InputMismatchException("輸入錯誤！");	
				}
			} catch (Exception e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
				// System.out.println("您的輸入非法！");
			} finally {
				ballPlay = null;
				System.out.println("第" + (i + 1) + "場賽事已經結束！");
			}
		}
		
		scanner.close();
		System.out.println("好了，回家吧！");
		
	}
}

class BallPlay {
	public BallPlay() {
		System.out.print("球賽-->");
	}
	
	public void playStarting(String strPlayName) {
		try {
			Thread.sleep(2000);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			// e.printStackTrace();
			System.out.println("賽場出事故了，" + strPlayName + "賽事提前結束！");
		}
	}
}

class PingPangBall extends BallPlay {
	public PingPangBall() {
		System.out.println("乒乓球賽正在直播。。。");
	}
	
	public void playStarting(String strPlayName) {
		super.playStarting(strPlayName);
	}
}

class FootBall extends BallPlay {
	public FootBall() {
		System.out.println("足球賽正在直播。。。");
	}
	
	public void playStarting(String strPlayName) {
		super.playStarting(strPlayName);
	}
	
	public void lookGiftBox() {
		System.out.println("足球賽場大，送你一臺望遠鏡！");
	}
}

class Badminton extends BallPlay {
	public Badminton() {
		System.out.println("羽毛球賽正在直播。。。");
	}
	
	public void playStarting(String strPlayName) {
		super.playStarting(strPlayName);
	}
}

class BilliardBall extends BallPlay {
	public BilliardBall() {
		System.out.println("檯球賽正在直播。。。");
	}
	
	public void playStarting(String strPlayName) {
		super.playStarting(strPlayName);
	}
}
 

執行結果示例：