###通过对两个类DVD,CD所拥有的共同属性进行合并，得到父类Item-->主要讨论了继承，向上造型等 ####Project名为Dome，Package名为dome，有四个类，名字分别为：Database,CD,DVD,Item

————————————————————Database————————————————————

package dome;

import java.util.ArrayList;

public class Database { // private ArrayList listCD= new ArrayList(); // private ArrayList listDVD= new ArrayList(); private ArrayList listItem = new ArrayList();

// public void add(CD cd) { // listCD.add(cd);
// } // // public void add(DVD dvd) { // listDVD.add(dvd); // }

 public void add(Item item) { 
	 listItem.add(item); 
 }

 public void list() {

// for( CD cd : listCD ) { // cd.print(); // } // for( DVD dvd : listDVD ) { // dvd.print(); // } for( Item item : listItem ) { item.print(); } }

 public static void main(String[] args) {
Database db = new Database();
db.add(new CD("abc","abc",4,10,"good"));
db.add(new CD("def","def",4,10,"good"));
db.add(new DVD("xxx","aaa",60,"nice"));
db.list();
 }

}

————————————————————CD————————————————————

package dome;

public class CD extends Item { private String artist; private int numofTracks;

public CD(String title, String artist, int numofTracks, int playTime,
    String comment) {
	super(title,playTime,false,comment); 
	this.artist = artist; 
	this.numofTracks = numofTracks; 

// this.playTime = playTime; // this.comment = comment; }

public void print() {
System.out.print("CD: ");
super.print();
System.out.println(artist);
}

}

————————————————————DVD————————————————————

package dome;

public class DVD extends Item { private String director;

public void print() {
    System.out.print("DVD: ");
    super.print();
    System.out.println(director);
}

public DVD(String title, String director, int playTime, String comment) {
    super(title,playTime,false,comment);

// this.title = title; this.director = director; // this.playTime = playTime; // this.comment = comment; } }

————————————————————Item————————————————————

package dome;

public class Item { private String title; private int playTime; private boolean gotIt = false; private String comment;

public Item(String title, int playTime, boolean gotIt, String comment) {
	super();
	this.title = title;
	this.playTime = playTime;
	this.gotIt = gotIt;
	this.comment = comment;
}

public void print() {
	System.out.print(title);
}

}