?? CD? DVD? ??? ? ?? ??? ?????? ??? ?????. ??? ?????? CD ? DVD? ???? ??? ?????? ?? CD ? DVD? ??? ? ????.

??? ?? ??? ?????. CD? ???? DVD? ?????. ? ???? ??, ?? ??, ?? ? ?? ???? ??? ????. ?? ?? ? ???? ???? ?? ?? CD? DVD?? ? ?? ?? ???? ?? Item??? ?? ???? ??????.

?? ?? ?? ???? ?? ???(??)? ???? ???. ? ???? ??? ?????? ArrayList? ?? ??? CD ?? DVD? ???? ?? ?? ? ?? ??? ????. ??? ?????? ?? ?. ? ???? ??? ??????? ?????.

?? ???? ?? 4?? Java ?? ??? ???? ???.

?? ??:

package dome;

import java.util.ArrayList;

public class Database {
    
    private ArrayList<Item> listItem = new ArrayList<Item>();
    
    public void add(Item item) {
        listItem.add(item);
    }
    
    public void list() {
        for(Item item:listItem) {
            item.print();
        }
    }
    
    public static void main(String[] args) {
        Database db = new Database();
        db.add(new CD("黑夢",600,true,"Classic Album","竇唯",10));
        db.add(new DVD("功夫",900,false,"Let&#39;s do this!","周星馳"));
        db.add(new CD("Wall",700,false,"Psychedelic Rock","PinkFloyd",15));
        db.list();
    }
    
}

Database.java

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+":"+playTime+":"+gotIt+":"+comment);
    }
    
}

Item.java

package dome;

public class CD extends Item {

    private String artist;
    private int numofTracks;
    
    public CD(String title, int playTime, boolean gotIt, String comment, String artist, int numofTracks) {
        super(title, playTime, gotIt, comment);
        this.artist = artist;
        this.numofTracks = numofTracks;
    }
    
    public void print() {
        System.out.print("CD:");
        super.print();
        System.out.println(":"+artist+":"+numofTracks);
    }
    
}

CD.java

package dome;

public class DVD extends Item {
    
    private String director;

    public DVD(String title, int playTime, boolean gotIt, String comment, String director) {
        super(title, playTime, gotIt, comment);
        this.director = director;
    }
    
    public void print() {
        System.out.print("DVD:");
        super.print();
        System.out.println(":"+director);
    }
    
}

 DVD.java

??:

???? ? ?? ??? ????.

??? ??: ???? ?? ??? ? ?? ?? ???? ??? ?? ? ????. parent class, ??????? 9?: Item ??? ?? ???? ?? ?? ??? ?????. ? ??? ?? ???? ??? ?????? ??? ?? ??? ????. ?? ????? ??? CD? DVD? ??? ? ???? ?? ??? ????? ???? ??? ?? ????? ??? ? ? ????. ? ??? ??? ?????.

?? ???: ?? ???? ?? ???? ?? ??? ?? ???? ??? ????? Database? db.add? ?? ?? ??? ??? ?? ???? ??? ???? ??? ???? ???. CD? DVD? ???? Item ??? ?? ??? ? ? ????. ???? ???? ???? ?? ?? ?????.

------------------------------- ------ ------------------ ------ --------

??? ???? ????:

??????? ???? ???? , 15?? ?? Sentence item.print

? ????. ?? Item?? ?? ??? ???? ?? ? ? ??? ???? ?? ??? ?? ??? ????. ??? ????. ?? ??? ??? CD.print() ? DVD.print();? ??? ?????.

Item.print():

CD.print():

DVD. print():

??? ???? ???? ???? ???. ?? ?? ???? ?? ???? ??? ??? ?? ??? ?? ?? ?? ???? ??? ?? ????? ??? ??? ???? ??? ?????? ?? ???? ??? ???? ???? ????. ?? ?? ??????? 15?? ?? ?? ??? ??? Item ???? ????? ??? ?? ???? ????? ?????. ?? ???? ?? ?? ??? ?? ?? ???? ?? ??? ?????.