如何基于抽象类Card向ArrayList添加对象以创建牌组

我正尝试将Card对象添加到Card类型的ArrayList中以创建一副牌组。为此遍历了牌的花色和数值，尝试添加Card对象时，出现无法实例化Card类型的错误。

public abstract class Card
{
public static final String FACES[] = {"ZERO","ACE","TWO","THREE","FOUR",
        "FIVE","SIX","SEVEN","EIGHT","NINE","TEN","JACK","QUEEN","KING"};
private String suit;
private int number;

//constructors
public Card(){
   
   }

public Card ( int num, String type) {
    
    number = num; 
    suit = type;
    
    }



// modifiers



//accessors

public String getSuit() {
    return suit; 
   }


public int getNumber() {
    return number; 
    }



public abstract int getValue();

public boolean equals(Object obj)
{
    if (getNumber()==((Card) obj).getNumber()) {
        
        return true;
    }
    
    
    return false;
}

//toString
public String toString () {
int var = getNumber(); 
return FACES[var] + " of " + getSuit() + " | Value " + getValue();

}

import java.util.ArrayList;
import java.util.Collections;

public class Deck
{
    public static final int NUMFACES = 13;
    public static final int NUMSUITS = 4;
    public static final int NUMCARDS = 52;

    public static final String SUITS[] = {"CLUBS","SPADES","DIAMONDS","HEARTS"};

    private int topCardIndex;
    private ArrayList<Card> stackOfCards;

    public Deck ()
    {
        //initialize data - stackOfCards - topCardIndex
        topCardIndex = 0; 
        
        for (int i=0;i<NUMSUITS;i++){
            for( int j=0; j<NUMFACES; j++) {
                stackOfCards.add(new Card(j,SUITS[i]));
            }
        }//loop through suits
            //loop through faces
                //add in a new card
    }

    //modifiers
    public void shuffle ()
    {
        //shuffle the deck
        //reset variables as needed
    }

    //accessors
    public int size ()
    {
        return 0;
    }

    public int numCardsLeft()
    {
        return stackOfCards.size();
        
    }

    public Card nextCard()
    {
        return stackOfCards.get(topCardIndex--);
    }

    public String toString()
    {
        return stackOfCards + "   topCardIndex = " + topCardIndex;
    } 
}

核心问题 #

错误根源是Card被定义为抽象类，Java中抽象类不能通过new关键字直接实例化——抽象类包含未实现的抽象方法（比如getValue()），它的设计目的是作为父类被继承，而非直接创建对象。

解决步骤 #

1. 创建Card的具体子类，实现抽象方法getValue()
   以标准扑克牌为例，创建StandardCard类：

   public class StandardCard extends Card {
       public StandardCard(int num, String type) {
           super(num, type);
       }
   
       @Override
       public int getValue() {
           // 按扑克牌规则实现点数：J/Q/K返回10，其余返回自身数值
           int num = getNumber();
           return num >= 10 ? 10 : num;
       }
   }
   

2. 修改Deck类的实例化代码
   将Deck构造方法中的new Card(j,SUITS[i])替换为子类对象，同时补充stackOfCards的初始化（否则会触发空指针异常）：

   public Deck ()
   {
       topCardIndex = 0; 
       stackOfCards = new ArrayList<>(); // 初始化ArrayList
       
       for (int i=0;i<NUMSUITS;i++){
           for( int j=0; j<NUMFACES; j++) {
               stackOfCards.add(new StandardCard(j,SUITS[i]));
           }
       }
   }
   

3. 可选方案
   如果不需要抽象类的设计特性，也可以直接移除Card类的abstract修饰符，并实现getValue()方法，这样就能直接实例化Card对象，但会丢失抽象类强制子类规范实现的优势。

内容的提问来源于stack exchange，提问作者mishifishy