为什么这个HashMap.get返回null？

Question

我正在尝试创建一个Hashmap为我执行查找.但是,当我运行此测试代码时,输​​出为空.我认为它必须与密钥的存储方式有关,但我并不积极.也许它是一个类似的怪癖,就像var1 == var2它们不等于除非它们指向内存中的同一个对象,而你必须使用var1.equals(var2)？

有两个类可以测试它.

TestCard.java

import java.util.HashMap;

public class TestCard {

     // HashMap for SpecialK Lookup
    private static HashMap<Card, Integer> specialKLookup = new HashMap<Card, Integer>();

    // Constructor
    public TestCard(){
    }

    public static void main(String[] args) {
        Card[] cards = new Card[3];
        cards[0] = new Card((short)12, (short)0);
        cards[1] = new Card((short)0, (short)1);
        cards[2] = new Card((short)5, (short)2);

        /* Build SpecialK Lookup HashMap.
         * Ace of Spades = 0
         * Ace of Hearts = 1
         * Ace of Diamonds = 2
         * Ace of Clubs = 3
         * ...
         * Two of Clubs = 51
         */
        Integer specialKCounter = 0;
        for(int i=12;i>=0;i--){
                for (int j=0;j<4;j++){
                        specialKLookup.put(new Card((short)i, (short)j), specialKCounter++);
                }
        }

        System.out.println(specialKLookup.get(cards[0]));
    }
}

Card.java

public class Card{
    private short rank, suit;

    private static String[] ranks = {"2", "3", "4", "5", "6", "7", "8", "9", "10", "Jack", "Queen", "King", "Ace"};
    private static String[] suits = {"Spades", "Hearts", "Diamonds", "Clubs"};

    //Constructor
    public Card(short rank, short suit){
        this.rank = rank;
        this.suit = suit;
    }

    // Getter and Setters
    public short getSuit(){
        return suit;
    }

    public short getRank(){
        return rank;
    }

    protected void setSuit(short suit){
        this.suit = suit;
    }

    protected void setRank(short rank){
        this.rank = rank;
    }   
}

Answer 1

类(Card)丢失的正确实施equals(Object)和hashCode()

如果没有这两个定义,它将无法正常工作.(它编译得很好,因为这些方法都是虚拟的,并且在所有对象中都是继承的,因为它们是Object的一部分:HashMap在编译时不能强制执行此操作.)请参阅上面的链接以获取所需的合同.

这两种方法都需要实现,因为hashCode确定HashMap实现中使用的哈希桶并且equals确保对象是值等于(多个对象可以具有相同的hashCode,这也是为什么equals也是必需的).有关更多常规哈希详细信息,请参阅哈希表.

如果这些方法没有重载,则使用Object中定义的实现.也就是说,x.equals(y)具有近似x == y语义并且hashCode每个合约返回一个稳定的数字.这有效地使地图像身份地图一样工作(当Card对象是键时):只有完全相同的对象才能检索先前存储的值 - 每个其他get将返回null,如所观察到的那样.

快乐的编码.