Question

我无法在此代码中获取某张卡片的属性，但始终会打印null并且我不知道为什么。我来自Php-HTML世界，我是JAVA的新手。

这是来自Opportunity.java

package opportunity;
import java.io.*;
import java.util.Collections;
import java.util.List;
import static java.util.stream.Collectors.toList;
import java.util.stream.IntStream;
import java.util.stream.Stream;
import opportunity.Card.CardType;

public class Opportunity {
    /*Player 1 */
    public int p1_money = 10000;
    public int p1_card_d = 40;
    public int p1_card_h = 0;
    /*Player 2 */
    public int p2_money = 10000;
    public int p2_card_d = 40;
    public int p2_card_h = 0;

    /**
     * @param args the command line arguments
     * @throws java.io.IOException
     */
    public static void main(String[] args) throws IOException {


        System.out.print("***************Opportunity***************\n");

        final Card card1 = new Card(Card.CardType.EVENT, "Get Tax Returns").
        setProperty("cost", "0.00").
        setProperty("Effect", "Effect: Earn money equal to the\n"
                + "maximum income each of your\n"
                + "properties can give you,\n"
                + "depending on their level.");
        final Card card2 = new Card(Card.CardType.EVENT, "BIR Hunting Begins").
        setProperty("cost", "0.00").
        setProperty("Effect", "Effect: An opponent loses\n"
                + "money equal to 50% of the\n"
                + "maximum income each of\n"
                + "their properties can give him or her,\n"
                + "depending on the level of the\n"
                + "property.");
        final Card card3 = new Card(Card.CardType.EVENT, "Restore Balance").
        setProperty("cost", "10000.00").
        setProperty("Effect", "Effect: The total income of all\n"
                + "the players becomes equal to\n"
                + "the income of the player\n"
                + "with the lowest income.");

        final List<Card> deck = Stream.of(CardType.values()).
        flatMap(type -> IntStream.rangeClosed(1, 4).mapToObj(num -> new Card(type, "CardName" + num))).
        collect(toList());
        Collections.shuffle(deck);

        System.out.println(deck.get(0).getProperty("Effect"));


    }

}

这是来自Card.java

的代码

package opportunity;

import java.util.*;


public class Card {
    public enum CardType {
        EVENT,
        PROPERTY,
        ASSET;
    }
    private final CardType cardType;
    private final String cardName;
    private final Map<String, String> properties = new HashMap<>();

    Card(final CardType cardType, final String cardName) {
        this.cardType = cardType;
        this.cardName = cardName;
    }

    public Card setProperty(final String name, final String value) {
        properties.put(name, value);
        return this;
    }

    public String getProperty(final String name) {
        return properties.get(name);
    }
}

它总是会返回：

run:
***************Opportunity***************
null
BUILD SUCCESSFUL (total time: 0 seconds)

即使我在get()

上放了一个索引

Answer 1

要建立@Luiggi门多萨的答案，您不会将您的卡片对象添加到deck列表。

替换

final List<Card> deck = Stream.of(CardType.values()).
flatMap(type -> IntStream.rangeClosed(1, 4).mapToObj(num -> new Card(type, "CardName" + num))).
collect(toList());

用

final List<Card> deck = new ArrayList<>();
deck.add(card1);
deck.add(card2);
deck.add(card3);

不是在列表中添加新的Cards，而是添加已创建的对象，并设置属性。

编辑：解决您的评论

final List<Card> deck = new ArrayList<>();
for (int i=0; i<4; i++) {
    deck.add(card1);
    deck.add(card2);
    deck.add(card3);
}

这会将三张卡中的每张卡的四份副本添加到列表中。

Answer 2

下面：

final List<Card> deck = Stream.of(CardType.values()).
    flatMap(type -> IntStream.rangeClosed(1, 4).mapToObj(num -> new Card(type, "CardName" + num))).
    collect(toList());
    Collections.shuffle(deck);

您使用新的List<Card> deck填充Card，不添加任何其他cardX变量。因此，在检索此"Effect"属性时，您会得到null，因为Card中的deck之前没有设置"Effect"属性。

Answer 3

前人的评论是完全正确的。您没有将创建的卡添加到列表中。

尝试替换您的代码

final List<Card> deck = Stream.of(CardType.values()).
    flatMap(type -> IntStream.rangeClosed(1, 4).mapToObj(num -> new Card(type, "CardName" + num))).
    collect(toList());

用于标准列表创建和添加。例如（并更新为4张卡）：

List<Card> deck = new ArrayList<Card>();
for (int i = 0; i < 4; i++){
    deck.add(card1);
    deck.add(card2);
    deck.add(card3);
}

如果要最小化代码行数，可以使用内容初始化新列表。试试这篇文章Initialization of an ArrayList in one line。

无论如何，总是尝试编写最简单的解决方案，在使用新语言时更是如此。

打印对象的属性

3 个答案: