如何实现枚举类型的某些方法

Question

我是编程和Java的新手，目前正在学习枚举类型。

我创建了一个枚举类型的Card ，它将每张卡的值存储在一副纸牌中，例如2 = 2、3 = 3 ... Ace =11。每张图片卡具有相同的值10。

我正在尝试实现方法getPrevious() ，该方法将返回列表中的上一个枚举值。 例如，如果在SIX上调用了该方法，则它将返回FIVE ，而在TWO被调用时，将返回ACE 。 但是，我正在努力寻找一种方法来做到这一点。

在下面，您可以看到我所编写的当前代码，有关如何实现方法getPrevious()任何帮助或提示都将非常有用。

public class EnumPractice {

    public enum Card {
        TWO(2), THREE(3), FOUR(4), FIVE(5), SIX(6), SEVEN(7), EIGHT(8), 
        NINE(9), TEN(10), JACK(10), QUEEN(10), KING(10), ACE(11), ;

        private int value;
        private Card card;

        Card(int value){
        this.value = value;
        }

        public int getValue(){
            return value;
        }

        public String toString(){
            return "The value of this card is: " + value;
        }
    }

    public static void main(String[] args) {
        System.out.println(Card.ACE.getValue());
    }
}

Answer 1

您的getPrevious()方法如下所示

public enum Card {
    TWO(2), THREE(3), FOUR(4), FIVE(5), SIX(6), SEVEN(7), EIGHT(8),
    NINE(9), TEN(10), JACK(10), QUEEN(10), KING(10), ACE(11);

    private int value;

    Card(int value) {
        this.value = value;
    }

    public int getValue() {
        return value;
    }

    public Card getPrevious() {
        int previous = ordinal() - 1;
        if (previous < 0 ) { // if negative, then we have to go to the last element
            previous += values().length;
        }

        return Card.values()[previous];
    }
}

Answer 2

这是您的getPrevious()实现

import java.util.Arrays;

public class EnumPractice {

    public enum Card {

        TWO(2), THREE(3), FOUR(4), FIVE(5), SIX(6), SEVEN(7), EIGHT(8), 
        NINE(9), TEN(10), JACK(10), QUEEN(10), KING(10), ACE(11), ;

        private int value;
        private Card card;

        Card(int value){
        this.value = value;
        }

        public int getValue(){
            return value;
        }

        public String toString(){
            return "The value of this card is: " + value;
        }

        public Card getPrevious() {
            int pos = ordinal();
            if (pos == 0)
                return ACE;
            return values()[pos - 1];
        }
    }

    public static void main(String[] args) {

    System.out.println(Card.THREE.getPrevious());
    System.out.println(Card.TWO.getPrevious());

    }

}

上面的程序输出是

The value of this card is: 2
The value of this card is: 11

Answer 3

其他方法可能是保留对先前卡的引用。 您可以在所有卡都在静态块中创建后进行设置，例如：

public enum Card {
    TWO(2), THREE(3), FOUR(4), FIVE(5), SIX(6), SEVEN(7), EIGHT(8), 
    NINE(9), TEN(10), JACK(10), QUEEN(10), KING(10), ACE(11);

    private int value;
    private Card previous;

    static{
        //lets set previous element for cards
        //we will iterate starting from TWO so its previous should be:
        Card previous = ACE;//yes ACE
        for (Card card : values()){
            card.previous = previous;
            previous = card; 
        }
    }

    Card(int value){
        this.value = value;
    }

    public int getValue(){
        return value;
    }

    public Card getPrevious(){
        return previous; //<-- simple getter is enough since we took care 
                         //    of setting this value earlier in static 
                         //    initialization block
    }

    public String toString(){
        return "The value of this card is: " + value;
    }
}