为什么我在这段代码中得到nullPointerException？

Question

所以我正在研究处理卡片和卡片的实验室。我在deck.java中编写的代码应该初始化一个套牌。但是，我得到一个nullPointException，我不知道为什么。这是代码的一小部分（我在第＃34; cards.add（thisCard）行中得到了错误;＆＃34;：

import java.util.List;
import java.util.ArrayList;
import java.lang.Math;

/**
 * The Deck class represents a shuffled deck of cards.
 * It provides several operations including
 *      initialize, shuffle, deal, and check if empty.
 */
public class Deck {

    /**
     * cards contains all the cards in the deck.
     */
    private List<Card> cards;

    /**
     * size is the number of not-yet-dealt cards.
     * Cards are dealt from the top (highest index) down.
     * The next card to be dealt is at size - 1.
     */
    private int size;


    /**
     * Creates a new <code>Deck</code> instance.<BR>
     * It pairs each element of ranks with each element of suits,
     * and produces one of the corresponding card.
     * @param ranks is an array containing all of the card ranks.
     * @param suits is an array containing all of the card suits.
     * @param values is an array containing all of the card point values.
     */
    public Deck(String[] ranks, String[] suits, int[] values) {
        for (int i = 0; i < ranks.length; i++){
                    Card thisCard = new Card(ranks[i], suits[i], values[i]);
                    cards.add(thisCard);
                }
                size = cards.size();
                shuffle();
    }
}