ArrayList的HashMap导致nullpointerexception

Question

我有一个ArrayLists的HashMap用于值,但是当我添加ArrayLists时HashMap保持为空,然后当我尝试get()ArrayList时抛出NullPointerException.非常困惑.

Random rand = new Random();
HashMap<String,ArrayList<Integer>> hands = new HashMap<String,ArrayList<Integer>>();
HashMap<Integer, Boolean> deck = new HashMap<Integer, Boolean>();

for(int x=0;x<4;x++){
    for(int y=0;y<4;y++){
    hands.put(x+SUITS[x], new ArrayList<Integer>());
    }
}       
    for(int x=0;x<4;x++){
        for(int y=0;y<13;y++){
            int randCard = rand.nextInt(52)+1;
            if(!deck.containsKey(randCard)){
                deck.put(randCard, true);

                hands.get(x+cardSuit(randCard)).add(randCard);

            }else y--;
        }
    }

Answer 1

您正在使用如下所示的键将值放入地图中:

someInt + ""

您将从地图中获取具有如下所示键的值:

someInt + cardSuit(randCard)

除非cardSuit总是返回一个空字符串,否则它们将成为不同的键.