我刚刚实现了一个循环，用纸牌（从堆栈中）填充玩家的手（队列），但是在LinkedStack.pop（）上遇到NullPointerException。我没有更改提供的LinkedStack代码，并且检查了甲板上是否填充了元素，因此我无法立即看到NullPointerException的来源。以下是我的代码段和错误输出。如果您需要任何其他代码，请告诉我。
（SimpleUnoGame.java）
45 LinkedQueue<UnoCard> cardHand = new LinkedQueue<UnoCard>();
46 //draw cards for player
47 for(int j = 0; j < INITIAL_CARDS; j++) {
48 cardHand.enqueue(faceDownCards.pop());
49 }
50 newPlayer.setCards(cardHand);
51 playerCollection.enqueue(newPlayer);
（LinkedStack.pop（））
43 T result = top.getElement();
44 top = top.getNext();
45 count--;
46 return result;
（例外）
Exception in thread "main" java.lang.NullPointerException
at LinkedStack.pop(LinkedStack.java:43)
at SimpleUnoGame.<init>(SimpleUnoGame.java:48)
这是完整的代码，根据要求：
（LinkedStack.java）
/**
* Represents a linked implementation of a stack
*/
public class LinkedStack<T> implements StackADT<T> {
//indicates number of elements stored
private int count;
//pointer to top of stack
private LinearNode<T> top;
/**
* Creates an empty stack
*/
public LinkedStack() {
count = 0;
top = null;
}
/**
* Adds the specified element to the top of this stack
* @param element element to be pushed on stack
*/
public void push(T element) {
LinearNode<T> temp = new LinearNode<T>(element);
temp.setNext(top);
top = temp;
count++;
}
/**
* Removes the element at the top of this stack and returns a reference to it
* Throws an EmptyCollectionException if the stack is empty
* @return T element from top of stack
* @throws EmptyCollectionException on pop from empty stack
*/
public T pop() throws EmptyCollectionException {
if (isEmpty()) {
throw new EmptyCollectionException("Stack");
}
T result = top.getElement();
top = top.getNext();
count--;
return result;
}
/**
* Returns a reference to the element at the top of this stack
* Throws an EmptyCollectionException if the stack is empty
* (the element is not removed from the stack)
* @return T element on top of stack
* @throws EmptyCollectionException on peek at empty stack
*/
public T peek() throws EmptyCollectionException {
if (isEmpty()) {
throw new EmptyCollectionException("Stack");
}
return top.getElement();
}
/**
* Returns true if this stack is empty and false otherwise
* @return boolean true if stack is empty
*/
public boolean isEmpty() {
if (count == 0) {
return true;
} else {
return false;
}
}
/**
* Returns the number of elements in this stack
* @return int number of elements in this stack
*/
public int size() {
return count;
}
/**
* Returns a string representation of this stack
* @return String representation of this stack
*/
public String toString() {
String output = "";
T result;
for (int i = 0; i < this.size(); i++) {
result = top.getElement();
output = output + result;
top = top.getNext();
}
return output;
}
}
（SimpleUnoGame.java）
public class SimpleUnoGame {
private final int INITIAL_CARDS = 7;
private LinkedQueue<UnoPlayer> playerCollection = new LinkedQueue<UnoPlayer>();
private LinkedStack<UnoCard> faceUpCards = new LinkedStack<UnoCard>();
private LinkedStack<UnoCard> faceDownCards = new LinkedStack<UnoCard>();
private int cardCount = 0;
public SimpleUnoGame(int numberOfPlayers, int highestRank) {
//create cards and add to faceDownCards
for(int i = 1; i <= highestRank; i++) {
for(int j = 0; j <= 1; j++) {
//blue cards
UnoCard cardB = new UnoCard('B', i);
faceDownCards.push(cardB);
//green cards
UnoCard cardG = new UnoCard('G', i);
faceDownCards.push(cardG);
//red cards
UnoCard cardR = new UnoCard('R', i);
faceDownCards.push(cardR);
//yellow cards
UnoCard cardY = new UnoCard('Y', i);
faceDownCards.push(cardY);
}
}
System.out.println("Cards: " + faceDownCards.toString()); //debug check
//shuffle cards randomly
shuffleCards(faceDownCards);
System.out.println("Cards: " + faceDownCards.toString()); //debug check
//create players and add to playerCollection
for (int i = 0; i < numberOfPlayers; i++) {
//ask for name
String name = "Taylor";
UnoPlayer newPlayer = new UnoPlayer(name);
LinkedQueue<UnoCard> cardHand = new LinkedQueue<UnoCard>();
//draw cards for player
for(int j = 0; j < INITIAL_CARDS; j++) {
cardHand.enqueue(faceDownCards.pop()); //PROBLEM OCCURS HERE
}
newPlayer.setCards(cardHand);
playerCollection.enqueue(newPlayer);
}
System.out.println("There are " + numberOfPlayers + " players in the game.");
System.out.println(playerCollection.toString()); //debug check
//draw one face up card
faceUpCards.push(faceDownCards.pop());
}
最佳答案
您的顶级变量为null。您如何初始化它？
从代码示例中，您似乎在构造对象时将top设置为null，然后仅在推送项目时将其设置为非null。
如果真是这样，那么您似乎还没有向LinkedStack中推送内容。
尽管我必须承认，我很难弄清楚确切的问题，因为您只提供了代码的一部分，而这样做却没有多大意义。
编辑
方法toString（）修改应该保留队列头的top字段。调用toString方法应该没有副作用。更改为：
public String toString() {
String output = "";
T result;
LinearNode<T> tempHead = top;
for (int i = 0; i < this.size(); i++) {
result = tempHead.getElement();
output = output + result;
tempHead = tempHead.getNext();
}
return output;
}
我不知道谁为您写了这堂课，但这是菜鸟的错误。通过调用toString（），您实际上可以清空队列。