所以我知道Null Point异常发生在哪里（Eclipse有帮助）
我有以下课程：
/**
* Card class - a typical playing card.
*
* @author Colleen
* @version 2010.03.09
*/
public class Card
{
private String suit;
private int value;
private String description;
private static final String[] SUITS = {"Hearts", "Diamonds", "Spades", "Clubs"};
private static final String[] DESCRIPTIONS = {
"Ace", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten",
"Jack", "Queen", "King"};
public Card(){
suit = "Spades";
value = 0;
description = "Joker";
}
public Card(String description, String suit){
setSuit(suit);
setValue(description);
setDescription(description);
}
/**
* @return the suit
*/
public String getSuit() {
return suit;
}
/**
* @param suit the suit to set
*/
public void setSuit(String suit) {
int count = 0;
boolean check = false;
while(count < SUITS.length){
if(suit == SUITS[count]){
check = true;
}
count ++;
}
if(check == true){
this.suit = suit;
} else {
suit = "Spades";
}
}
/**
* @return the value
*/
public int getValue() {
return value;
}
/**
* @param value the value to set
*/
public void setValue(String description) {
if(description == "Two"){
value = 2;
} else if(description == "Three"){
value = 3;
} else if(description == "Four"){
value = 4;
} else if(description == "Five"){
value = 5;
} else if(description == "Six"){
value = 6;
} else if(description == "Sever"){
value = 7;
} else if(description == "Eight"){
value = 8;
}else if(description == "Nine"){
value = 9;
} else if(description == "Ten" || description == "Jack" || description == "Queen" || description == "King"){
value = 10;
} else if(description == "Ace"){
value = 11;
} else {
description = "Joker";
value = 0;
}
}
/**
* @return the description
*/
public String getDescription() {
return description;
}
/**
* @param description the description to set
*/
public void setDescription(String description) {
int count = 0;
boolean check = false;
while(count < DESCRIPTIONS.length){
if(description == DESCRIPTIONS[count]){
check = true;
}
count ++;
}
if(check == true){
this.description = description;
} else {
description = "Joker";
}
}
}
下一课：
import java.util.ArrayList;
import java.util.Collections;
import java.util.Random;
/**
* Deck of cards.
*
* @author Bullwinkle J. Moose
* @version (June 11, 2012)
*/
public class Deck
{
private ArrayList<Card> deck;
private static final int TIMES_TO_SHUFFLE = 5;
private static final String[] SUITS = {"Hearts", "Diamonds", "Spades", "Clubs"};
private static final String[] DESCRIPTIONS = {
"Ace", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten",
"Jack", "Queen", "King"};
/**
* Constructor for objects of class Deck
* Creates a new container for Card objects
*/
public Deck()
{
deck = new ArrayList<Card>();
}
/**
* Add a card to the deck.
* @param Card to be added
*/
public void addCard(Card cardToAdd)
{
deck.add(cardToAdd);
}
/**
* Take the first card from the deck.
* @return Card or null
*/
public Card takeCard()
{
if(deck.isEmpty()) {
return null;
}
else { // get the top card
return deck.remove(0);
}
}
public int deckSize(){
int deckSize = deck.size();
return deckSize;
}
/**
* Show the contents of the deck.
*/
public void showDeck(){
for(Card eachCard : deck) {
System.out.println(eachCard.getDescription() + " of " + eachCard.getSuit());
}
}
/**
* Method that switch the index position in the deck of the two cards.
*/
public void swap(int firstNumber, int secondNumber){
if (firstNumber >= 0 && firstNumber < deck.size()) {
if (secondNumber >= 0 && secondNumber < deck.size()) {
Collections.swap(deck,firstNumber,secondNumber);
} else {
System.out.println("Invalid index entry ...");
}
} else {
System.out.println("Invalid index entry ...");
}
}
/**
* Method that randomly selects two numbers between 0 (inclusive) and the size of the deck (exclusive),
* and passes those numbers as parameters to swap(). This must be within a loop, so that the
* swap() method is called TIMES_TO_SHUFFLE times.
*/
public void shuffle(){
Random randomGenerator = new Random();
int counter = 0;
int firstNumber, secondNumber = 0;
while (counter++ < TIMES_TO_SHUFFLE){
firstNumber = randomGenerator.nextInt(deck.size());
secondNumber = randomGenerator.nextInt(deck.size());
swap(firstNumber,secondNumber);
}
}
public void loadDeck() {
for (int suit = 0; suit < SUITS.length; suit++) {
for (int description = 0; description < DESCRIPTIONS.length; description++) {
Card card = new Card(DESCRIPTIONS[description], SUITS[suit]);
deck.add(card);
}
}
}
}
最后一堂课：
import java.util.ArrayList;
/**
* @author jaimefenton
*
*/
public class Game {
private Deck aDeck;
private InputReader reader;
private ArrayList<Card> hand;
private String commandChoice;
/**
* Method to run the game.
* First while loop will run until the player chooses "no" for another round.
* Second while look will keep running until the player chooses to stand, has 21 or busts.
* the last while loop is to make sure that the player chooses either "Hit" or "Stand". If neither is choosen, it will keep requesting it.
*/
public void Play(){
int playerPoints = 0;
int totalRounds = 0;
intro();
aDeck = new Deck();
aDeck.loadDeck();
while(anotherRound() == false){
dealCard();
dealCard();
showHand();
report();
while(isStanding() == false){
if(hasBlackjack() == true){
hasBlackjack();
System.out.println("BlackJack!");
}else if (isBusted() == true){
isBusted();
System.out.println("You have Busted!");
}else {
dealCard();
report();
System.out.println("Your choice: Hit or Stand? ");
String inputChoice = reader.getInput();
while(inputChoice != "Hit" || inputChoice != "Stand"){
System.out.println("That is not a correct choice.");
}
isStanding();
}
} totalRounds ++;
}
System.out.println("Player Points: " + playerPoints);
System.out.println("Total Rounds: " + totalRounds);
}
/**
* intro message to player
*/
public void intro(){
System.out.println("Welcome to 1451 Blackjack!");
System.out.println("You will start with two cards.");
System.out.println("You will be prompted to 'hit' or 'stand' 'hit' means you want another card, 'stand' not.");
System.out.println("");
System.out.println("You are trying to get Blackjack with exactly 21 points.");
}
/**
* deals a card to the player
*/
public void dealCard(){
int deckSize = aDeck.deckSize();
if(deckSize == 0){
System.out.println("Time for some more cards");
aDeck.loadDeck();
aDeck.shuffle();
} else {
Card tempCard = aDeck.takeCard();
hand.add(tempCard);
}
}
/**
* calculates the hand value of the player
* @return handValue
*/
public int getHandValue(){
int handValue = 0;
for(Card eachCard : hand) {
int tempValue = eachCard.getValue();
handValue = handValue + tempValue;
}
return handValue;
}
/**
* displays contents of players hand
*/
public void showHand(){
System.out.println("Your cards:");
for(Card eachCard : hand) {
System.out.println(eachCard.getDescription()+
" of " + eachCard.getSuit());
}
}
/**
* displays contents of hand
* @return true or false
*/
public boolean hasBlackjack(){
int bjValue = getHandValue();
if(bjValue == 21){
return true;
} else {
return false;
}
}
/**
* is hand value above 21?
*/
public boolean isBusted(){
int bjValue = getHandValue();
if(bjValue > 21){
return true;
} else {
return false;
}
}
/**
* has player chosen to "stand"?
* @return true or false
*/
public boolean isStanding(){
if(commandChoice == "Hit"){
return true;
} else {
return false;
}
}
public boolean anotherRound(){
if(commandChoice == "no"){
return true;
} else {
return false;
}
}
/**
* final report of points
*/
public void report(){
showHand();
System.out.println("Hand Value: " + getHandValue());
System.out.println("");
}
}
我没有包含InputReader，因为它对这个问题并不重要。
我使用的驱动程序类彻底测试了Deck和Card，并且都返回了正面结果。请记住，我知道play()可能搞砸了，但在我解决它调用的方法之前，我无法修复它。
我遇到的第一个错误是＆＃34; public void dealCard()＆＃34;。它会提取与＆＃34; if(deckSize == 0)＆＃34;相关的零点异常错误。我不能告诉它为什么不能正常工作，因为当我直接从甲板上运行时，它完美无缺。一旦问题得到解决，我相当肯定我可以解决剩下的问题了。
感谢您查看此内容。
答案 0 :(得分：1)
我很确定hand为空。 reader也可能为空。我没有看到他们分配任何内容（类似hand =或reader =这样的表达）并且它们是私密的。
这就是为什么你应该初始化你的成员变量或使用构造函数，除非你有什么理由不能：
public class Game {
private Deck aDeck = new Deck();
private InputReader reader = /* ??? */;
private ArrayList<Card> hand = new ArrayList<Card>();
private String commandChoice;
/*
*/
通常，对象成员的生命周期应该与对象相同，换句话说，您应该始终在创建时实例化成员。在可能被多次调用的某些方法（如Play）中执行此操作会增加对象最终处于某种无效状态的可能性。 （或者在这种情况下，从无效状态开始。）
我也注意到你这样做了：
String inputChoice = reader.getInput();
while(inputChoice != "Hit" || inputChoice != "Stand"){
答案 1 :(得分：0)
您永远不会初始化 hand ，这是 Game 类的成员变量。
必须先初始化ArrayLists ，然后才能调用它们的方法。
dealCard（）的以下行将始终抛出 NullPointerException ：
hand.add(tempCard);
修改声明 hand 的行如下所示：
private ArrayList<Card> hand = new ArrayList<Card>();
正如Radiodef所提到的，使用标准构造函数会更好。