我遇到了java.lang.NullPointerException错误，希望了解有关如何解决该错误的信息。我知道这是一个零值问题，但是我无法弄清楚是什么。
基本上，只有在我尝试将Do-While循环放到一起时，问题才开始出现。如果删除循环并编辑代码，以便仅在调用getHistogram（）方法时运行一次，则不会发生错误。
这是我的一些代码，以查看我目前正在执行的操作以及发生错误的位置。我该怎么做才能阻止此错误的发生？
第1页。
Main第2页。
public class MainClass {
/**
* @param args the command line arguments
*/
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
int optionSelected;
int NumberOfSuits;
int NumberOfRanks;
int numberOfCardsDealt = 0;
int SizeOfDeck;
int numberOfCards;
System.out.printf( "How many suits? " );
NumberOfSuits = getDecision();
System.out.printf( "How many ranks? " );
NumberOfRanks = getDecision();
SizeOfDeck = NumberOfSuits * NumberOfRanks;
DeckofCards newDeck = new DeckofCards();
newDeck.setDeckofCards(NumberOfRanks, NumberOfSuits);
newDeck.createDeckofCards();
do {
System.out.println ( newDeck.toString() );
System.out.printf ( "1=Shuffle, 2=Deal 1 Hand, 3= Deal 100000 Times,"
+ "4=Quit: " );
optionSelected = getDecision();
switch ( optionSelected ) {
case 1:
newDeck.shuffleDeckofCards();
break;
case 2:
numberOfCards = getCardDecision();
numberOfCardsDealt = numberOfCardsDealt + numberOfCards;
if ( SizeOfDeck > numberOfCardsDealt ) {
String[] dealCards = newDeck.dealCards ( numberOfCards );
for ( String printCardsDealt : dealCards ) {
System.out.println ( printCardsDealt );
}
}
else {
System.out.println ( "Deck must be re-shuffled" );
numberOfCardsDealt = 0;
break;
}
break;
case 3:
numberOfCards = getCardDecision();
int[] histogram = newDeck.getHistogram( numberOfCards );
for ( int printHistogram : histogram ) {
System.out.println ( printHistogram );
}
break;
}
} while ( optionSelected != 4 );
}
public static int getDecision() {
String numberMatch;
int optionSelected = 0;
Scanner in = new Scanner(System.in);
do {
numberMatch = in.next();
if( numberMatch.matches("[0-9]+" ) ){
optionSelected = Integer.parseInt( numberMatch );
}
if ( optionSelected < 1 || optionSelected > 9 ){
System.out.println("Try again!");
}
} while ( optionSelected < 1 || optionSelected > 9 );
return optionSelected;
}
public static int getCardDecision() {
String numberMatch;
int optionSelected = 0;
Scanner in = new Scanner(System.in);
do {
System.out.printf ( "How many cards? " );
numberMatch = in.next();
if( numberMatch.matches("[0-9]+" ) ){
optionSelected = Integer.parseInt( numberMatch );
}
if ( optionSelected < 1 || optionSelected > 9 ){
System.out.println("Try again!");
}
} while ( optionSelected < 1 || optionSelected > 9 );
return optionSelected;
}
}
第3页。
public class DeckofCards {
private int NumberOfRanks;
private int NumberOfSuits;
private int SizeOfDeck;
private String newDeck[];
private String dealThisDeck[];
private int howManyCards;
private int cardsDealtTotal = 0;
private int numberOfCardsDealt;
private int numberOfCards;
private String dealtDeck[];
private int CardsDealtInTotal = 0;
private int histoSum[];
public void setDeckofCards ( int NumberOfRanks, int NumberOfSuits ) {
this.NumberOfRanks = NumberOfRanks;
this.NumberOfSuits = NumberOfSuits;
}
public void createDeckofCards() {
SizeOfDeck = NumberOfRanks * NumberOfSuits;
Cards newCard = new Cards();
newCard.setCards ( NumberOfRanks, NumberOfSuits );
newDeck = new String [ SizeOfDeck ];
int counter = 0;
for (int whatSuit = 1; whatSuit <= NumberOfSuits; whatSuit++) {
for (int rank = 1; rank <= NumberOfRanks; rank++) {
newDeck[counter++] = newCard.createCard(rank, whatSuit);
}
}
}
public void shuffleDeckofCards() {
int index;
int length = newDeck.length;
String temp;
Random random = new Random();
for ( int i = length - 1; i > 0; i-- ) {
index = random.nextInt(i+1);
temp = newDeck[index];
newDeck[index] = newDeck[i];
newDeck[i] = temp;
}
}
public String[] dealCards( int howManyCards ) {
this.howManyCards = howManyCards;
dealtDeck = new String[ howManyCards ];
System.arraycopy(newDeck, cardsDealtTotal, dealtDeck, 0, howManyCards);
cardsDealtTotal = cardsDealtTotal + howManyCards;
return dealtDeck;
}
@Override
public String toString() {
return ( "Deck of " + SizeOfDeck + "cards: low = 1 high = " +
SizeOfDeck + "top = Card " + newDeck [ cardsDealtTotal ] );
}
public int[] getHistogram( int numberOfCardsDealt ) {
int sum = 0; //initialize sum as 0
int count = 0; //initialize count 0
this.numberOfCards = numberOfCardsDealt - 1; //Equivalent array position to cards dealt
DeckofCards histoDeck = new DeckofCards();
do {
CardsDealtInTotal = CardsDealtInTotal + numberOfCards;
if ( SizeOfDeck < CardsDealtInTotal ) {
shuffleDeckofCards();
CardsDealtInTotal = 0;
}
for ( int i = 0; i <= numberOfCards; i++ ) {
sum = sum + histoDeck.getSumOfHand( dealtDeck[i]); //Check and send each position of hand to be added
histoSum[sum] +=1; //Based on sum add 1 to that position in the array
}
count+= 1;
} while ( count != 2 );
return histoSum;
}
public int getSumOfHand( String a) {
int value = 0;
String s;
s = a;
Pattern pattern = Pattern.compile("^S([\\d]+)R([\\d]+)$");
Matcher matcher = pattern.matcher(s);
if(matcher.find()){
value = (Integer.parseInt(matcher.group(1))*Integer.parseInt(matcher.group(2)));
}
return value;
}
}
StackTrace？
public class Cards {
private int numberOfRanks;
private int numberOfSuits;
private int createCard;
private String theNumber;
private int whatSuit;
private String theSuit;
private String theCard;
public void setCards( int numberOfRanks, int numberOfSuits ) {
this.numberOfRanks = numberOfRanks;
this.numberOfSuits = numberOfSuits;
}
public String createCard( int newCard, int whatSuit ) {
createCard = newCard;
theNumber = Integer.toString( createCard );
theSuit = Integer.toString ( whatSuit );
theCard = ( "S" + theSuit + "R" + theNumber );
return theCard;
}
}
这是在对case3和整个其他方法调用进行更改之后，getHistogram（）的替代选项。使用此方法，我遇到了ArrayIndexOutOfBoundsException的问题。
Exception in thread "main" java.lang.NullPointerException
at MainClass.DeckofCards.getHistogram(DeckofCards.java:105)
at MainClass.MainClass.main(Assignment4.java:72)
C:\Users\mikel\AppData\Local\NetBeans\Cache\8.2\executor-snippets\run.xml:53: Java returned: 1
这些错误发生在
case 3:
numberOfCards = getCardDecision();
newDeck.getHistogram( numberOfCards );
//for ( String printHistogram : histogram ) {
// System.out.println ( printHistogram );
//}
break;
public void getHistogram( int numberOfCardsDealt ) {
int sum = 0;
int count = 0;
this.numberOfCardsDealt = numberOfCardsDealt;
this.numberOfCards = numberOfCardsDealt - 1;
CardsDealtInTotal = numberOfCardsDealt + numberOfCards;
DeckofCards histoDeck = new DeckofCards();
do {
if ( SizeOfDeck > CardsDealtInTotal ) {
String[] histogramDeck = dealCards ( numberOfCardsDealt );
for ( int i = 0; i <= numberOfCards; i++ ) {
sum = sum + histoDeck.getSumOfHand( histogramDeck[i]);
System.out.println ( sum );
}
}
else {
shuffleDeckofCards();
CardsDealtInTotal = 0;
}
count+= 1;
} while ( count != 6 );
}
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException
at java.lang.System.arraycopy(Native Method)
at MainClass.DeckofCards.dealCards(DeckofCards.java:74)
at MainClass.DeckofCards.getHistogram(DeckofCards.java:101)
at MainClass.MainClass.main(MainClass.java:72)
C:\Users\mikel\AppData\Local\NetBeans\Cache\8.2\executor-snippets\run.xml:53: Java returned: 1
答案 0 :(得分：0)
我已经编译并运行了程序...我将所有文本合并到一个文件中，所以我显示带有错误的整行。
摘要：您需要研究DealdDeck-初始化时，它包含什么值以及在哪里更新。
Exception in thread "main" java.lang.NullPointerException
at DeckofCards.getHistogram(MainClass.java:207)
at MainClass.main(MainClass.java:64)
这207行：
sum = sum + histoDeck.getSumOfHand( dealtDeck[i]); //Check and send each position of hand to be added
我初始化数组以进一步：
private String dealtDeck[] = new String[1000];
下一个错误
Exception in thread "main" java.lang.NullPointerException
at java.util.regex.Matcher.getTextLength(Unknown Source)
at java.util.regex.Matcher.reset(Unknown Source)
at java.util.regex.Matcher.<init>(Unknown Source)
at java.util.regex.Pattern.matcher(Unknown Source)
at DeckofCards.getSumOfHand(MainClass.java:224)
at DeckofCards.getHistogram(MainClass.java:207)
at MainClass.main(MainClass.java:64)
出现在这里：
Pattern pattern = Pattern.compile("^S([\\d]+)R([\\d]+)$");
Matcher matcher = pattern.matcher(s); //<----- here from histoDeck.getSumOfHand( dealtDeck[i]);
P.S。重现错误：运行程序，然后输入花色和等级，然后选择选项3-接收NullPointerException。