为什么我得到java.lang.NullPointerException错误?
我一直在试图解决过去一小时的java.lang.NullPointerException。当我调用play()方法并输入no时会发生此错误。我已经评论错误指向下面的位置。我希望得到一些帮助。谢谢。为什么我得到java.lang.NullPointerException错误?
import java.util.ArrayList;
public class Game
{
private InputReader input ;
private Deck newDeck;
private ArrayList <Card> hand;
public Game(Deck deckToAdd)
{
input = new InputReader();
newDeck = deckToAdd;
hand = new ArrayList <Card>();
}
public void dealCard()
{
hand.add(newDeck.takeCard());
}
public void showHand()
{
for(Card showCards: hand){
if(hand == null){
System.out.println("(Warning, the deck may have been empty the last time you dealt a card)");
}
System.out.println(showCards.getDescription() + " of " + showCards.getSuit());
// Error points to above line
}
}
public int getHandValue()
{
int counter = 0;
int handValue = 0;
while(counter < hand.size()){
Card checker = hand.get(counter);
handValue += checker.getValue();
counter++;
}
return handValue;
}
public void play() //Error occurs when invoking this method and selecing no, points to showHand() method
{
boolean userWantsToPlay = true;
while(userWantsToPlay){
dealCard();
showHand();
System.out.println("Hand Value : " + getHandValue());
System.out.println("Do you want to continue? (yes or no)");
String userInput = input.getInput();
if(userInput == "no"){
userWantsToPlay = false;
}
}
}
}
你的条件是错误的:
if (hand == null) {
// do your stuff
}
else {
// do your stuff
}
在你的情况,你的第二个System.out.println
因为不在状态,这两种情况下(NULL,NOT NULL)将被应用将一直执行。
注:此外,我看到你的代码更“脏”的代码,例如你是比较Strings
与==
,它不会工作,因为它比较的参考,而不是内容。始终当你想比较Strings
你需要使用的equals()
代替==
所以
userInput.equals("no") {
// do your stuff
}
感谢您的帮助 – 2013-03-10 11:12:25
@JoshuaBaker欢迎您。 – Sajmon 2013-03-10 11:13:03
,而不是你的代码:
for(Card showCards: hand){
if(hand == null){
System.out.println("(Warning, the deck may have been empty the last time you dealt a card)");
}
System.out.println(showCards.getDescription() + " of " + showCards.getSuit());
// Error points to above line
}
应该不会是
if(hand!=null){
for(Card showCards: hand){
if(showCards== null){
System.out.println("(Warning, the deck may have been empty the last time you dealt a card)");
}else{
System.out.println(showCards.getDescription() + " of " + showCards.getSuit());
}
}
}
检查showCards代替hand.But调试将有帮助
你也应该更换:
userInput == "no"
有了:
userInput.equals("no")
请尽量让你的例子贴尽可能小。这将帮助我们帮助您调试问题。 – jontro 2013-03-10 10:55:28
play()从哪里调用?你可以显示该代码吗? – angelatlarge 2013-03-10 10:57:03