我想通过让大家知道这是我第一次出现堆栈溢出来引导这个问题，所以如果我不符合问题标准，请告诉我。
我正在制作一个与你合作播放Rock，Paper，Scissors的程序，当我接近项目的后端时，出现了这个错误：
Exception in thread "main" java.lang.NullPointerException
at RockPaperScissors.getPlayerThrow(RockPaperScissors.java:93)
at RockPaperScissors.main(RockPaperScissors.java:26)
我不知道我会在哪里使用null，但这就是你在这里的目的。
这是目前编译的整个项目：
public class RockPaperScissors {
//sets the constants
static final int ROCK = 1;
static final int PAPER = 2;
static final int SCISSORS = 3;
//creates some variables
static int playerThrow, computerThrow, result, timesPlayed, playerWins, computerWins;
static String playAgain;
static Scanner fru;
/*
* The Results
* 0 = tie
* 1 = Player win
* 2 = Computer win
*/
public static void main(String[] args) {
//this do while loop is the whole game
do {
//decides the throws of the players
playerThrow = getPlayerThrow();
computerThrow = (int)(Math.random() * 3 + 1);
switch(playerThrow) {
//compares and displays the computer and player
//choices if the player chooses rock
case ROCK:
switch(computerThrow) {
case ROCK:
result = 0;
System.out.println("You threw rock and the computer threw rock!");
break;
case PAPER:
result = 2;
System.out.println("You threw rock and the computer threw paper!");
break;
case SCISSORS:
result = 1;
System.out.println("You threw rock and the computer threw scissors!");
break;
} break;
//compares and displays the computer and player
//choices if the player throws paper
case PAPER:
switch(computerThrow) {
case ROCK:
result = 1;
System.out.println("You threw paper and the computer threw rock!");
break;
case PAPER:
result = 2;
System.out.println("You threw paper and the computer threw paper!");
break;
case SCISSORS:
result = 3;
System.out.println("You threw paper and the computer threw scissors!");
break;
} break;
//compares and displays the computer and player
//choices if the player throws scissors
case SCISSORS:
switch(computerThrow) {
case ROCK:
result = 2;
System.out.println("You threw scissors and the computer threw rock!");
break;
case PAPER:
result = 1;
System.out.println("You threw scissors and the computer threw paper!");
break;
case SCISSORS:
result = 0;
System.out.print("You threw scissors and the computer threw scissors!");
break;
} break;
}
timesPlayed ++;
// will compare and decide the winner of the two players
finish();
} while (timesPlayed < 3);
}
public static int getPlayerThrow() {
//prompts weapon choice and stores said choice
System.out.println("Choose your weapon of choice:\n(1 for rock, 2 for paper, 3 for scissors)");
int choice = fru.nextInt();
//checks for validity and returns the choice
if (choice != 1 && choice != 2 && choice != 3) {
System.out.print("Not a valid input!\n Please try again: ");
choice = fru.nextInt();
}
return choice;
}
//compares and decides the winner of the two players
public static void finish() {
//displays the winner of the round accourding to aforementioned possible results
switch(result) {
case 0:
System.out.println("Its a tie!"); break;
case 1:
System.out.println("You are victorious! Man over machine!");
playerWins++; break;
case 2:
System.out.println("The computer has taken the round! Technological singularity approaches!");
computerWins++; break;
}
//cheks if the match is over and displays messages accordingly
switch(timesPlayed) {
case 1: break;
case 2:
if (playerWins == 2 || computerWins == 2) {
if (playerWins == 2) {
System.out.println("You win the match! Congratulations!\nWould you like to play another match?\n(y for yes, n for no)");
timesPlayed = 5;
playAgain = fru.nextLine();
//checks for validity
if (playAgain != "y" || playAgain != "n") {
System.out.print("Not a valid input!\n Please try again: ");
playAgain = fru.nextLine();
}
}
else if (computerWins == 2) {
System.out.println("The computer wins the match!\nPlay again! I know you can beat it.\n(y for yes, n for no)");
timesPlayed = 5;
playAgain = fru.nextLine();
//checks for validity
if (playAgain != "y" || playAgain != "n") {
System.out.print("Not a valid input!\n Please try again: ");
playAgain = fru.nextLine();
}
}
} break;
//will happen for any amount of times played over 2
default:
if (playerWins == 2) {
System.out.println("You win the match! Congratulations!\nWould you like to play another match?\n(y for yes, n for no)");
playAgain = fru.nextLine();
//checks for validity
if (playAgain != "y" || playAgain != "n") {
System.out.print("Not a valid input!\n Please try again: ");
playAgain = fru.nextLine();
}
}
else if (computerWins == 2) {
System.out.println("The computer wins the match!\nPlay again! I know you can beat it.\n(y for yes, n for no)");
playAgain = fru.nextLine();
//checks for validity
if (playAgain != "y" || playAgain != "n") {
System.out.print("Not a valid input!\n Please try again: ");
playAgain = fru.nextLine();
}
}
}
}
}
我既不了解错误的含义，也不了解它的来源。关于它的唯一信息来自于浏览谷歌的错误，但是当问题或发布的示例并非特定于我的项目时，这很难。我已经采取了多个步骤来修复它，但它们似乎都没有做任何事情。
这可能是一个复杂的编码问题或我错过的单个字符，但是任何和所有帮助都表示赞赏！谢谢！
答案 0 :(得分：0)
我看到很多地方你使用 fru扫描仪，但绝对没有你实际初始化它的地方。你有什么基本归结为：
import java.util.Scanner;
public class Test {
static Scanner fru;
public static void main(String[] arg) {
int x = fru.nextInt();
System.out.println(x+1);
}
}
并且，当你运行它时，你会看到异常。您需要执行以下操作：
fru = new Scanner(System.in);
在您尝试进行任何扫描之前。在你这样做之前，尝试取消引用它会导致你得到的异常。换句话说，比如：
import java.util.Scanner;
public class Test {
static Scanner fru;
public static void main(String[] arg) {
fru = new Scanner(System.in);
int x = fru.nextInt();
System.out.println(x+1);
}
}
成功运行。
在将来发现这些问题方面，您最好查看实际获得的错误消息：
Exception in thread "main" java.lang.NullPointerException
at RockPaperScissors.getPlayerThrow(RockPaperScissors.java:93)
at RockPaperScissors.main(RockPaperScissors.java:26)
尽管您的错误消息完全与您的代码相匹配（行号被三个关闭），但检查堆栈跟踪以追踪错误的位置和错误是一项至关重要的技能。
在这种情况下，它是行：
int choice = fru.nextInt();
因此您认为这是因为fru未正确设置。从那里开始，追溯到实际将fru设置为有用的地方。在这种特殊情况下，这种情况不存在，因此相对容易理解。