java - Rock，Paper，Scissors程序中的空指针异常

Question

这个问题在这里已经有了答案:





What is a NullPointerException, and how do I fix it?

(12 个回答)


6年前关闭。




我想通过让每个人都知道这是我第一次遇到堆栈溢出来引导这个问题，所以如果我不符合提问标准，请告诉我。

我正在制作一个和你一起玩摇滚、纸、剪刀的程序，就在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)

我不确定我会在哪里使用 null，但这就是你来这里的目的。

这是当前编译的整个项目:

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。我所拥有的唯一信息来自浏览谷歌的错误，但是当提出的问题或发布的示例不是我的项目特定的时，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，但感谢您提供任何和所有帮助!谢谢!

Answer 1

我看到你在很多地方使用 fru扫描仪，但在您实际初始化它的地方绝对没有。你所拥有的基本上归结为:

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);
    }
}

而且，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，试图取消引用它会导致你得到的异常。换句话说，类似:

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。

就将来发现这些问题而言，您最好查看您实际收到的错误消息:

Exception in thread "main" java.lang.NullPointerException
  at RockPaperScissors.getPlayerThrow(RockPaperScissors.java:93)
  at RockPaperScissors.main(RockPaperScissors.java:26)

尽管您的错误消息与您的代码不完全匹配(行号相差三)，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();

因此你会认为这是因为 fru未正确设置。从那里开始，需要追溯到您实际设置 fru 的位置。对有用的东西。在这种特殊情况下，它不存在，因此相对容易弄清楚。