java - 程序无法运行，Java、StringBuilder、Array

Question

我正在尝试让我的程序运行。但是，我收到了 NullPointerException。

如有任何帮助，我们将不胜感激。我不想包含太多代码；所以，我只包含了 eclipse 说错误的 block 。

这是拼图类的构造函数。 (java.lang.NullPointerException)

我纠正了第一个问题并添加了“StringBuilder puzzle = new StringBuilder();”。我现在在同一行收到这些错误。

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.lang.AbstractStringBuilder.setCharAt(Unknown Source)
at java.lang.StringBuilder.setCharAt(Unknown Source)
at Puzzle.<init>(Puzzle.java:55)
at PuzzleGameConsole.main(PuzzleGameConsole.java:22)

第 55 行是原始问题行。它标有“//eclipse 在这一行有错误”

代码:

public class Puzzle
{

    private StringBuilder puzzle;

    public Puzzle(String theSolution)
    {
        for (int index = 0; index < theSolution.length(); index++) 
        {
            if (theSolution.charAt(index) != ' ') 
            {
                puzzle.setCharAt(index, '-');  //eclipse had an error on this line
            }
            else if (theSolution.charAt(index) == ' ')
            {
                puzzle.setCharAt(index, ' ');
            }
        }
    }
}


public int guessLetter(char letter)
    {   
        int count = 0;

        for(int index = 0; index < solution.length(); index++)
        {
            if(Character.toUpperCase(letter) == solution.toUpperCase().charAt(index))
            {
                if (puzzle.charAt(index) == '-') 
                {
                    puzzle.setCharAt(index, letter);
                }
                else
                {
                    count = 0;
                    break;
                }
                count++;
            }
        }
        return count;
    }

    public String getPuzzle()
    {
        return "";
    }


    public boolean solvePuzzle(String guess)
    {
        if(guess.toUpperCase().equals(solution.toUpperCase()))
        {
            return true;
        }
        else
        {
            return false;
        }       
    }

}

测试发布在下面。没有任何问题。

import java.util.Arrays;
import java.util.Scanner;


public class PuzzleGameConsole
{
    private static final String NEWLINE = "\n";
    private static final String MENU = "(1) GUESS A LETTER (2) SOLVE THE PUZZLE (3) QUIT >> ";
    private static final String PROMPT_ENTER_LETTER = "\nENTER A LETTER> ";
    private static final String PROMPT_SOLVE_PUZZLE = "\nSOLVE THE PUZZLE> ";

    private static final int SELCTION_ENTER_LETTER = 1;
    private static final int SELCTION_SOLVE_PUZZLE = 2;
    private static final int SELCTION_QUIT = 3;


    public static void main(String[] args) 
    {

        Scanner in = new Scanner(System.in);

        Puzzle puzzle = new Puzzle("JAVA IS FUN");


        int selection = 0;

        while(selection != SELCTION_QUIT)
        {
            System.out.printf("PUZZLE: %s\n\n", puzzle.getPuzzle());

            System.out.print(MENU);
            selection = in.nextInt();
            in.nextLine();

            if(selection == SELCTION_ENTER_LETTER)
            {
                System.out.print(PROMPT_ENTER_LETTER);
                char letter = in.next().charAt(0);

                int letterCount = puzzle.guessLetter(letter);
                if(letterCount > 0)
                {
                    printMessageBox(String.format("YES! The letter %s was found %d time(s).", letter, letterCount));
                }
                else
                {
                    printMessageBox(String.format("Sorry, the letter %s is not available in the puzzle.", letter));                 
                }
            }
            else if(selection == SELCTION_SOLVE_PUZZLE)
            {
                System.out.print(PROMPT_SOLVE_PUZZLE);
                String guess = in.nextLine();

                if(puzzle.solvePuzzle(guess))
                {
                    printMessageBox("Congratulations, you solved the puzzle!"); 
                    selection = 3;
                }
                else
                {
                    printMessageBox(String.format("Sorry, '%s' is not the puzzle.  Keep trying!", guess));
                }
            }
            else if (selection != SELCTION_QUIT)
            {
                System.out.println("INVALID MENU OPTION");
            }
        }

        System.out.println("\nGood Bye!");

        in.close();
    }

    public static void printMessageBox(String message)
    {
        char [] border = new char[message.length() + 4];
        Arrays.fill(border, '-');

        StringBuilder result = new StringBuilder();
        result.append(NEWLINE);
        result.append(border).append(NEWLINE);
        result.append("| ").append(message).append(" |").append(NEWLINE);
        result.append(border).append(NEWLINE);

        System.out.println(result);
    }

}

Answer 1

您的问题可能是您从未初始化拼图，也从未为其分配new StringBuilder()。即，

StringBuilder puzzle = new StringBuilder();  

例如，

public class Puzzle
{
    private StringBuilder puzzle = new StringBuilder(); // **** add this!

    public Puzzle(String theSolution)
    {
        for (int index = 0; index < theSolution.length(); index++) 
        {
            if (theSolution.charAt(index) != ' ') 
            {
                puzzle.setCharAt(index, '-');  //eclipse had an error on this line
            }
            else if (theSolution.charAt(index) == ' ')
            {
                puzzle.setCharAt(index, ' ');
            }
        }
    }
}

<小时/>

更重要的是，您需要学习如何调试 NPE(NullPointerException)的一般概念。您应该仔细检查抛出它的行，找出哪个变量为空，然后追溯到您的代码以查看原因。你会一次又一次地遇到这些，相信我。