Question

我在CS1分配中遇到麻烦，希望您能帮助我！ This is my assignment

我已经开始工作，但是运行play.java时出现Null Pointer Exception错误。我对play.java和FMP.java的代码如下

FMP.java：

public class FMP {

    // ATTRIBUTES: ******************************************************************
    private int[][] puzzle;
    private String[][] path;
    private boolean solvable;

    // CONSTRUCTORS: ******************************************************************
    //default constructor
    //...........YOUR CODE GOES HERE
     public FMP(){

    }

    //constructor that takes in a 2D-array of integers representing a maze 
    public FMP(int[][] maze) {
        // Initialize puzzle
        //...........YOUR CODE GOES HERE
        int[][] puzzle = new int [10][10];
        // Initialize path to be a table of the same dimensions as puzzle 
        //...........YOUR CODE GOES HERE
        String[][] path = new String[10][10];
        // fill path with spaces
        //...........YOUR CODE GOES HERE
        for(int i=0; i < 9; i++){
            for(int j=0; j < 9; j++){
                path[i][j] = " ";
            }
        }
        // call the method buildPath on your current instance: given to you
        this.buildPath();
    }

    // SETTERS: ******************************************************************
    public void setPuzzle(int[][] maze) {
        // Same code as for the second constructor
        // copy and paste here
         int[][] puzzle = new int [10][10];

        String[][] path = new String[10][10];

        for(int i=0; i < 9; i++){
            for(int j=0; j < 9; j++){
                path[i][j] = " ";
            }
        }
    }

    // GETTERS:  ******************************************************************
    // One getter method per attribute (lines 13, 14, and 15)

    // YOUR GETTER METHODS GO HERE
    public int[][] getPuzzle(){
        return puzzle;
    }
    public String[][] getPath(){
        return path;
    }
    public boolean getSolvable(){
        return solvable;
    }


    // OTHER METHODS:  ************************************************************

    /* buildPath:
     ***************************************************************************/
    public String[][] buildPath() {

        // HERE GOES CODE FROM YOUR PREVIOUS METHOD buildPath
        String[][] result = new String[puzzle.length][puzzle[0].length];
        for (int i=0; i<result.length; i++) {
            for (int j=0; j<result[0].length; j++) {
                result[i][j]=" ";   
            }
        }

        int i = 0;
        int j = 0;
        result[0][0] = "X";
            while (i < puzzle.length && j < puzzle[0].length) {
                if (puzzle[i][j] % 2 == 0 && result[i][j - 1].equals(" ")){
                    j--;
                    result[i][j] = "X";
                }
                else if (puzzle[i][j] % 5 == 0 && result[i][j + 1].equals(" ")){
                    j++;
                    result[i][j] = "X";
                }
                 else if (puzzle[i][j] % 3 == 0 && result[i - 1][j].equals(" ")){
                    i--;
                    result[i][j] = "X";
                 }
                 else if (puzzle[i][j] % 7 == 0 && result[i + 1][j].equals(" ")){
                    i++;
                    result[i][j] = "X";
                 }
                 if( i == 9 && j == 9){
                    break;
                }
            }
        if ((i!=puzzle.length-1)||(j!=puzzle[0].length-1)){
            solvable = false;
        }
        else{
            solvable = true;
        }
        return result;
    }


        // At the end of your code, if you have reached the bottom right corner 
        // of the maze, the puzzle is solvable (true). Otherwise it is not (false). 
        // Your code may look like the following two commented lines:
        // if ((i!=puzzle.length-1)||(j!=puzzle[0].length-1)) solvable = false;
        // else solvable = true;

    /* printPath:
     ***************************************************************************/
    public void printPath() {

        int nextLine = 0;
        System.out.println("_______________________________");
        for( int i = 0; i < path.length; i++){
            for ( int j = 0; j < path[0].length; j++){
                System.out.print("| " + path[i][j]);
                nextLine++;
                if( nextLine % 10 == 0){
                    System.out.println("|");
                    System.out.println("_______________________________");
                }
            }        
        }
    }   
}

play.java：

public class play {
    public static void main(String[] args) {
        int[][] puzzle = {{225,    200,	70,	   14,	    35,	    14,	    35,	    14,	    25,	    14},
                       {35,	    40,	    6,	   15,	    28,	    15,	    18,	    15,	    20,	     42},
                       {75,	    20,	    14,	    35,	    40,	    50,	    10,	    14,	    5,	     42},
                       {35,	    14,	    21,	    21,	    35,	    200,	2,	    105,	14,	     21},
                       {21,	    15,	    12,	    225,	49,	    35,	    14,	      9,	21,	      9},
                       {115,	50,	    20,	    40,	    77,	    9,	    15,	    14,	    225,	 14},
                       {28,	    35,	    200,	14,	    35,	    20,	    50,	    14,	    35,	     6},
                       {21,	    3,	     35,	42,	    49,	    20,	    20,	    14,	    105,	 14},
                       {15,	    50,	     28,	21,	    5,	    70,	    14,	    21,	    9,	     21},
                       {5,	    40,	     205,	300,	40,	    25,	    15,	    45,	    20,	     42}};
        FMP p1 = new FMP(puzzle);
        p1.printPath();
        
    }
}

play.java不需要修改，只有FMP.java。为什么会出现Null指针异常？

Answer 1

问题是您在FMP(int[][])构造函数中正在屏蔽您的字段。这个

int[][] puzzle = new int [10][10];
// Initialize path to be a table of the same dimensions as puzzle 
//...........YOUR CODE GOES HERE
String[][] path = new String[10][10];

应该是

this.puzzle = new int [10][10];
this.path = new String[10][10];

Java中的空指针异常（CS1分配）

1 个答案: