空指针异常 - 打印二维数组

Question

我有一个试图打印2维数组的问题，我不断地在printArray（）和printArray（）的第一个for循环中得到一个空指针异常。我确实知道什么是虚假的感觉，我只是很难理解为什么我会得到它。在尝试打印数组之前，我遇到过这个问题，最后我会很好地理解这一点。

import java.lang.*; 
import java.util.*; 

public class Board { 

int N = 3; 
static int [][] unittest; 
//construct a board from an N-by-N array of blocks 
//(where blocks[i][j] = block in row i, column j) 
public Board(int[][] blocks){ 
    blocks = new int[N][N]; //creates array of size N 
    //generates random numbers 0 inclusive to # exclusive 
    //Random r = new Random(); uses blocks[i][j] = r.nextInt(); 
    for (int i = 0; i < blocks.length; i++){ 
     for (int j = 0; j < blocks[i].length; j++){ 
      blocks[i][j] = randInt(0,9); 
     } 
    } 
    unittest = blocks.clone(); 
} 
//generates random numbers in a range 
private static int randInt(int min, int max){ 
    Random rand = new Random(); 
    int randomNum = rand.nextInt((max - min) + 1) + min; 
    return randomNum; 
} 
//board size N 
public int size(){ 
    return 0; //placeholder 
} 
//number of blocks out of place 
public int hamming(){ 
    return 0; //placeholder 
} 
//sum of manhattan distances between blocks and goal 
public int manhattan(){ 
    return 0; 
} 
//is this board the goal board? 
public boolean isGoal(){ 
    return true; //placeholder 
} 
//is the board solvable? 
public boolean isSolvable(){ 
    return true; //placeholder 
} 
//does this board equal y? 
//Object because can only take objects 
//does it use Comparable? 
public boolean equals(Object y){ 
    return true; //placeholder 
} 
//all neighboring boards 
public Iterable<Board> neighbors(){ 
    Stack<Board> placeholder = new Stack<Board>(); 
    return placeholder; 
} 
//string representation of the board 
public String toString(){ 
    return "placeholder"; 
} 
//unit test 

private static void printArray(){ 
    for (int i = 0; i < unittest.length; i++){ //**NULL POINTER EXCEPTION 
     for (int j = 0; j < unittest[i].length; j++){ 
      System.out.print(unittest[i][j]); 
      if (j < unittest[i].length - 1){ 
       System.out.print(" "); 
      } 
     } 
     System.out.println(); 
    } 

} 
public static void main(String[] args){ 
    //prints out board 
    printArray(); //**NULL POINTER EXCEPTION 


} 

}

Answer 1

的问题出在测试条件printArray（）功能：

for (int i = 0; i < unittest.length; i++) 

这里，unitest是一个空对象，当您尝试对其应用长度方法时，它的引发和异常。

基本上，你没有初始化unittest对象（在你的情况下是2D数组）。你可以做这样的事情，以避免该异常：

private static void printArray(){ 
    if(unittest == null) 
     System.out.println("its NULL"); 
    else{ 
     for (int i = 0; i < unittest.length; i++){ //**NULL POINTER EXCEPTION 
      for (int j = 0; j < unittest[i].length; j++){ 
       System.out.print(unittest[i][j]); 
       if (j < unittest[i].length - 1){ 
       System.out.print(" "); 
       } 
      } 
      System.out.println(); 
     } 
    } 
} 

希望它能帮助:)

Answer 2

static int [][] unittest; 

为空时，你怎么称呼它

你从来没有初始化数组，也没有将任何东西放入它

Answer 3

超出了一个错误

public Board(int[][] blocks){ 
    blocks = new int[N][N]; //creates array of size N 
    //generates random numbers 0 inclusive to # exclusive 
    //Random r = new Random(); uses blocks[i][j] = r.nextInt(); 

    for (int i = 0; i < blocks.length; i++){    <----------------- blocks length should be blocks.length-1 

     for (int j = 0; j < blocks[i].length; j++){  <---------------------also blocks [i].length - 1 
      blocks[i][j] = randInt(0,9); 
     }