Java新手: How to solve a NullPointerException?

Question

我正在尝试用 Java 创建康威的生命游戏，但似乎遇到了障碍。我创建了一个名为“Cell”的类，它包含一个 boolean 变量，该变量本质上确定细胞是活的还是死的，以及在需要时杀死或创建细胞的方法。在我的主要方法中，我获取用户在游戏中想要的行数和列数，并尝试创建一个 Cell 对象数组，每个对象都命名为“位置”。当我尝试运行代码并打印每个单元格的初始值时，我收到空指针异常错误，并且不确定如何修复它。据我所知，每个数组不应该有空值，但是我对此很陌生......

//Create a Cell class for the purpose of creating Cell objects.

公共(public)类 Cell

{


private boolean cellState;

//Cell Constructor. Initializes every cell's state to dead.
public Cell()
{
    cellState = false;
}

//This function kills a cell.
//Should be called using objectName[x][y].killCell.
public void killCell()
{
    cellState = false;
}

//This function creates a cell.
//Should be called using objectName[x][y]createCell.
public void createCell()
{
    cellState = true;
}

public void printCell()
{
    if (cellState == true)
            {  
               System.out.print("1");
            }
            else if
            {
               System.out.print("0");
            }

}


//End Class Cell//
}

这是我的 Cell 类(class)。如果一个单元格还活着，就会在它的位置打印一个单元格。如果死亡，则 0 将代替它。

这是我的主要方法。该错误发生在我创建 Cell 对象数组的行处。难道我做的这一切都是错的吗？

//GAME OF LIFE//


import java.util.Scanner;
import java.lang.*;
public class GameOfLife
{
    public static void main(String[] args)
    {
    Scanner scan = new Scanner(System.in);
    System.out.println("\t\tWelcome to the Game of Life!");
    System.out.println("\n\nDeveloped by Daniel Pikul");

    System.out.print("\n\nHow many rows would you like your game to have?");
    int numRows = scan.nextInt();

    System.out.print("How many columns would you like your game to have?");
    int numColumns = scan.nextInt();

    //Create an array of cell objects.
    Cell[][] location = new Cell[numColumns][numRows];

    //This for loop will print out the cell array to the screen.//
    for (int i = 0; i < numRows; i++)
    {
        for (int j = 0; j < numColumns; j++)
        {
            location[i][j].printCell();
        }
        System.out.print("\n");
    }

    //Prompt the user to enter the coordinates of cells that should live.
    System.out.println("Input coordinates tocreate active cells.");

    int xCo, yCo;
    char userChoice;

    //This do loop takes coordinates from the user. Every valid coordinate
    //creates a living cell in that location.
    do
    {
        System.out.print("Enter an x coordinate and a y coordinate: ");
        xCo = scan.nextInt();
        yCo = scan.nextInt();
        location[xCo][yCo].createCell();
        System.out.print("Enter another coordinate? (Y/N) ");
        String tempString = scan.next();
        userChoice = tempString.charAt(0);
    }while(userChoice == 'Y');

    //THIS IS AS FAR AS I HAVE GOTTEN IN THE PROGRAM THUS FAR//
}

}

Answer 1

//This for loop will print out the cell array to the screen.//
for (int i = 0; i < numRows; i++)
{
    for (int j = 0; j < numColumns; j++)
    {
        location[i][j].printCell();  // location[i][j] not instantiated
    }
    System.out.print("\n");
}

在上面的 for 循环中: - 您尚未在数组中实例化 Cell 对象:-

location[i][j].printCell();

在此代码之前您需要执行以下操作:-

location[i][j] = new Cell();