为什么当我把这个指针推到Deque/ArrayList上时,它显然不是Null时,我会得到NullPointerException

所以我尝试做一个简单的迷宫解算器(深度优先)。我不想帮助解决递归方法,但由于某种原因,这会导致单元格的ArrayList的.add上出现NullPointerException,有人能帮我知道为什么吗?(MazeExample类制作了一个指向N S E W的简单迷宫式指针,由我的老师提供,这样我们就可以尝试测试我们的代码)。

public static void main(String[] args)
{
    MazeSolver solver = new MazeSolver();
    ExampleMaze example = new ExampleMaze();

    System.out.println(solver.stepsToSolveMaze(example.getStart()));    

}

这是主要的,这里是MazeSolver对象(到目前为止,它只计算移动到最后的次数,一次一步)。

public class MazeSolver 
{
private int steps=0;
private ArrayList<MazeCell> visitedCells;
private Deque<MazeCell> breadCrumbs;

public int stepsToSolveMaze(MazeCell cell)
{
    visitedCells.add(cell); //this is where the exception is getting thrown.
    breadCrumbs.push(cell);

    if (cell.isFinish())
    {
        return 1;
    }


    if (cell.east() != null && !visitedCells.contains(cell.east()))
    {
        steps += stepsToSolveMaze(cell.east());
    }
    if (cell.south() != null && !visitedCells.contains(cell.south()))
    {
        steps += stepsToSolveMaze(cell.south());
    }
    if (cell.west() != null && !visitedCells.contains(cell.west()))
    {
        steps += stepsToSolveMaze(cell.west());
    }
    if (cell.north() != null && !visitedCells.contains(cell.north()))
    {
        steps += stepsToSolveMaze(cell.north());
    }
    else
    {
        steps--;
        stepsToSolveMaze(breadCrumbs.pop());            
    }

    return steps;

}