无法寻址对象的 2D 数组

Question

我在寻址一个对象的2D数组时遇到了麻烦…

我有一个类GameEngine，我声明:

Tile[][] theBoard;

后来在课堂上，我设置了黑板:

theBoard = new Tile[8][8];
prepareTheBoard();

prepareTheBoard方法:(也在同一个类中声明- GameEngine)

public void prepareTheBoard(){
    int n = 0;
    for(n = 0; n < 8; n++){
        System.out.println("n: " + n + " length: " + theBoard[1].length);
        System.out.println("theBoard : " + theBoard[1][1].isEmpty());
        theBoard[1][n].setPiece(new Piece(PieceColor.WHITE, Pieces.PAWN, theBoard[1][n]));
        theBoard[6][n].setPiece(new Piece(PieceColor.BLACK, Pieces.PAWN, theBoard[6][n]));
    }
...
}

第一次打印给我(如预期的):

n: 0长度:8

但是第二次打印给出了一个错误:

main线程异常java.lang.NullPointerException

我做错了什么?为什么它能看到数组的长度，但我不能处理它?

Answer 1

您没有实例化2d数组单元格。

theBoard = new Tile[8][8];

它将创建2d的空数组。您需要使用如下的new操作符实例化每个单元格。

theBoard[i][j] = new Tile();

Answer 2

在调用setPiece()方法之前，必须像这样在for循环中初始化数组中的对象:

for(n = 0; n < 8; n++) {
    theBoard[1][n] = new Tile();
    theBoard[6][n] = new Tile();
    System.out.println("n: " + n + " length: " + theBoard[1].length);
    System.out.println("theBoard : " + theBoard[1][1].isEmpty());
    theBoard[1][n].setPiece(new Piece(PieceColor.WHITE, Pieces.PAWN, theBoard[1][n]));
    theBoard[6][n].setPiece(new Piece(PieceColor.BLACK, Pieces.PAWN, theBoard[6][n]));
}