引用变量到Java中另一个类中实例化/初始化的对象 [英] Reference variable to an object instantiated/initialized in another class in Java
问题描述
我问的原因是因为我收到了NullPointerException。
我现在这很容易，但我很新编程，发现这有点令人困惑。
所以说我已经在一个类中初始化了一个对象，并希望从另一个类中访问同一个对象。
The reason I'm asking is because I'm getting NullPointerException. I now this is very easy but I'm pretty new programming and find this a bit confusing. So say I have initialized an object in a class and want to access that same object from another class.
比如现在我正在研究一个小型国际象棋游戏，在我的模型游戏类我有一个Board实例，一个对象。反过来，董事会有一系列Squares。 Square [] []。
Like now for instance I'm working on a small Chess game, in my model Game class I have an instance of Board, an object. Board, in turn, has an array of Squares. Square[][].
游戏有棋盘，棋盘有Square [] []。
Game has board, board has Square[][].
现在如果我想通过Board类型的对象板（在游戏中）访问Square [] []。
我只是声明一个具有相同名称和类型的变量，还是我必须再次初始化它？
Now if I want to access the Square[][] through the object board (in Game) of type Board. Do I just declare a variable with the same name and type or do I have to initialize it again?
董事会董事会或董事会董事会=新董事会（） ;
Board board OR Board board = new Board();
注意，我已经在Game类中初始化了板，所以如果我再次执行它，它们不会是两个完全不同的Board对象吗？
Note, I have already initialized board in the class Game so if I do it again, won't they be two totally different Board objects?
引用board的类：
public class View extends JFrame {
Board board;
JFrame gameWindow = new JFrame("Chess");
JPanel gamePanel = new JPanel();
JPanel[][] boardPanel = new JPanel[8][8];
JMenuBar gameMenu = new JMenuBar();
JButton newGame = new JButton("New game");
JButton pauseGame = new JButton("Pause");
JButton actionLog = new JButton("Action log");
View(){
gameWindow.setDefaultCloseOperation(EXIT_ON_CLOSE);
gameWindow.setSize(400, 400);
gameWindow.getContentPane().add(gamePanel);
gameWindow.setVisible(true);
gamePanel.setVisible(true);
gameMenu.add(newGame);
gameMenu.add(pauseGame);
gameMenu.add(actionLog);
for(JPanel[] row : boardPanel){
for(JPanel box : row){
gamePanel.add(box);
}
}
}
public void drawBoard(){
for(int y = 0; y < 8; y++){
for(int x = 0; x < 8; x++){
Box box = new Box();
box.setColour(board.getSquare(x, y).getColour());
box.setCol(x);
box.setRow(y);
box.repaint();
boardPanel[x][y].add(box);
}
}
}
}
class Box extends JComponent{
JPanel[][] boardPanel;
Board board;
Color boxColour;
int col, row;
public Box(){
repaint();
}
public void paint(Graphics drawBox){
drawBox.setColor(boxColour);
drawBox.drawRect(50*col, 50*row, 50, 50);
drawBox.fillRect(50*col, 50*row, 50, 50);
}
public void setColour(Color boxColour){
this.boxColour = boxColour;
}
public void setCol(int col){
this.col = col;
}
public void setRow(int row){
this.row = row;
}
}
......以及实例化董事会的类：
...and the class that instantiates "board":
public class Game {
@SuppressWarnings("unused")
public static void main(String[] args)
throws Throwable{
Board board = new Board();
View view = new View();
}
}
此处发生异常：
for(JPanel[] row : boardPanel){
for(JPanel box : row){
gamePanel.add(box);
}
}
推荐答案
注意，我已经在Game类中初始化了板，所以如果我再次执行它，它们不会是两个完全不同的Board对象吗？
Note, I have already initialized board in the class Game so if I do it again, won't they be two totally different Board objects?
是的，你将有两个完全不同的实例。你得到了基本的想法 - 你的程序中有对象的实例，现在你必须让它们一起工作。
Yes, you'll then have two totally different instances. You're getting the basic idea - you have instances of objects in your program and now you have to get them to work together.
现在如果我想通过Board类型的对象板（在游戏中）访问Square [] []。我只是声明一个具有相同名称和类型的变量，还是我必须再次初始化它？
Now if I want to access the Square[][] through the object board (in Game) of type Board. Do I just declare a variable with the same name and type or do I have to initialize it again?
你有2种方法可以给对正方形的游戏访问（可能超过2只取决于你如何看待它）：
You have 2 ways to give Game access to the squares (probably more than just 2 depending on how you look at it):
1让董事会提供对Squares的访问权限（例如，getter方法返回Squares数组的棋盘）所以游戏可以访问它们。然后，董事会可以保存参考（有自己的实例变量来保存对方块的引用，或者可以每次询问董事会参考）。
1 Have the Board provide access to the Squares (e.g. a getter method on Board that returns the Squares array) so Game can access them. Board can then save the reference (have its own instance variable to hold the reference to squares, or can ask Board each time for the reference).
2让董事会提供做游戏想要在广场上做的事情的方法，即游戏要求棋盘对方块做一些事情，棋盘在广场上做动作。
2 Have the Board provide methods that do the things that Game wants to do on the the squares, i.e. the game asks the board to do something to the squares and the board does the action on the squares.
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