java - 全局修改 Java 方法内的全局变量

Question

我正在制作《康维斯生命游戏》。在鼠标监听器中，我希望单击一次时单元格出现/消失在屏幕上。我使用 20x20 像素单元的 40x40 boolean 数组 (gameState)。我想使用我在单击方法中获得的鼠标坐标在绘制方法中绘制正方形。但是，我在第 71 行遇到空指针异常，并且不知道如何解决它。

主要

import javax.swing.*;
import java.awt.*;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.image.BufferStrategy;

public class mainApplication extends JFrame implements Runnable, MouseListener {


    private static final Dimension windowsize = new Dimension(80, 600);
    private BufferStrategy strategy;
    private Graphics offscreenGraphics;
    private static boolean isGraphicsInitialised = false;
    private static int rows = 40;
    private static int columns = 40;
    private static int height = windowsize.height;
    private static int width = windowsize.width;
    private static Cells cells;
    private int xArrayElement,yArrayElement, xPosition, yPosition;
    private static boolean gameState[][] = new boolean[rows][columns];

    public mainApplication() {
        System.out.println(System.getProperty("user.dir"));

        setDefaultCloseOperation(EXIT_ON_CLOSE);
        Dimension screensize = java.awt.Toolkit.getDefaultToolkit().getScreenSize();

        int x = screensize.width / 2 - windowsize.width / 2;
        int y = screensize.height / 2 - windowsize.height / 2;

        setBounds(x, y, screensize.width, screensize.height);

        setVisible(true);

        createBufferStrategy(2);

        strategy = getBufferStrategy();

        offscreenGraphics = strategy.getDrawGraphics();

        isGraphicsInitialised = true;

       // MouseEvent mouseEvent = new MouseEvent();
        addMouseListener(this);
       // addMouseMotionListener(MouseEvent);



        Thread t = new Thread(this);

        t.start();

    }


    public void mousePressed(MouseEvent e) { }

    public void mouseReleased(MouseEvent e) { }

    public void mouseEntered(MouseEvent e) { }

    public void mouseExited(MouseEvent e) { }

    public void mouseClicked(MouseEvent e) {

        if(e.getClickCount() == 1){

               xPosition = e.getX();
               yPosition = e.getY();

           cells.setPosition(xPosition,yPosition);

            xArrayElement = (xPosition/20);
            yArrayElement = (yPosition/20);

            if(gameState[xArrayElement][yArrayElement]){
                gameState[xArrayElement][yArrayElement] = false;
            }
            else if (!gameState[xArrayElement][yArrayElement]) {
                gameState[xArrayElement][yArrayElement] = true;
            }
        }
    }

    @Override
    public void run() {
            while (true) {

                try { //threads entry point
                    Thread.sleep(20); //forces us to  catch exception
                }

                catch (InterruptedException e) {
                }
            }
        }


    public void paint(Graphics g) {

        if (isGraphicsInitialised) {
            g = strategy.getDrawGraphics();

            g.setColor(Color.BLACK);

            g.fillRect(0, 0, 800, 800);


                    if (gameState[xArrayElement][yArrayElement]) {
                        g.setColor(Color.WHITE);
                        cells.paint(g);
                    }

                    else if (!gameState[xArrayElement][yArrayElement]) {

                        g.setColor(Color.BLACK);
                        g.fillRect(xPosition, yPosition, 20, 20);
                    }

                strategy.show();
            }
        }

    public static void main(String[]args){

        mainApplication test = new mainApplication();

    }
}

细胞类别

import java.awt.*;

public class Cells {

    int x;
    int y;


    public Cells(){

    }

    public void setPosition(int xi, int xj){

        x = xi;
        y = xi;
        System.out.println(xi);
        System.out.println("sjdkgffdjv" + y);
    }

    public boolean cellState(boolean visible){

        return visible;
    }

    public void paint(Graphics g){

        g.drawRect(x, y, 20,20);
    }
}

Answer 1

这是因为您尚未在 Main 类中初始化 cells 变量。所以试试这个

private static Cells cells = new Cells();