Question

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.image.BufferedImage;
import java.io.Console;
import java.util.Scanner;
import javax.swing.JComponent;
import javax.swing.JFrame;


public class CA extends JComponent{

    int[] cells;
    int[] ruleset;
    int w = 10;
    int generation = 0;
    int width = 600; //Pixel length of the window
    private BufferedImage caImage;

    CA()
    {       
        cells = new int[width/w];
        //Rule 90 of Wolfram
        ruleset = new int[]{0,1,0,1,1,0,1,0};


        for (int i = 0; i < cells.length; i++) 
            cells[i] = 0;

        cells[cells.length/2] = 1;
        print(cells, cells.length);
        generate();
    }

    public void generate()
    {
        int [] nextgen = new int[cells.length];

        while(generation<6)
        {

            for(int i=1; i<cells.length-1; i++)
            {
                int left = cells[i-1];
                int middle = cells[i];
                int right = cells[i+1];
                nextgen[i] = rules(left, middle, right);
            }
            cells = nextgen;

            print(cells, cells.length);
            System.out.println();

            paintComponent(this.getGraphics());
            generation++;
        }
    }

    public void print(int[] a, int length)
    {
        System.out.println();
        for(int i=0; i<length; i++)
        {
            System.out.print(a[i]+" ");
        }
    }

    public int rules(int a, int b, int c)
    {
        String s = ""+a+b+c;
        int index = Integer.parseInt(s, 2);
        return ruleset[index];
    }


    public void paintComponent(Graphics g)
    {       
        super.paintComponent(g);

        Graphics2D g2 = (Graphics2D) g;
        for(int i=0; i<cells.length; i++)
        {
            if(cells[i] == 0)       
            {
                Color color = Color.WHITE;
                g2.setColor(color);
            }

            else if(cells[i] == 1)
            {
                Color color = Color.BLACK;
                g2.setColor(color);
            }

            //Rectangle rect = new Rectangle(i*w, generation*2, w, w);
            //g2.fill(rect);
            g2.drawRect(i*w, generation*2, w, w);
        }
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        JFrame frame = new JFrame();
        frame.setSize(600, 600);
        frame.setTitle("Cellular Automaton");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setBackground(Color.RED);
        frame.setVisible(true);

        //Scanner input = new Scanner(System.in);
        //String pause = input.nextLine(); 



        CA cellularAutomaton = new CA();
        frame.add(cellularAutomaton);
        frame.setVisible(true);

    }

}

我希望通过从60个元素的1D数组中获取1或0来显示10x10像素的白色或黑色矩形。对于1D阵列的不同更新元素，我重复相同的操作6次。（实施细胞自动机）。

问题：当我想在600x600的窗口上显示结果时，会出现此问题。 g2.setColor(Color)行抛出NullPointerException。我找不到发生此错误的原因。

Answer 1

您的Graphics对象为null。不要在组件上调用getGraphics()来获取Graphics实例，因为它不会持久存在。可以在BufferedImage上调用它，但如果从组件中获取它，那么任何重绘都会变为null。而是使用JVM在paintComponent(...)方法参数中提供的Graphics实例或BufferedImage中的Graphics实例。

另外，请勿直接致电paintComponent(...)。那是JVM的工作。

另外，你的while循环不起作用，因为它会占用并锁定Swing Event Dispatch Thread或EDT。您应该使用SwingWorker在后台线程上生成BufferedImage，然后显示在Swing事件线程上创建的Image。

Graphics2D：设置颜色时出现空指针异常

1 个答案: