Java 我的编程项目将无法工作

这是模具类，它应该根据用户规范绘制模具，如主视图所示

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JPanel;

public class Die extends JPanel
{
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    int xCoordinate;
    int yCoordinate;
    int num;


    Graphics pen, g;

    private static final int DIE_LENGTH = 60;        //Set's the size
    private static final int DIE_WIDTH = 60;        //of the die
    private static final int DIE_HEIGHT = 60;
    private static final int DOT_LENGTH = 10;
    private static final int DOT_WIDTH = 10;
    private static final Color LINE_COLOR = Color.black;
    private static final Color DIE_COLOR = Color.white;
    private static final Color DOT_COLOR = Color.black;
    private static final int DIE_VERT = 10;
    private static final int DIE_HORI = 10;


    public Die(int dieNum, int x, int y)
    {
        xCoordinate = x;
        yCoordinate = y;

        num = dieNum;
    }

    @Override protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        //Creates the square
        g.drawRect(getX(), getY(), DIE_WIDTH, DIE_HEIGHT);
        g.setColor(LINE_COLOR);
        g.fillRect(xCoordinate, yCoordinate, DIE_LENGTH, DIE_WIDTH);
        if (num <= 0)
            drawBlank();
        else if (num == 1)
            drawOne();
        else if (num == 2)
            drawTwo();
        else if (num == 3)
            drawThree();
        else if (num == 4)
            drawFour();
        else if (num == 5)
            drawFive();
        else if (num == 6)
            drawSix();



    }

    private void drawBlank ()
    {
        pen.fillRect(xCoordinate, yCoordinate, DIE_LENGTH, DIE_WIDTH);
        pen.setColor(DIE_COLOR);
    }

    private void drawDot ( int x, int y)
    {
        super.paintComponent(g);
        g.setColor(DOT_COLOR);
        g.fillOval(x, y, DOT_LENGTH, DOT_WIDTH);

    }
    private void drawOne ()
    {
        super.paintComponent(g);
        drawBlank();
        drawDot(xCoordinate, yCoordinate);

    }
    private void drawTwo ()
    {
        super.paintComponent(g);
        drawBlank();
        pen.fillOval((xCoordinate - DIE_HORI), (yCoordinate + DIE_VERT), DOT_LENGTH, DOT_WIDTH);
        pen.fillOval((xCoordinate + DIE_HORI), (yCoordinate - DIE_VERT), DOT_LENGTH, DOT_WIDTH);

    }
    private void drawThree ()
    {

        drawBlank();
        drawOne();
        drawTwo();

    }

    private void drawFour ()
    {
        super.paintComponent(g);
        drawBlank();
        drawDot((xCoordinate - DIE_HORI),(yCoordinate + DIE_VERT));
        drawDot((xCoordinate + DIE_HORI),(yCoordinate + DIE_VERT));
        drawDot((xCoordinate - DIE_HORI),(yCoordinate - DIE_VERT));
        drawDot((xCoordinate + DIE_HORI),(yCoordinate - DIE_VERT));

    }

    private void drawFive ()
    {
        drawBlank();
        drawFour();
        drawOne();


    }


    private void drawSix()
    {
        drawBlank();
        drawFour();
        drawDot((xCoordinate - DIE_HORI), yCoordinate);
        drawDot((xCoordinate + DIE_HORI) , yCoordinate);

    }

}

import javax.swing.JFrame;
import java.awt.Color;
import java.util.Scanner;

public class RollTheDie
{

    public static void main(String[] args)
    {

        final int WINDOW_HEIGHT = 350;
        final int WINDOW_WIDTH = 300;

        System.out.println("Hi! Let's play dice!\n");

        Scanner kb = new Scanner( System.in );

        System.out.print("Enter the number on the face of the die:");
        int num = kb.nextInt();

        System.out.print("Enter the location of the die:");
        int x = kb.nextInt();
        int y = kb.nextInt();

        System.out.println("I hope you had fun! Bye!");

        JFrame dieWindow = new JFrame();
        dieWindow.setBackground(Color.gray);
        dieWindow.setSize(WINDOW_HEIGHT, WINDOW_WIDTH);
        dieWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        dieWindow.setTitle("Roll the Die");

        Die newDie = new Die(num,x,y);
        dieWindow.add(newDie);
        dieWindow.setVisible(true);

        kb.close();
    }
}

当我运行代码时，我得到了这个空异常错误，我似乎无法修复，有什么提示吗？错误指向绘图方法中的第一行，具体取决于用户输入的内容（模具上的点数），窗口打开，但没有绘制任何内容，背景为白色。下面是我运行代码时控制台中显示的内容

嗨！让我们掷骰子吧

在模具表面输入数字：5

输入模具的位置：50

五十

我希望你玩得开心！再见

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Die.drawBlank(Die.java:67)--(This Points to the drawBlank() method first line.)
    at Die.drawFive(Die.java:115)--(This points to the drawBlank() in the method drawFive().)
    at Die.paintComponent(Die.java:57)--(This points to my paintComponent(Graphics g) method, the line that has drawFive().)
    at javax.swing.JComponent.paint(Unknown Source)
    at javax.swing.JComponent.paintChildren(Unknown Source)
    at javax.swing.JComponent.paint(Unknown Source)
    at javax.swing.JComponent.paintChildren(Unknown Source)
    at javax.swing.JComponent.paint(Unknown Source)
    at javax.swing.JLayeredPane.paint(Unknown Source)
    at javax.swing.JComponent.paintChildren(Unknown Source)
    at javax.swing.JComponent.paintToOffscreen(Unknown Source)
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(Unknown Source)
    at javax.swing.RepaintManager$PaintManager.paint(Unknown Source)
    at javax.swing.RepaintManager.paint(Unknown Source)
    at javax.swing.JComponent.paint(Unknown Source)
    at java.awt.GraphicsCallback$PaintCallback.run(Unknown Source)
    at sun.awt.SunGraphicsCallback.runOneComponent(Unknown Source)
    at sun.awt.SunGraphicsCallback.runComponents(Unknown Source)
    at java.awt.Container.paint(Unknown Source)
    at java.awt.Window.paint(Unknown Source)
    at javax.swing.RepaintManager$3.run(Unknown Source)
    at javax.swing.RepaintManager$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.access$1100(Unknown Source)
    at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$200(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

---

您声明变量

pen

，但从不初始化它

您的代码中有：

Graphics pen, g;
// ...
private void drawBlank ()
{
    pen.fillRect(xCoordinate, yCoordinate, DIE_LENGTH, DIE_WIDTH);
    pen.setColor(DIE_COLOR);
}

但是没有任何地方真正初始化或设置为实例。执行

pen.fillRect（）

时，将抛出NullPointerException

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有关NullPointerException的更多信息，请参见@Sotirios_Delimanolis对您的问题的评论。

我可能是瞎子，但您是否曾将其分配给g或pen

Graphics g, pen;

---

编辑：在对g和pen调用方法之前，您必须将图形对象实际分配给它们。

能否将标题更改为更好地描述问题的内容，以方便将来的搜索用户？