面板 - 为什么我不能将代码移动到另一种方法？

Question

我正在写一些代码，现在画一个圆圈，稍后按下按钮时会移动圆圈。创建面板时，我希望已经在中心绘制了一个圆。我在actionPerformed方法中编写的代码在按下按钮时会这样做，但我希望在面板初始化时已经绘制好了，所以我将代码移到了createGUI方法中。然而，当我把它移到那里时，它不起作用，并且我得到了一个空指针异常。

有人能向我解释一下原因吗？谢谢

import java.awt.Color;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class MovingArrows extends JFrame implements ActionListener {
    private JButton buttonUp, buttonDown, buttonLeft, buttonRight;
    private JPanel panel;
    private int xCircleCentre, yCircleCentre;
    final int xCircleCentreStarting = 250, yCircleCentreStarting = 250;
    final int RADIUS = 20;
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        MovingArrows frame = new MovingArrows();
        frame.setSize(550, 600);
        frame.createGUI();
        frame.setVisible(true);
    }
    private void createGUI() {
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        Container window = getContentPane();
        window.setLayout(new FlowLayout());
        panel = new JPanel();
        panel.setPreferredSize(new Dimension(500, 500));
        panel.setBackground(Color.white);
        window.add(panel);
        buttonUp = new JButton("Up");
        buttonDown = new JButton("Down");
        buttonLeft = new JButton("Left");
        buttonRight = new JButton("Right");
        window.add(buttonUp);
        window.add(buttonDown);
        window.add(buttonLeft);
        window.add(buttonRight);
        buttonUp.addActionListener(this);
        buttonDown.addActionListener(this);
        buttonLeft.addActionListener(this);
        buttonRight.addActionListener(this);
        Graphics paper = panel.getGraphics();
        paper.setColor(Color.black);
        paper.fillOval(xCircleCentreStarting - RADIUS, yCircleCentreStarting
                - RADIUS, RADIUS * 2, RADIUS * 2);
    }
    @Override
    public void actionPerformed(ActionEvent event) {
        /*
         * Graphics paper = panel.getGraphics(); paper.setColor(Color.black);
         * paper.fillOval(xCircleCentreStarting - RADIUS, yCircleCentreStarting
         * - RADIUS, RADIUS * 2, RADIUS * 2);
         */
    }
}

Answer 1

panel.getGraphics（）返回null，因为组件尚未渲染。

另请参阅调用getGraphics（）的任何替代方法，该方法返回空