为什么这个 Swing Timer 代码会抛出 NullPointerException？

Question

我正在尝试使用 Swing Timer 在 Java 中为椭圆设置动画。我相信下面的代码应该可以完成这项工作，但是当我运行我的程序时，计时器会抛出一个NullPointerException。知道为什么会这样吗？

我已经排除了包含主线的代码，因为它不相关。这是我的错误，发生在actionlistener 的actionperformed 方法的第一行：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Timer$MoveListener.actionPerformed(Timer.java:18)

面板类：

import java.awt.geom.*;

public class Glitchpanel extends javax.swing.JPanel {

    private Ellipse ellipse;
    private java.awt.Dimension size;
    private Timer timer;

    public Glitchpanel() {
        super();

        ellipse = new Ellipse();
        size = new java.awt.Dimension(1000, 1000);
        timer = new Timer(ellipse, this);
        timer.start();

        this.setSize(size);
        this.setPreferredSize(size); 
        this.setBackground(java.awt.Color.WHITE);
}

    public void paintComponent(java.awt.Graphics g) {
        super.paintComponent(g);
        java.awt.Graphics2D brush = (java.awt.Graphics2D) g;
        brush.draw(ellipse);
    }
}

定时器类：

import java.awt.geom.*;

public class Timer extends javax.swing.Timer {

    private Glitchpanel glitch;
    private Ellipse ellipse;

    public Timer(Ellipse ellipse, Glitchpanel glitch) {
        super(100, null);
        ellipse = ellipse;
        glitch = glitch;
        this.addActionListener(new MoveListener());
    }

    private class MoveListener implements java.awt.event.ActionListener {

        public void actionPerformed(java.awt.event.ActionEvent e) {
            ellipse.setX(ellipse.getX()+1);
            ellipse.setY(ellipse.getY()+1);
            glitch.repaint();
        }
    }
}

形状类（试图为这个对象设置动画）：

public class Ellipse extends java.awt.geom.Ellipse2D.Double {

    private Glitchpanel glitch;
    private double x, y, w, h;

    public Ellipse() {
        super();

        double x = 100;
        double y = 100;
        double w = 100;
        double h = 100;
        this.setFrame(x, y, w, h);
    }

    public void setX(double x2) {
        x = x2;
    }

    public void setY(double y2) {
        y = y2;
    }
}

Answer 1

变化：

public Timer(Ellipse ellipse, Glitchpanel glitch) {
    super(100, null);
    ellipse = ellipse;
    glitch = glitch;
    this.addActionListener(new MoveListener());
}

到：

public Timer(Ellipse ellipse, Glitchpanel glitch) {
    super(100, null);
    this.ellipse = ellipse;
    this.glitch = glitch;
    this.addActionListener(new MoveListener());
}