Java Applet 错误，NullPointerException

Question

我刚开始学习 Java 小程序，就遇到了这个错误。有人可以帮助我吗？

import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;


public class MakeDots extends Applet implements MouseListener{

    private Graphics graphics = null;

    public void init() {
        this.addMouseListener(this);
    }

    public void paint(Graphics g) {
        this.setSize(800, 600);
        g.create();
    }
    public void drawDot(int x, int y) {
        int r = (int)(Math.random()*256);
        int b = (int)(Math.random()*256);
        int g = (int)(Math.random()*256);
        Color color = new Color(r, g, b);
        graphics.setColor(color);
        graphics.fillOval(x, y, 3, 3);
    }

    @Override
    public void mouseClicked(MouseEvent e) {
        int mouseX = e.getX();
        int mouseY = e.getY();
        drawDot(mouseX, mouseY);
    }

    @Override
    public void mouseEntered(MouseEvent arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void mouseExited(MouseEvent arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void mousePressed(MouseEvent arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void mouseReleased(MouseEvent arg0) {
        // TODO Auto-generated method stub

    }

}

出现此错误:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at MakeDots.drawDot(MakeDots.java:26)
    at MakeDots.mouseClicked(MakeDots.java:34)
    at java.awt.Component.processMouseEvent(Component.java:6417)
    at java.awt.Component.processEvent(Component.java:6179)
at java.awt.Component.dispatchEventImpl(Component.java:4776)
    at java.awt.Container.dispatchEventImpl(Container.java:2142)
    at java.awt.Component.dispatchEvent(Component.java:4604)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:717)
    at java.awt.EventQueue.access$400(EventQueue.java:82)
    at java.awt.EventQueue$2.run(EventQueue.java:676)
    at java.awt.EventQueue$2.run(EventQueue.java:674)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:86)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:690)
    at java.awt.EventQueue$3.run(EventQueue.java:688)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:86)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:687)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

我不知道为什么!

Answer 1

这不是您进行 Swing 或 AWT 绘图的方式:

private Graphics graphics = null;

相反，如果是 AWT 应用程序，您应该在组件或此类的子组件的 paint 方法中绘制)并使用 JVM 提供的 Graphics 对象。

更好的方法是创建一个 Swing 应用程序并在 JComponent 或其子组件的 paintComponent(Graphics g) 方法中绘制，再次使用 JVM 提供的 Graphics 对象。最重要的是，谷歌并阅读教程。我可以根据经验告诉你，你不应该猜测这些东西，因为你总是会猜错。