Java Applet錯誤，NullPointerException

Question

我剛剛開始學習Java小程序，並遇到了此錯誤。 有人能幫我嗎？

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

    }

}

出現此錯誤：

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)

我不知道為什么！

Answer 1

這不是您執行Swing或AWT繪圖的方式：

private Graphics graphics = null;

相反，如果是AWT應用程序，則應繪制Component的paint方法或此類的子級），並使用JVM提供的Graphics對象。

最好使用JVM提供的Graphics對象，創建一個Swing應用程序並繪制JComponent的paintComponent(Graphics g)方法或該方法的子級。 最重要的是，Google並閱讀了教程。 我可以通過經驗告訴您，您不應該猜這些東西，因為您總是會猜錯。

Answer 2

您試圖在空指針graphics上調用方法。 圖形必須實例化並指向實例，然后才能在其上調用方法。

在此處查看更多信息什么是NullPointerException，以及如何解決？

Answer 3

盡管兩個答案都很有幫助，但沒有一個給我解決方案。 康的回答給了我什么地方出了錯：
g.create();
然后我發現應該是：
graphics = g.create();

再次感謝您的所有建議！