[英]Java Applet Error, NullPointerException
我刚刚开始学习Java小程序,并遇到了此错误。 有人能帮我吗?
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!
这不是您执行Swing或AWT绘图的方式:
private Graphics graphics = null;
相反,如果是AWT应用程序,则应绘制Component的paint
方法或此类的子级),并使用JVM提供的Graphics对象。
最好使用JVM提供的Graphics对象,创建一个Swing应用程序并绘制JComponent的paintComponent(Graphics g)
方法或该方法的子级。 最重要的是,Google并阅读了教程。 我可以通过经验告诉您,您不应该猜这些东西,因为您总是会猜错。
您试图在空指针graphics
上调用方法。 图形必须实例化并指向实例,然后才能在其上调用方法。
在此处查看更多信息什么是NullPointerException,以及如何解决?
尽管两个答案都很有帮助,但没有一个给我解决方案。 康的回答给了我什么地方出了错:
g.create();
然后我发现应该是:
graphics = g.create();
再次感谢您的所有建议!
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