蔡鸟问题，怎么在一个Panel上画图，一条线就行

[yukia]

一个同学说：
public class ActiveCanvas extends JPanel{
public ActiveCanvas() {
}
public void paint(Graphics g){
g.drawLine(10,10,100,100);
}
}
我试了试，还是不会！我知道这是对的。但是初学，没接触过。
我试着在他的下面写
void button1_actionPerformed(ActionEvent e) {
Graphics g;
ActiveCanvas canvas1;
canvas1.paint(g);
}
编译不过，提示canvas1和g可能没有初始化！
怎么初始化呀？求救，刚开始学，请讲得细一点，谢谢！

[wchq1998]

ActiveCanvas canvas1;//应在Jinit方法中定义和声明和初始化
//ActiveCanvas canvas1= new ActiveCanvas();


void button1_actionPerformed(ActionEvent e) {
Graphics g;
g=canvas1.getGraphics();//获得canvas1的图形句柄
g.drawLine(10,10,100,100);

}

[yukia]

oh,谢谢，这个解决了，编译通过！
可还是不能在我指定的panel上画
怎么指向我的panel
java.lang.NullPointerException

at eidtor.Frame1$ActiveCanvas.paint(Frame1.java:116)

at eidtor.Frame1.button1_actionPerformed(Frame1.java:124)

at eidtor.Frame1_button1_actionAdapter.actionPerformed(Frame1.java:158)

at java.awt.Button.processActionEvent(Button.java:381)

at java.awt.Button.processEvent(Button.java:350)

at java.awt.Component.dispatchEventImpl(Component.java:3639)

at java.awt.Component.dispatchEvent(Component.java:3480)

at java.awt.EventQueue.dispatchEvent(EventQueue.java:450)

at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:197)

at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:144)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:136)

at java.awt.EventDispatchThread.run(EventDispatchThread.java:99)