Null Graphics objects

Question

Hello, I am attempting to write a java program that places a graph within a panel that the user can interact with. I have chosen Canvas() as the method to acieve this, but am running into problems with the instantiation of the graphics object- it keeps returning null, and I haven't the faitest idea why. Here is my code in question:
Canvas canvas = new Canvas();
Graphics g = canvas.getGraphics();
'g' is null- when I try to call g.drawLine(), it gives me a nullPointerException. Any help or information about why this is occuring would be greatly appreciated.

Answer 1

Well, I figured out that I needed to add the canvas to the contentPane before it had a Graphics object associated with it. But now, where the canvas is, there is a just a gray box. SetColor doesn't work, drawLine doesn't work, nothing. Here is the code:
Canvas canvas = new Canvas();
contentPane.add(canvas, BorderLayout.WEST);
show();
Graphics g = canvas.getGraphics();
Color white = new Color (255,255,255);
g.setColor(white);
setSize(300,200);
//Sets up the graph lines and tic marks
int size = 100;
g.drawLine(0, 0, size, 0);
g.drawLine(0, 0, 0, size);
//Horizontal tics
g.drawLine((size/4), 0, (size/4), (size/10));
g.drawLine((size/2), 0, (size/2), (size/10));
g.drawLine(((3*size)/4), 0, ((3*size)/4), (size/10));
g.drawLine(size, 0, size, (size/10));
//Vertical tics
g.drawLine(0, (size/4), (size/10), (size/4));
g.drawLine(0, (size/2), (size/10), (size/2));
g.drawLine(0, ((3*size)/4), (size/10), ((3*size)/4));
g.drawLine(0, size, (size/10), size);
show();
As I said, it compiles and runs fine, but there are no lines set up. I even added a bunch of show() statements as a desperate effort to get something to show up. Can anyone help? Thanks
I hate programming.