Java source and binaries out of sync

[WhatNow46]

Well, here goes ...
I was debugging an application that uses classes such as the following:
Graphics, Graphics2D, Plot2D, GeneralPath, ..., methods such as
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g.create();
...
...
g2.draw(path);
}
When I got to paintComponent I noticed that the actual class was sun.java2D.SunGraphics2D, not Graphics2D as I'd thought, and the debugger skips over g2.draw(path) when I try to drill in to it, obviously it can't find the source. Among other things SunGraphics2D inherits from Graphics2D so clearly, my source code is out of sync with my runtimes. And when I look in the src/java/awt directory of my jdk installation which is jdk1.6.0_18, Graphics2D.java is there, but not SunGraphics2D. Can anyone tell me which source files are missing from my installation, and how to get them? If you need more info on my runtimes, please let me know.

My directories look like this:
Java
-Java3D
--1.5.2
--j3d-1_5_2-api-docs
--src
-jdk1.6.0_18
--bin
--demo
--include
--jre
--lib
--sample
--src
-jre6
--bin
--lib

If the above is confusing, I've attached a bitmap.

Thanks.