The source attachments does not contain the file for GContainer.class

The source attachments does not contain the file for GContainer.class

I am new to Java and have been trying to follow a course provided by
Stanford.
Installed the JRE 6 from their website. Probably along the way, my
computer has prompted me to upgrade the JRE, since now I m noticing the
JRE 7 version in the "Libraries" in the Java Build path of my project. The
projects that I used to run perfectly fine, and now is has stopped running
giving me these errors:
Source not found
The sources attachment does not contain the file for GContainer.class
Source not found
The source attachment does not contain the file for Program.class
Source not found
The source attachment does not contain the file for GraphicsProgram.class
Source not found
The source attachment does not contain the file for GCanvas.class
Since the program used to work perfectly and all of these files must be in
the imported file (acm.program.* , acm.graphics.* , acm.util.* , acm.awt.*
, acm.awt.event.*), I suspect the new errors I m encountering might be due
to the discrepancy or disconnect that was created when I inadvertently
upgraded to JRE7. Or maybe something else. I have spent a ton of time on
various sites including here, but the discussions are way above my head as
a beginner. I m thinking there must be a relatively easy fix to this, as I
did nt change the Build Path or anything else. Would appreciate any
guidance.