Eclipse: false error with jdk7 - eclipse

Question

I've been having a problem with eclipse that's been stressing me a lot. Whenever I start a new Java porject, everything goes smoothly and perfect, until I "clean" the project (Project -> Clean...) and everything goes to ##~%&...
I keep getting:
XYZ cannot be resolved to a type
Multi-catch parameters are not allowed for source level below 1.7
<> operator is not allowed for source level below 1.7
That's very nice, only that I am importing all required classes and I do have 1.7 since the begginig. It's the only version I've installed. I've already tried this and other similar responses for the last few days with no avail.
A solution was to create a new project, copy and paste all packages/clases, which worked for a while, but I added UMLet to the project and again... Removing UMLet plugin did nothing for me.
BTW, even with the "errors", my project runs without any problem, therefore I know they're not real. I could keep working, but then, I can't see when a real error arises...
Does anyone got a final solution to get rid of this false errors?
EDIT:
I forgot to mention that I'm working with Kepler.
EDIT 2:
Here are screen captures with the requested information.

Answer 1

You can solve this by setting up correct JRE environment in Eclipse as below.
Go to Project => Properties => Java Build Path
Click on Libraries
Select JRE System Library
Click on Edit button at right hand side
Set Execution Environment to Jave SE-1.7(jre7)
Clean project or refresh project.

Answer 2

A solution was to create a new project, copy and paste all packages/clases, which worked for a while, but I added UMLet to the project and again...
One possibility is that the UMLet plugin is the cause of your problems. Try a clean Eclipse install without UMLet. (The fact that uninstalling the UMLet plugin didn't fix the problem is in no way conclusive.)
Note that UMLet will run as a stand-alone tool.
BTW, even with the "errors", my project runs without any problem, therefore I know they're not real.
The likely explanation for that is that the compilation errors come from the incremental Java compiler that is built into Eclipse. A different compiler (i.e. the one from your installed / configured JDK) is used to generate the ".class" files.

Answer 3

Multi-catch parameters are not allowed for source level below 1.7
not a solution, to this issue, but if you want to have two exceptions you can code it like this:
try {
// code here
} catch {
// code here
} catch {
// code here
}
it worked for me, instead of having catch (exception1 | exception2 e)