Q:

In Eclipse, the changes of my source code have no effect

I have a strange problem in Eclipse Helios. When I alter my source code, my changes have no effect. Even if I alter the code in a way that looks uncompilable, it still can be run. 
It seems that Eclipse keeps its state somewhere in the background while I edit and save my sources and commit them to svn. 
I don't know what setting to look up or what else to do. I'd be glad to receive any help.
Best regards,
David

A:

Note : Please try this only if you have run out of other options in this thread.
So Just in case the other options like clean/rebuild listen in the other answers, do not address the problem do the following,
Close eclipse 
Go to the project folder on the hard drive.
Delete the .metadata file and restart eclipse and revisit the workspace.
The class path and libraries will be re validated and the project fresh built.

A:

In the Project menu, verify that Build Automatically is checked.

A:

I was having the same problem while running a Maven project in Spring Tool Suit (which is based on Eclipse). I needed to clean and build the maven project every time to make the STS detect the new changes (what is obviously a slow and inefficient process).
So my solution was:
Pressing Alt+F5 and check Force update of Snapshots/Releases

