I think what you mean is that you're having problems when you open a class file in that you cannot see the class Java source.
That is normal if you are trying to open a class that you don't have source code for, such as a library class. The source for many of the standard JDK/JRE files will be installed with Eclipse, but other external classes generally won't have source installed. In which case you can always download the source manually if it's available. Make sure that it's the EXACT same version as the class itself, though. Otherwise you'll see strange things if you try to step a debugger through the class.
Note that having the class source on your local machine isn't enough. You will have to tell Eclipse where the class source code is located.
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