How to decide whether a statement requires citing a reliable source When writing a paper, I usually face the problem of deciding whether a sentence is cite-able or not. Let's say I am writing about optimization of an algorithm. And I used Java to implement it. So, in the introduction, I would say something like "Java is a class based object oriented language..."  This information is available in many java books and online tutorials. Does the ubiquity of the sentence make it un-citeable? Or do I cite that fact. I have read many prominent papers that simply use that sentence without quoting a source. 

Or do we only cite numerical facts that go like "Java is x times slower/faster than c++". I once read an article that says "when writing a paper, you should cite every sentence that you happen to read some where." But this is unrealistic; and we might have almost as many citations as the number of sentences in the paper. 

Is there any rule that dictates on what is quotable or not?
Yes, there are rules on what requires citation.

Generally, publicly-known information (widely known, common knowledge, etc.) does not need to be cited. Your sentence about Java being class-based would fall under this category.

Information which is based on someone else's work (analysis, research, pictures, etc.), whether quoted or paraphrased, should always be cited. Your sentence about Java being x time slower/faster than C++ would fall under this category.

If you are unsure, cite it to be safe.