Problem: Eclipse is not highlighting variables if select one variable
Solution: Searched in Eclipse Help and got the answer.
Locate variables and their read/write access
You can locate variables and see their read/write status by selecting an identifier (variable, method or type reference or declaration) and invoking Search > Occurrences in File > Identifier. This marks all references of this identifier in the same file. The results are also shown in the search view, along with icons showing the variable’s read or write access.