Read data: I can use classes bufferedReader and bufferwriter. 
Lets say I have a text in Java.
I use bufferedReader to read the files if it is a random file 
bufferedReader is only the functionality to read a file.
For, Excel files I use the special library, Apache POI library. 
These are external libraries. 
I use properties class to read the properties files. 
Scanner class also for read the data from files