Refactored code to follow OOD, Simplicity, Readability, Maintainability, Clean code principles etc.

Source code modified to reflect proper encapsulation, string concatenation, accessors, singleton pattern, variable & method naming,  removed unwanted complexity etc. 
JUnit Test cases modified to reflect the refactored code.
All source & test code compiled successfully.
All JUnit test cases ran successfully.

Further refactorings (not done yet) would be to provide test cases in BankTest for withdrawal from account, TransactionTest valiating amount, Modifying testApp() in CustomerTest to a better name etc.