What is the difference after I change my code to
Did you read linked material? Put it simple, since you have setter methods it would be silly not to use them and instead do another direct assign. Doing such thing is also called "code duplication". {Want t learn more? Read Refactoring: Improving the Design of Existing Code}
By the way, when I run the Employee test, my decimal is like 660.000000, how to change decimal place into 2?
As for this you need to rad something on formatting. Like in your case DecimalFormat