Using method of java : Text Field API

http://docs.oracle.com/javase/tutorial/uiswing/components/textfield.html

This is the best guide way to understand about Java

Text Field API 

some other links are:

http://www.leepoint.net/notes-java/GUI/components/30textfield/11textfield.html

https://compilr.com/pricing?referer=course-build-your-own-calculator

http://www.homeandlearn.co.uk/java/calculator_plus_button.html