Add test for Mile equals null
Refactor equals of Mile, shorten the code.
Test equality for Yard
Add test and implemention for equlity of Yard
Add test for Yard != null
Test equality between Mile and Yard and the simplest implementation
Test 1760Yard = 1Mile and simplest implementation
Extract equals into parent class Length to decrease duplicate code.
Add test for 1Yard == 3Feet and 3Feet == 1Yard
Refactor equals for different length unit in Length, to decress if else if else if....
Put map prepare into construtor, because it needs to be there once the object initaited.
Add test for 1Feet == 12Inch and 12Inch == 1Feet
Make constructor of Length private, so no new Length Unit can be derived.
Add test and implementation for convert mile yard, and mile feet