Why String concatenate null with + operator and throws NullPointerException with concate() method

Question

Here is my class, where i am concatenating two string. String concatenate with null using + operator execute smoothly but throws NullPointerException

Answer 1

concate() method creates new String() object under the hood.

where as + combines\ concatenates values by using toString() method. That's why when we use + it continents "somevalue".append(null).toString().

for reference see this question.