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

Case 1:

 System.out.println(strNull+str);  // will not give you exception

From the docs(String conversion)

If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l).

Otherwise, the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments; but if the result of invoking the toString method is null, then the string "null" is used instead.

Case 2:

str.concat(strNull);  //NullPointer exception

If you see the source of concat(String str) it uses str.length(); so it would be like null.length() giving you a NullPointerException.