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

String.concat() need an object of type String as a parameter.

There is no type which null is an instanceof. Refer JLS:

15.20.2 Type Comparison Operator instanceof

RelationalExpression: RelationalExpression instanceof ReferenceType At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.