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

The String class holds an array (probably an ArrayList) of characters. When you call .concat() it goes through and adds every character from the second string to the first.

If the first String is null, there is nothing to add to, causing a NullPointer Exception. Try initializing Strings with "".