Why I'm getting this output.?? can anyone explain plz ...(Note :- plz don't give unnecessary comments) i know you all are genius

Question

https://code.sololearn.com/cW0zkvXl9I3k/?ref=app

Answer 1

You might want:
a = a.concat(b);
Concat returns a new string, doesn't modify the original.

Answer 2

concat() doesn't work in place, so "a" isn't changed.
If you want to change "a", reassign:
a = a.concat(b);

Answer 3

compare with

StringBuilder a = new StringBuilder("10");
String b = "20";
a.append(b);

System.out.println( a);