How to increase word with symbols?

Assume we get any word.
For example "apple"

String a = "apple"

This String got length 5 symbols.
How to increase this word to 15 symbols added "#" symbol.

Should convert to:

String a = "apple##########"

How to achieve it? Knowing that string in java immutable. I don’t find standart method to do it.

>Solution :

String a = "apple";
String b = a.concat("#".repeat(10));;