Java – StringBuilder using string instead of StringBuilder

I use StringBuilder instead of string in my code

But when I look at its source code, the substring () method abstractstringbuilder, so StringBuilder returns a string instead of StringBuilder

What's behind this?

thank you.

Solution

The reason why the substring method returns an immutable string is that once you get a part of the string in StringBuilder, it must be copied It can't give you a variable "real-time view" into the middle of the contents of StringBuilder, because otherwise you will encounter conflicts, and the changes apply to what string

Because you have to make a copy anyway, it may also be immutable: if you want to build a StringBuilder around it, you can easily make it variable and fully understand its separation from the variable original