Why there is no reverse method in
Since you have it in StringBuilder, there's no need for it in String, right? :-)
Seriously, when designing an API there's lots of things you could include. The interfaces are however intentionally kept small for simplicity and clarity. Google on "API design" and you'll find tons of pages agreeing on this.
Here's how you do it if you actually need it:
str = new StringBuilder(str).reverse().toString();