There is a subtle issue here that is a bit of a gotcha.
The toString() method has a base implementation in Object. CharSequence is an interface; and although the toString() method appears as part of that interface, there is nothing at compile-time that will force you to override it and honor the additional constraints that the CharSequence toString() method's javadoc puts on the toString() method; ie that it should return a string containing the characters in the order returned by charAt().
Your IDE won't even help you out by reminding that you that you probably should override toString(). For example, in intellij, this is what you'll see if you create a new CharSequence implementation: http://puu.sh/2w1RJ. Note the absence of toString().
If you rely on toString() on an arbitrary CharSequence, it should work provided the CharSequence implementer did their job properly. But if you want to avoid any uncertainty altogether, you should use a StringBuilder and append(), like so:
final StringBuilder sb = new StringBuilder(charSequence.length());
sb.append(charSequence);
return sb.toString();