JLS 15.18.1, "String Concatenation Operator +" does not mandate any specific implementation for String concatenation.

The Javadoc for java.lang.String makes an unnecessarily stronger claim:
  * The Java language provides special support for the string
  * concatenation operator ( + ), and for conversion of
  * other objects to strings. String concatenation is implemented
  * through the {@code StringBuilder}(or {@code StringBuffer})
  * class and its {@code append} method.

We should relax this in Javadoc too.