Method String.join new in java8 (not in java7)

We try to implement version 2.1.0 of sdk. It's not possible with Java 7 in opposition with you documentation **Minimum Requirements : Java Development Kit 7**.

There are in source code specific instructions from Java 8, see command lines in the class LocationCache.java :
logger.trace("Current writeEndpoints = ({}) readEndpoints = ({})",
**String.join**(", ", nextLocationInfo.writeEndpoints.toString()),
**String.join**(", ", nextLocationInfo.readEndpoints.toString()));
The instruction **String.join** was only **introduced in Java 8** (JDK 1.8) :
https://docs.oracle.com/javase/**7**/docs/api/java/lang/String.html
https://docs.oracle.com/javase/**8**/docs/api/java/lang/String.html

FYI, same issue in version of SDK 2.1.1.

Also:
https://stackoverflow.com/questions/23413092/error-under-ubuntu-with-the-method-string-join

Let me know how you will fix it.
Thanks,