AbstractStringBuilder is not visible

Hi,

I am uploading source code in NWDS and i have resolved all the exceptions that occur due to jar files.

I am receiving one Exception :

Abstact String Builder is not visible.

Am I missing any jar files, i also checked with the compatible versions.

jdk1.6.0_26

jre1.6.0_07

NwDS : 7.0.17