Most likely you downloaded the wrong version of the module, those imports aren't even present in Community Commons v1.2 which is ment for Mendix 2.5.3., they only appear in later versions which are ment for Mendix 3.x.
To download the right version, go to the 'older versions' tab and find your Mendix version there. Or consider upgrading to Mendix 3.x
Did you download the correct version of the Community Commons module? I don't even have a StringUtils.java in Community Commons version v1.2 (which is for Mendix 2.5.3)