Apache's StringUtils.isNumeric() method specification says:
Checks if the String contains only unicode digits. A decimal point is not a unicode digit and returns false.
Null will return false. An empty String ("") will return true.
Is this logically right? Why do they see empty string as numeric?
There was not only me who asked this question :) People were opening this defect in Apache's Jira: https://issues.apache.org/jira/browse/LANG-428
They closed without fixing it only to keep backwards compatibility (to follow method specification).
But everybody agreed that current behavior of method is wrong.