This statement contains a redundant check of a known non-null s against the constant null.
http://findbugs.sourceforge.net/bugDescriptions.html#RCN_REDUNDANT_NULLCHECK_OF_NONNULL_VALUE

based on patch by Kui Liu

based on pytch by Kui Liu

The current source code accesses the key and value of a Hashtable entry, using a key that is retrieved from a keySet iterator.
It is more efficient to use an iterator on the entrySet of the Hashtable, to avoid the Map.get(key) lookup.

The method is building a String using concatenation in a loop.
In each iteration, the String is converted to a StringBuilder, appended to, and converted back to a String.
This can lead to a cost quadratic in the number of iterations, as the growing string is recopied in each iteration.
Better performance can be obtained by using a StringBuilder explicitly.
http://findbugs.sourceforge.net/bugDescriptions.html#SBSC_USE_STRINGBUFFER_CONCATENATION

This instanceof test will always return true because DataType is the superclass of variable nested's class ResourceComparator. The variable nested is not initialized, it would be better to do a null test rather than an instanceof test.
http://findbugs.sourceforge.net/bugDescriptions.html#BC_VACUOUS_INSTANCEOF

Also add test