FULL PRODUCT VERSION :
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 10.0.14393]
(any system really, it's in the jdk's source)

A DESCRIPTION OF THE PROBLEM :
The FXMLLoader class overrides the Object#equals method to set it so that it is dependent not on the instance of the FXMLLoader but on the field FXMLLoader#location.

This isn't reflected by the FXMLLoader#hashcode method, which is not implemented and is therefore defaulting to Object#hashcode, which means that it is instance-bound only.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Create two instances of FXMLLoader.

Assert their equalities as per equals() and hashCode() are the same value.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
That the assert succeeds.
ACTUAL -
The assert fails.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.lang.AssertionError ...

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
FXMLLoader loader1 = new FXMLLoader();
FXMLLoader loader2 = new FXMLLoader();

boolean equalsEquals = loader1.equals(loader2);
boolean equalsHashcode = loader1.hashCode() == loader2.hashCode();

assert equalsEquals == equalsHashcode;
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Just never use FXMLLoader#equals.
Also do not use maps to store them or check their presence.

Would be nice to have a warning of sorts in the class' JavaDoc.