OneDefinitionRule is more strict on global variables (which have static duration). You can’t have 2 global variables sharing the same name. Devil is in the details:
As explained in various posts, you declare the same global variable in a header file that’s included in various compilation units, but you allocate storage in exactly one compilation unit. Under a temporary suspension of disbelief, let’s say there are 2 allocated storage for the same global var, how would you update this variable?
With free function f1(), ODR is more relaxed. http://www.drdobbs.com/cpp/blundering-into-the-one-definition-rule/240166489 explains the Lessor ODR vs Greater ODR. My focus today is the Greater ODR therein. The same function can be included via a header file into multiple compilation units and goes into multiple binaries. Two copies of this function are usually identical definitions. If they actually have different definitions, compiler/linker can’t easily notice and are not required to verify, so no build error (!) but you get could strange run time errors.
Java linker is simpler and never cause any problem so I never look into it.