Revert "Merge remote-tracking branch 'upstream/trunk' into GIRAPH-723"

This reverts commit 8b224040f5dc86381cbcd7a9ec0357852780a9d3, reversing
changes made to 6fabdaaca9c540e1b2b08af996ba2201fdb2358a.
Revert "fix an error for Java6"

This reverts commit 6fabdaaca9c540e1b2b08af996ba2201fdb2358a.