Fix compile error on JDK 1.6.
Fix compile error on JDK 1.6.

dindin5258/calcite,minji-kim/calcite,amoghmargoor/incubator-calcite,dindin5258/calcite,jcamachor/calcite,arina-ielchiieva/calcite,julianhyde/calcite,vlsi/calcite,yeongwei/incubator-calcite,jcamachor/calcite,julianhyde/calcite,vlsi/calcite,julianhyde/calcite,jcamachor/calcite,sudheeshkatkam/incubator-calcite,glimpseio/incubator-calcite,looker-open-source/calcite-avatica,looker-open-source/calcite,looker-open-source/calcite-avatica,amansinha100/incubator-calcite,datametica/calcite,datametica/calcite,glimpseio/incubator-calcite,arina-ielchiieva/calcite,wanglan/calcite,dindin5258/calcite,glimpseio/incubator-calcite,looker-open-source/calcite,hsuanyi/incubator-calcite,wanglan/calcite,adeshr/incubator-calcite,apache/calcite-avatica,b-slim/calcite,joshelser/incubator-calcite,minji-kim/calcite,googleinterns/calcite,glimpseio/incubator-calcite,wanglan/calcite,julianhyde/calcite,mehant/incubator-calcite,b-slim/calcite,julianhyde/optiq,minji-kim/calcite,hsuanyi/incubator-calcite,minji-kim/calcite,jinfengni/optiq,jinfengni/incubator-optiq,apache/calcite,apache/calcite,joshelser/incubator-calcite,googleinterns/calcite,looker-open-source/calcite-avatica,apache/calcite,YrAuYong/incubator-calcite,sreev/incubator-calcite,datametica/calcite,julianhyde/optiq,looker-open-source/calcite,vlsi/calcite,arina-ielchiieva/calcite,apache/calcite-avatica,julianhyde/calcite,apache/calcite-avatica,vlsi/calcite,jcamachor/calcite,dindin5258/calcite,googleinterns/calcite,jcamachor/calcite,vlsi/incubator-calcite,xhoong/incubator-calcite,joshelser/calcite-avatica,dindin5258/calcite,apache/calcite,apache/calcite,xhoong/incubator-calcite,googleinterns/calcite,dindin5258/calcite,devth/calcite,datametica/calcite,googleinterns/calcite,yeongwei/incubator-calcite,joshelser/incubator-calcite,mapr/incubator-calcite,joshelser/calcite-avatica,sreev/incubator-calcite,sreev/incubator-calcite,apache/calcite,datametica/calcite,xhoong/incubator-calcite,joshelser/calcite-avatica,yeongwei/incubator-calcite,YrAuYong/incubator-calcite,looker-open-source/calcite-avatica,sreev/incubator-calcite,YrAuYong/incubator-calcite,googleinterns/calcite,amoghmargoor/incubator-calcite,amoghmargoor/incubator-calcite,adeshr/incubator-calcite,looker-open-source/calcite-avatica,amoghmargoor/incubator-calcite,jinfengni/optiq,hsuanyi/optiq,hsuanyi/optiq,xhoong/incubator-calcite,sudheeshkatkam/incubator-calcite,joshelser/incubator-calcite,mapr/incubator-calcite,b-slim/calcite,vlsi/incubator-calcite,looker-open-source/calcite,vlsi/calcite,adeshr/incubator-calcite,adeshr/incubator-calcite,devth/calcite,sudheeshkatkam/incubator-calcite,arina-ielchiieva/calcite,looker-open-source/calcite,looker-open-source/calcite,sudheeshkatkam/incubator-calcite,apache/calcite-avatica,wanglan/calcite,mprudhom/incubator-optiq,b-slim/calcite,datametica/calcite,julianhyde/calcite,YrAuYong/incubator-calcite,vlsi/calcite,amansinha100/incubator-calcite,mprudhom/incubator-optiq,jcamachor/calcite,hsuanyi/incubator-calcite,yeongwei/incubator-calcite,apache/calcite-avatica,b-slim/calcite,hsuanyi/incubator-calcite,jinfengni/incubator-optiq,mehant/incubator-calcite,minji-kim/calcite,arina-ielchiieva/calcite