fix some warnings

[javeme]

Change-Id: If743a47269ac769c7108c697ad8cfdd50225dc26

[codecov]

Codecov Report

Merging #1076 into master will increase coverage by 8.42%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1076      +/-   ##
============================================
+ Coverage     60.97%   69.40%   +8.42%     
- Complexity     4718     5562     +844     
============================================
  Files           331      331              
  Lines         26877    26877              
  Branches       3842     3842              
============================================
+ Hits          16389    18654    +2265     
+ Misses         8719     6410    -2309     
- Partials       1769     1813      +44     

Impacted Files	Coverage Δ	Complexity Δ
...ugegraph/backend/cache/CachedGraphTransaction.java	88.54% <ø> (+3.05%)	38.00 <0.00> (+1.00)
...gegraph/backend/cache/CachedSchemaTransaction.java	96.72% <ø> (+11.47%)	31.00 <0.00> (+3.00)
.../java/com/baidu/hugegraph/task/HugeServerInfo.java	68.70% <ø> (ø)	18.00 <0.00> (ø)
...m/baidu/hugegraph/backend/tx/GraphTransaction.java	80.62% <0.00%> (+0.22%)	302.00% <0.00%> (+14.00%)
...du/hugegraph/backend/tx/GraphIndexTransaction.java	80.50% <0.00%> (+0.35%)	193.00% <0.00%> (+3.00%)
...u/hugegraph/backend/store/mysql/MysqlSessions.java	68.94% <0.00%> (+0.45%)	27.00% <0.00%> (ø%)
.../baidu/hugegraph/backend/tx/SchemaTransaction.java	91.54% <0.00%> (+0.49%)	70.00% <0.00%> (+1.00%)
...n/java/com/baidu/hugegraph/structure/HugeEdge.java	75.12% <0.00%> (+0.51%)	84.00% <0.00%> (+3.00%)
...om/baidu/hugegraph/task/StandardTaskScheduler.java	80.10% <0.00%> (+0.52%)	86.00% <0.00%> (+5.00%)
...ava/com/baidu/hugegraph/structure/HugeElement.java	70.52% <0.00%> (+0.52%)	70.00% <0.00%> (+3.00%)
... and 122 more

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